1
In 430 B.C. Democritus postulated that matter is made up of very
small particles called Atomswhich means "indivisible".Later,
Antoine Lavoisier, from his experimental observations, established
laws of chemical combinations.
2. LAWS OF CHEMICAL COMBINATION
As mentioned above whenever reactants (elements) react together
to form a compound they do so according to certain laws. These laws
are called laws of chemical combination. There are two important
laws of chemical combination. These are
(i)Law of conservation of mass
(ii)Law of constant proportions
2.1LAW OF CONSERVATION OF MASS
In any chemical reaction the total mass of the reactants is
equal to the total mass of the products
2.1.1When matter undergoes a physical change mass does not
change or in other words mass is conserved during the physical
change.
Water
Ice
Heat
After heating, the ice changes into water. When we weighed the
flask the mass does not change though a physical change has taken
place
2.1.2When matter undergoes a chemical change, mass remains the
same or unchanged
When barium chloride reacts with sodium sulphate barium sulphate
and sodium chloride is formed.
Barium chloride + Sodium sulphate
Barium sulphate + sodium chloride
(solution)(solution)(white ppt) (solution)
It was observed that the mass of the reactants (x) comes out to
be same as that of the products (y). This is in accordance with the
law of conservation of mass.
2.2LAW OF CONSTANT PROPORTIONS
A chemical compound is always made up of the same elements
combined together in the same fixed proportion by mass.
Example : In water, hydrogen and oxygen combined together in the
same fixed proportion of 1 : 8 by mass, irrespective of the source
of water (like river, rain or tap water).
If we decompose 9 g of pure water by electrolysis i.e. passing
electricity through it, then 1 gm of hydrogen and 8 gm of oxygen
are obtained. Now, This experiment shows that water always consists
of hydrogen and oxygen combined together in the same constant
proportion of 1 : 8 by mass.
% of an element in the compound =
compound
the
of
Mass
element
that
of
Mass
3. DALTONS ATOMIC THEORY
Atom
atom is defined as the smallest particle of an element that can
take part in a chemical reaction and which may or may not be
capable of free existence.
3.1POSTULATES OF DALTONS ATOMIC THEORY
The main postulates of Daltons atomic theory are as follows
:
(i)All matter is made up of very small particles called
atoms.
(ii)Atoms are indivisible particles, which can not be created or
destroyed in a chemical reaction.
(iii)Atoms of a given element are identical in all respects i.e.
size, shape, mass and chemical properties.
(iv)Atoms of different elements have different size and masses
and also posses different properties.
(v)Atoms of the same or different elements combine in the ratio
of small whole numbers to form compounds.
(vi)The relative number and kinds of atoms are constant in a
given compound.
(vii)Atoms of the same elements or two different elements may
combine in different ratios to form more than one compound.
3.3LIMITATIONS OR DRAWBACKS OF DALTONS ATOMIC THEORY
With the advancement in scientific studies Daltons atomic theory
suffered from the following drawbacks :
(i)Atom is no longer considered as the smallest indivisible
particle.
(ii)According to Daltons atomic theory says that all the atoms
of an element have exactly the same mass. Though it is now known
that atoms of the same elements may have different masses.
(iii)Daltons atomic theory atoms of different elements have
different masses. However it is now known that even atoms of
different elements can have the same mass.
(iv)Substances made up of the same kind of atoms may have
different properties. For example charcoal, graphite and diamond
are all made up of carbon atoms but have different physical
properties.
4.1HOW BIG ARE THE ATOMS ?
Atoms are very-very small in size. They are so small that they
can not be seen even under a microscope. To imagine about their
size, it is very much interesting to note that if millions of atoms
are stacked one above the other, the thickness produced may not be
equal to the thickness of the sheet of a paper.
The size of an atom is indicated by its radius which is called
atomic radius. Atomic radius is measured in nanometers. Which is
represented by the symbol nm.
1 nm = 10(9 m
1m = 109 nm
Relative size
Radii (in meter)
Example
1010
molecule of water
109
atom of hydrogen
108
molecule of Hemoglobin
104
Grain of sand
103
ant
10 1
Watermelon
4.2WHAT ARE THE MODERN DAY SYMBOLS OF ATOMS OF DIFFERENT
ELEMENTS ?
Symbol is a short method of representing anything. In case of
elements a short method of representing the full name of an element
is knows as symbols.
4.2.1Daltons symbols of element
Dalton was the first scientist to suggest the symbols for
elements in a very specific way. Daltons symbol for an element
represent the element as well as one atom of that element. Thus we
can say that the symbol used by him also represent the quantity of
the element. A few of these symbols as proposed by Dalton are as
follows .
4.2.2Berzelius suggestion for symbols of elements
J.J. Berzelius a Swedish chemist, suggested a more scientific
method for representing an element, He suggested that the first one
or two letter of the name of an element can be used as its symbols.
This idea led to the development of modern symbols of elements.
4.2.3Modern symbols of elements
In all cases the symbol of an element is the first letter or the
first letter and another letter of the English or Latin name of the
element. For example :
The symbol of Hydrogen is H
The symbol of oxygen is O
So, in the case of hydrogen and oxygen the first letter of their
English names are taken as their symbols.
It should be noted that in a two letter symbol, the first letter
is the capital letter but the second letter is the small letter.
The necessity of adding another letter arises only in case of
elements whose names start with the same letter. For example the
name of the elements viz. carbon, chlorine calcium and copper
starts with the common letter C.
Hence,
chlorine is represented by the symbol Cl
calcium is represented by the symbol Ca
copper is represented by the symbol Cu
4.3ATOMIC MASS
The atomic mass can be defined as :
One atomic mass unit is a mass unit equal to exactly one twelfth
(1/12th) the mass of one atom of carbon-12. The relative atomic
masses of all elements have been found with respect to an atom of
carbon -12.
This is called one atomic mass unit (amu). Now it is represented
simply by u which stands for unified mass.
Atomic mass of an element may therefore also be defined as the
number of times an atom of that element is heavier than 1/12th of
the mass of an atom of C-12 isotope.
For example an atom of magnesium is found to be two times
heavier than an atom of C-12 i.e. 24 times heavier than 1/12th of
the mass of C-12 atom. Hence, atomic mass of magnesium = 24
amu.
Atomic masses of some common elements
Element
Symbol
Atomic mass
Element
Symbol
Atomic mass
1.Hydrogen
H
1
14.Sulphur
S
32
2.Helium
He
4
15.Chlorine
Cl
35.5
3.Lithium
Li
7
16.Argon
Ar
40
4.Boron
B
11
17.Potassium
K
39
5.Carbon
C
12
18.Calcium
Ca
40
5. HOW DO ATOMS EXIST ?
Atoms usually exist in two ways :
(i)In the form of molecules
(ii)In the form of ions.
Through we can not see individual atoms or molecule or ions but
we can see the matter. For example we cannot see the Na+ and Cl(
ions but we can see the sodium chloride compound
6. WHAT IS A MOLECULE ?
A molecule is the smallest particle of an element or a compound
which can exist freely and possesses all the properties of that
substance.
Atoms of the same element or of different elements can join
together to form molecules.
6.1MOLECULE OF AN ELEMENT
The molecules of an element contain two (or more) similar atoms
chemically combined together. Molecules of many elements such as
argon (Ar), Helium (He) etc. are made up of only one atom of that
element. But this is not the case with most of the elements.
Depending upon whether the molecule contains one, two, three or
four atoms they are called monoatomic, diatomic triatomic, tetra
atomic or polyatomic. A few examples of molecules of different
types are as follows:
(i)Monoatomic molecules : Noble gases like Helium Neon etc.
exist as single atoms i.e. He, Ne etc. Hence they are called
monoatomic.
(ii)Diatomic molecules : Molecules of Hydrogen, Oxygen Nitrogen
contain two atoms of each element respectively and are represented
by H2, O2, N2 etc.
(iii)Triatomic molecules : Molecules containing 3 atoms are
called triatomic molecules. For example, ozone contains 3 atoms of
oxygen element combined together.
(iv)Tetratomic molecules : Molecules containing 4 atoms of an
element are called tetratomic molecules. Most common example is
that of phosphorus represented by P4.
(v)Polyatomic molecules : Molecules containing more than four
atoms of particular element are called polyatomic molecules. For
example a molecule of sulphur contains 8 atoms of sulphur and is
represented by S8.
6.1.1Atomicity
The number of atoms present in one molecule of a substance is
known as its atomicity.
6.2Molecules of compounds
The molecules of a compound consists of two or more atoms of
different elements combined together in a definite proportion by
mass to form a compound that can exist freely.
For example carbon dioxide contain, atom of carbon and two atoms
of oxygen combined together in a fixed ratio of 3 : 8 by mass.
Molecules of some compounds
Compound
Combining Elements
Ratio by Mass
Water
Ammonia
Carbon dioxide
Hydrogen, Oxygen
Nitrogen, Hydrogen
Carbon, Oxygen
1:8
14:3
3:8
The atomic masses of different elements are H = 1.0u, O = 16.0u,
N = 14.0u, C = 12.0u. By comparing the data we can find out the
ratio by number of atoms of elements in the molecule of the
particular compound as follows :
S.No.
Compound
Element
ratio by mass
Atomic mass(u)
Mass ratio/ atomic mass
simplest ratio
1.
H2O
H
1
1
1
1
1
=
2
O
8
16
2
1
16
8
=
1
2.
NH3
N
14
14
1
14
14
=
1
H
3
1
3
1
3
=
3
Thus ratio by number of atoms for water is H:O = 2:1, for
ammonia is N:H = 1:3 and Thus we can say that in a compound, the
element are combined together in a simple whole number atomic
ratio.
7. WHAT IS AN ION?
The charged species are known as ions. Depending upon the charge
they carry ions can be of two types :
(i)Cation : A positively charged ion is known as the cation. For
example Sodium ion (Na+). Magnesium ion (Mg++) etc. A cation is
formed by the loss of one or more electrons by an atoms. This can
be represented as follows :
+
-
Na
Na
electron
1
(sodium ion cation)
(ii)Anion : A negatively charged ion is known as anion. For
example chloride ion (Cl(), oxide ion (O( () are anions as they are
negatively charged.
An anion is formed by the gain of one or more electrons.
-
+
Cl
Cl
electron
1
Chlorine atomchloride ion (anion)
7.1SIMPLE IONS AND COMPOUND IONS (POLYATOMIC IONS)
7.1.1Simple ions
Those ions which are formed from single atoms are called simple
ions.
For example
,
Al
,
Mg
,
Na
3
2
+
+
+
etc.
7.1.2Polyatomic ion
Ions formed from a group of atoms carrying a charge (either
negative or positive) is known as a polyatomic ion or compound
ion.
For example
-
-
+
2
4
2
3
4
SO
,
CO
,
NH
.
CHEMICAL FORMULAA chemical formula of a molecular compound
represents the actual number of atoms present in one molecule of
the compound.
For example: H2O is the chemical formula of water, NH3 is the
chemical formula of ammonia.
Chemical formula of an ionic compound represents the cations and
anions present in the structure of the compound.
For example: Na+Cl( represents that sodium chloride contains Na+
and Cl( ions in the ratio of 1:1
CONCEPT OF VALENCY
Valency can be defined as the combining capacity of that
particular element.
For example valency of oxygen is 2, this means that one atom of
oxygen can combine with 2 atoms of hydrogen or in other words we
can say that valency of hydrogen is one so 2 atoms of hydrogen can
combine with one atom of oxygen to form water molecule.
Valencies of some common non-metal elements
Element
Symbol
Valency
Element
Symbol
Valency
Hydrogen
H
1
Oxygen
O
2
Fluorine
F
1
Sulphur
S
2, 4, 6
Chlorine
Cl
1
Nitrogen
N
3, 5
Bromine
Br
1
Phosphorus
P
3, 5
Iodine
I
1
Carbon
C
4
For writing the chemical formula of an ionic compound valency of
an ion can be defined as the units of positive or negative change
present on the ion.
For example:Na+ ion has one unit positive charge
SO
-
2
4
ion has two unit negative charge
Depending upon whether the ions has 1, 2, 3 or 4 unit charge
(positive or negative) they are called monovalent, divalent,
trivalent and tetravalent ions respectively.
8.2RULES FOR WRITING THE CHEMICAL FORMULAE
While writing a chemical formulae of molecular or ionic
compounds the following steps are to be followed:
(i)In case of simple molecular compounds (compounds made up of
only two elements). The symbols of the two elements are written
side by side and their respective valencies are written below their
symbols.
(ii)In case of simple ionic compounds, the symbol of the cation
or metal atom is written first followed by the symbol of the anion
or non-metal atom and their respective valencies are written below
their symbols. For example in CaO, symbol of calcium (Ca, a metal)
must be written first followed by symbol of oxygen (which is a
non-metal).
(iii)The valencies or charges on the ion must be balanced.
(iv)In case of compounds containing polyatomic ions. The formula
of the polyatomic ion is written in brackets and the valencies are
written below.
(v)In any of the above cases, if there is a common factor
between the valencies of the cation and anion, the valencies are
divided by the common factor.
(vi)Finally we apply cross-over of the valencies so that they
appear on the lower right hand side of the symbols. However, 1
appearing on the lower right hand side of the symbol is omitted.
Similarly we also omit the + and ( signs of the charges of the
ions.
Formulae of Simple Compounds
The simplest compounds, which are made up of two different
elements are called binary compounds.
Example 1: Steps for writing the formula of Hydrogen
chloride:
(i)Elements present are:hydrogen and chlorine
(ii)Symbols of the elements:HCl
(iii)Valency of the elements:11
(iv)Cross-over of the valency:
H Cl
1 1
(v)We staled that one appearing on the lower right hand side of
the symbol is omitted. So the formula of the compound would be
HCl.
Chemical formulae of some simple ionic compound
Example 1: Steps for writing the chemical formula of Sodium
chloride
(i)Elements present in the compound:SodiumChlorine
(ii)Symbols of the elements:NaCl
(iii)Charge on the ions+1(1
(iv)Valency of the elements:11
(v)Cross-over of the valency:
H Cl
1 1
(vi)So the chemical formula of sodium chloride can be written as
NaCl (As we omit 1 appearing on the lower right hand side of Na and
Cl atoms and + and sign. of the charges of ions).
8.2.3Chemical formulae of compounds containing polyatomic
ions
White writing the chemical formulae compounds containing
polyatomic ions, same rules will apply except that the formula of
polyatomic ion is written in brackets.
Example 1: Steps for writing the formula of potassium
nitrate
(i)Symbols of the ions:K (NO3)
(ii)Charge on the ions:1+1(
(iii)Valency of the ions:11
(iv)Cross-over of the valency:
K (NO
3
)
1+
1
(v)So the chemical formula of Potassium nitrate can be written
as KNO3 (As we omit one appearing on the right hand side of the
irons).
9. MOLECULAR MASS AND MOLE - CONCEPT
Molecular mass of a substance (element or compound) is the
average relative mass of its molecule as compared with that of an
atom of C-12 isotope taken as 12.
For example the molecular mass of hydrogen is 2, which means
that a molecule of hydrogen is two times heavier than the 1/12th of
the mass of an atom of C-12 isotope.
9.1CALCULATION OF MOLECULAR MASS
(a)A molecule of water has the formula H2O. Hence molecular mass
of
H2O = (2 x atomic mass of hydrogen) + (1 x atomic mass of
oxygen)
= (2 x 1.0 u + 1 x 16.04) = 18 u
(b)A molecule of sulphuric acid has the formula H2SO4. Hence
molecular mass of
H2SO4 = (2 x atomic mass of hydrogen) + (1 x atomic mass of
sulphur)
+ (4 x atomic mass of oxygen)
= (2 x 1.0 u) + (1 x 32.0 u) + (4 x 16.0 u)
= (2.0 u + 32.0 u + 64.0 u)
= 98.0 u
9.2FORMULA UNIT MASS
Before describing the formula unit mass, we should be aware of
the meaning of Formula Unit of an ionic compound.
The formula unit mass or formula mass of an ionic compound is
the sum of the atomic masses of all the atoms present in one
formula unit of the compound.
Example:Calculate the formula unit mass of Na2SO4.10H2O Atomic
masses Na = 23.0u, S = 32.0u, O = 16.0u, H = 1.0u.
Solution:Formula unit mass = (2 x atomic mass of Na) + (2 x
atomic mass of S) + (4 x atomic mass of oxygen) + 10 (2 x atomic
mass of H + atomic mass of oxygen)
= [2 x 23.0u] + [32.0u] + [4 x 16.0u] + 10 [ 2 x 1.0u +
16.0u]
= [46.0u + 32.0u + 64.0u] + 10 [2.0u + 16.0u]
= [142.0u] + 10 [18.0u]
= 142.0u + 180.0u]
= [322.0u]
Thus the formula mass of Na2SO4.10H2O is 322.0u
9.3GRAM ATOMIC MASS AND GRAM MOLECULAR MASS
Gram Atomic Mass
Atomic mass expressed in grams is called gram atomic mass of
that element.
For example:
(a)Atomic mass of Na = 23.0u
Gram atomic mass of Na = 23.0g
(a)Atomic mass of Cl = 35.5u
Gram atomic mass of Cl = 35.5g.
9.3.2Gram Molecular Mass
Like gram atomic mass, gram molecular mass can also be defined
as follows:
Molecular mass expressed in grams is called gram molecular mass
of that element
For example:
(a) Molecular mass of H2O = 18.0u
Gram molecular mass of H2O = 18.0g.
9.3.3Gram Formula Unit Mass
Similarly we can define the gram formula unit mass as
follow:
Formula unit mass expressed in grams is called as gram formula
unit mass.
For example:
Formula unit mass of NaCl = 23.0u + 35.5u
= 58.5u
Gram formula unit mass of Nacl = 58.5u
9.4MOLE CONCEPT
A mole of atoms is equal to one gram atom of that particular
element.
For example:
1 mole of Hydrogen (H) atom= 1g atom of H = 1.0g
1 mole of Oxygen (O) atom= 1g atom of O = 16.0g
1 mole of Nitrogen (N) atom= 1g atom of N = 14.0g
A mole of molecule is defined as that amount of the substance
which has mass equal to gram molecular mass.
For example
1 mole of Hydrogen (H2) molecule= 1 g molecule of H2 = 2.0g
1 mole of Oxygen (O2) molecule= 1 g molecule of O2 = 32.0g
1 mole of Ammonia (NH3) molecule= 1 g molecule of NH3 =
17.0g
9.4.2Mole in Terms of Number
A mole of particles (atoms, molecules or ions) is defined as
that amount of the substance which contains the same number of
particles as there are C-12 atoms in 12g of carbon.
Experimentally, it has been found that 12g of C-12 isotope
contain 6.022 x 1223 atoms. This number is called Avogadros number
or Avogadros constant and is represented by the symbol N0. Thus
Avogadros number (N0) = 6.022 x 1023.
Thus a mole of particles can also be defined as follows:
A mole of particles (atoms, molecules or ions) is that amount of
the substance which contain 6.022 x 1023 particles.
For example:
1 mole of C atoms= 6.022 x 1023 C atoms
1 mole of H2O molecules= 6.022 x 1023 H2O molecules
1 mole of Na+ ions= 6.022 x 1023 Na+ ions
A mole represents the following:
(i)It represents 6.022 x 1023 particles of the substance
(ii)The mass of one mole of an element is equal to the mass of
6.022 x 1023 atoms of that element.
(iii)One mole of a substance represents one gram, formula mass
of that
9.4.5Formulae for calculation
(a)Number of moles =
element
the
of
mass
atomic
Gram
grams
in
element
the
of
Mass
(b)Number of moles =
number
s
Avogadro'
molecules
or
atom
of
number
Given
1. CHARGED PARTICLES IN ATOM OR SUB-ATOMIC PARTICLES
OF ATOM
Activity
(i)Comb dry hair. Does the comb then attract small pieces of
paper?
(ii)Rub the glass rod with a silk cloth and bring the rod near
the an inflated balloon. Observe what happens?
Discussion
(i)When we comb our dry hair, and put the comb near the small
pieces of paper. We observe that the small pieces of paper got
attracted towards the comb.
(ii)When the glass rod rubbed by a silk cloth was brought near
the inflated balloon. The inflated balloon got attracted towards
the rod.
Conclusion:
From the above observation, we conclude that on rubbing two
objects together. They become electrically charged. The charged
produced shown that atom consists of charged particles also known
as sub-atomic particles.
1.1DISCOVERY OF ELECTRON
The existence of electrons in an atom was shown by J.J. Thomson
in 1897. He passed electricity at high voltage through a gas at
very low pressure taken in a discharge tube.
A discharge tube is a long glass tube and closed at both ends.
Two circular metal plates A and B are sealed at the two ends of the
tube as shown in figure. These circular plates are called
electrodes. A side tube S is fused to the tube which can be
connected to a vacuum pump (to such out the air or gas present
inside the tube to reduce the pressure inside the tube).
. The plate A connected to the negative terminal is cathode.
Whereas, the plate B connected to the positive terminal is called
anode.
1.1.2Properties of cathode rays
At normal pressure air or any other gas is a non-conductor of
electricity, but at low pressures the gases become conductors of
electricity. When sufficiently high voltage is applied across the
electrodes, current starts flowing thorough a stream of particles,
moving in the tube from the cathode to the anode. These were called
cathode rays or cathode ray particles.
The cathode rays have been found to possess the following
properties:
(i) Cathode rays travel in straight lines: This is shown by the
fact that if a metal object is placed in the path of the cathode
rays. They cast a sharp shadow of the object at the back.
(ii)Cathode rays are made up of material particle: If a large
paddle wheel (e.g. that of mica) is placed in their path, the wheel
starts rotating. This shows that cathode rays are made up material
particles.
(iii)Cathode rays carry negative charge: When an electric field
is applied on the cathode rays, they get deflected towards the
positive plate of the electric field. This shows that they carry
negative charge.
(iv) When cathode rays strike a metal foil, the foil becomes
hot. This indicates that cathode rays produce heating effect.
(v)They causes ionization of the gas through which they
pass.
(vii) They produce green fluorescence on the glass walls of the
discharge tube as well as on certain other substance such as zinc
sulphide (ZnS).
(viii) They produces penetrating effect i.e. they can easily
pass through thin foils of metal.
From the study of above properties it was concluded that:
(a)cathode rays are made up of material particles.
(b)cathode rays carry negative charge.
These negatively charged material particles constituting the
cathode rays are called electrons.
Determination of charge and mass of electrons
Further experiments were carried out to determine the exact
charge and mass of electrons.
(i)Charge to mass ratio of electron: J.J. Thomson studied the
extent of deflection of cathode rays of cathode rays under
influence of electric fields and magnetic fields of different
strengths. He placed different gases in the tube. He found that
every time the ratio of charge to mass of the electron was the
same. This is usually represented by e/m, where e represents the
charge on the electron and m represents the mass of the electrons.
The value was found to be:
m
e
=
Mass
Charge
= 1.76 108 C/g (Coulombs/kg)
(ii) Charge on the electron: Charge on the electron was found by
R.A. Milliken. He devised a method known as oil drop experiment to
determine the charge on the electrons. He found that the charge on
the electron was equal to 1.60 1019 C (1 unit).
This is the smallest quantity of charge that could be measured.
Hence, it is also called one unit charge
By using the vale of e/m and e, the mass of an electron can also
be calculated.
C/g
10
76
.
1
C
10
60
.
1
e/m
e
m
8
19
=
=
-
= 9.1 1031 kg
As charge on electron is 1 unit and mass is negligible
1.1.4Electrons are constituent of all atoms
We studies in the discharge tube experiment conducted by J.J.
Thomson, that we may take electrodes of any material and we may
take any gas inside the discharge tube at low pressure. The cathode
ray particles have the same e/m ratio as well as the charge (e)
i.e. they carry the same charge and mass. This shows that electrons
are constituents of all atoms.
1.2DISCOVERY OF PROTON
The existence of positive charged particles in an atom was shown
by Goldstein. Electric discharge carried out in the modified
cathode ray tube led to the discovery of particles carrying charge.
He took a discharge tube with a perforated cathode and a gas at low
pressure was taken inside the discharge tube.
On applying high voltage between the anode and the cathode, it
is observed that like cathode rays produces a fluorescence on the
glass wall on the tube at E, a fluorescence is also observed on the
glass wall of the tube F. This shows that some rays are also coming
from the anode which passed through the holes in the cathode and
strike, the wall of the tube F. These rays are called Anode rays,
as they are coming from the side of anode. They are also known as
canal rays. Their deflection in an electric field indicate that
they carry positive charge.
1.2.1Properties of anode rays / canal rays
The characteristics or properties of the positively charged rays
or anode rays or canal rays are listed below
(i)They travel in straight line
(ii)They are made up of material particle
(iii)They carry positive charge
(iv) Determination charge / mass ratio of the positively charged
particles present in anode rays: Unlike cathode rays, the ratio of
e/m is found to be different for different gases or we can say that
e/m is not constant but depends upon the nature of the gas taken in
the discharge tube.
(v) The value of charge on the particles constituting the anode
rays is also dependent on the nature of the gas taken inside the
discharge tube.
(vi)Mass of the particle constituting the anode rays is also
found to be different for the different gases taken in the
discharge tube.
Determination of charge and mass of proton
The chare and mass of protons are also determined experimentally
like that of electrons.
The charge on these particles is found to be same as that of on
the electrons, i.e.
e = +1.60 1019 C
The ratio of charge / mass (i.e. e/m) = 9.58 108 C/kg
The mass of proton
kg
C/
10
58
.
9
C
10
60
.
1
e/m
e
m
8
19
=
=
-
= 1.67 1027 kg
That sub-atomic particle carrying one unit positive charge and
has mass nearly equal to that of hydrogen atom
1.2.3Protons are constituent of all atoms:
If any other gas (other than hydrogen) is taken in the discharge
tube, it is observed that the mass of positively charged particle
is nearly a whole number multiple of the mass of proton. Hence, it
can be concluded that protons are the fundamental particle present
in all atoms.
1.3DISCOVERY OF NEUTRON
In 1932, Chadwick discovered another sub-atomic particle called
neutron, by bomarding a their sheet of beryllium by (-particles.
Neutrons are electrically neutral particle i.e. they has no charge
and have mass equal to or slightly greater than that of like
protons. Neutrons are present in the nucleus of all atoms except
hydrogen. In general x neutron is represented by the symbol n.
A newton can be defined as the fundamental particle of an atom
which has n( charge but has a mass nearly equal to that of hydrogen
atom
Composition of the characteristics of electrons protons and
neutrons.
2. THE STRUCTURE OF AN ATOM (ATOMIC MODELS)
According to Daltons atomic theory atom was indivisible and
indestructible. But after the discovery of subatomic particles
(electrons, protons and neutrons). Various atomic models were
proposed by many scientists to explain their arrangement in the
atom.
THOMSONS MODEL OF AN ATOM
After the discovery of electrons and protons J.J. Thomson (1898)
tried to explain the arrangement of electrons and protons within
the tom. He proposed that an atom consists of a sphere of positive
electricity in which electrons are embedded like plum in pudding or
seeds evenly distributed in red spongy mass in watermelon. The
radius of the sphere is of the other 10-8 cm which is equal to the
size of the atom. Although Thmosons model could explain the
electrical neutrality of an atom but this model could not satisfy
experimental facts proposed by Rutherford and hence was
discarded.
Limitations: Though it could explain the overall neutrality of
the atom, it failed to explain the results of experiments carried
out by other scientist.
2.2RUTHERFORDS MODEL OF ATOM
Earnest Rutherford was interested in knowing how the electrons
are arranged within the atom. For this purpose, he performed some
experiments also known as Rutherfords scattering experiment.
Experiment: In this experiment, he bombarded a thin foil e.g.
gold foil (thickness: 100 nm) with a beam of fast moving
(-particles: Alpha particles are high energy, positively charged
helium ions (emitted during radioactive decay of unstable elements
such as uranium) having 2 units of positive charge and 4 units of
mass. He observed the scattering of the (-rays after hitting the
foil by placing a circular zinc sulphide screen around the metal
foil. The results of scattering experiment were quite unexpected.
Rutherfords famous alpha particle-scattering experiment is
represented in the given figure:
Observations: After the bombardment of (-particles on the thick
gold foil, Rutherford observed that
(i) Most of the fast moving (-particles passed through the gold
foil.
(ii)Some of the (-particles were deflected by small angles.
(iii)A very few particles (1 in 20,000) bounded back i.e. were
deflected by nearly 180.
Conclusion: On the basis of these observations, Rutherford draw
the following conclusions regarding the structure of atom.
(i)Most of the space in the atom is empty as most of the
(-particles passed through the foil undeflected.
(ii)A few (-particle were deflected from there path. The
deflection must be due to enormous repulsive force showing that the
positive charge of the atom is not spread throughout the atom, as
Thomson had thought. According to Rutherford, the positive charge
of the atom occupies very little space. This very small portion of
the atom was called Nucleus.
(iii)A very small fraction of the (-particles were deflected by
180. Showing that all the positive charge and mass of the gold atom
were concentrated in a very small volume within the atom. (Radius
of the atom is about 1010 m while that of nucleus is 1015 m)
2.2.1Rutherfords Model
On the basis of above observations. Rutherford proposed the
nuclear model of atom. Rutherford proposed the nuclear model of
atom. According to this model.
(i)An atom consists of a positively charged centre called
nucleus.
(ii)The positive charge of the nucleus is due to the protons. On
the other hand, the mass of the nucleus is due to the protons and
some other neutral particles called neutrons which were discovered
later on by Chadwick in 1932.
(iii)The electrons revolve around the nucleus in well, defined
orbits. Thus, Rutherfords model of atom resembles the solar system
in which the nucleus plays the role of sun and the electrons that
of revolving planets.
(iv)The atom is electrically neutral because total number of
protons in it is exactly equal to the total number of
electrons.
(v)The size of the nucleus is very small as compared to that of
atom.
(vi)Electrons and the nucleus are held together by electrostatic
force of attraction.
To explain that the electrons do not fall into the nucleus as a
result of attraction, Rutherford suggested that electrons were not
stationary but were moving around the nucleus in certain circular
orbits.
2.2.2Drawbacks of Rutherfords Model of an atom
(i)Rutherfords model could not explain the stability of an atom.
This is because when a particle is moving in a circular orbit, it
undergoes acceleration. During acceleration charged particles would
radiate energy. Thus, the orbit of the revolving electrons will
keep on shrinking or becoming smaller and smaller, following a
spiral path and will ultimately fall into the nucleus. However,
this actually does not happen and we know that atoms are quite
stable.
2.3BOHRS MODEL OF AN ATOM
In order to overcome the objections raised against Rutherfords
model of the atom. Neils Bohr, a proposed a new model of atom. To
explain the stability of the atom, he introduced the concept of the
stationary orbitals.
2.3.1Postulates of Bohrs model
The main points of this Bohrs model of an atom are as
follows:
(i)An atom consists of positively charged nucleus responsible
for almost the entire mass of the atom.
(ii)Electrons revolve around the nucleus in certain permitted
circular orbits of definite radius and while revolving they do not
radiate energy.
(iii)In a particular atom, the orbits in which electrons revolve
have fixed radii and energy. These orbits are, therefore called
shells or energy levels. These shells are also called stationary
states as they have fixed energy. In this manner, Bohr overcame
Rutherfords difficulty to account for the stability of the
atom.
(iv)The different energy levels were numbered as 1, 2, 3, 4, etc
and called as K, L, M, N etc. respectively. Greater the distance of
energy level from the nucleus, none is the energy associated with
it.
+
Nucleus
E
1
E
2
E
3
E
4
1 2 3 4
K L M N
However, the gaps decreases between the successive energy shells
as we move outwards from the nucleus.
(v)When electrons move in permitted discrete orbits they do not
radiate or lose energy, or gain energy. This stable state of atom
is called ground state.
(vi)When energy is given to the electron, it jumps to any higher
energy level and said to be in the excited stale. In the excited
stable, the atom is not stable. I tends to lose or emit energy and
jumps back to some inner energy level. In other words, when an
electron absorbs energy it jumps from inner shell to outer shell
whereas when an electron emits energy it jumps from outer shell to
inner shell as shown in figure.
(a)
+
E
2
Energy absorbed
by electron
Nucleus
(b)
+
E
2
Energy emitted
by electron
Nucleus
E
1
2.3.2Advantage of Bohrs model
Bohrs model of an atom explains the stability of an atom by
putting the concept of stationary stale or energy levels and thus
explains the drawback of Rutherfords model of an atom.
The distribution (arrangement) of the electrons in the different
energy shells of the atom is known as the electronic configuration
of that element.
3.1BOHR-BURY SCHEME OF DISTRIBUTION OF ELECTRONS
The following rules are given by Bohr and Bury for writing the
number of electrons in different energy levels or shells.
(i)The maximum number of electrons that can be present in a
given shell is equal to 2n2, where n = number of shell.
Hence, the maximum number of electrons in different shells can
be given as follows:
Shell
Maximum No. of electrons present
(a)1st shell or K-shell (n = 1)
2 (1)2 = 2
(b)2nd shell or L-shell (n = 2)
2 (2)2 = 8
(c)3rd shell or M-shell (n = 3)
2 (3)2 = 18
(d)4th shell or N-shell (n = 4)
2 (4)2 = 32
(iii)The maximum number of electrons that can be accommodated in
the outermost orbit is 8.
(iv)Electrons do not enter into a new shell until unless the
inner shells are completely filled or we can easy that shells are
filled in a step-wise manner.
Schematic atomic structure of the first eighteen elements
Diagrammatically, the nuclear structure and the distribution of
electrons can be represented as below:
4.1VALENCY OF AN ATOM
The concept of valency arises from the study of inert elements.
Inert elements are also called noble gases. They have 8 valency
electrons (acted) in their outermost orbit/shell or valency shell
except helium which has 2 electrons (double). Apart from these
elements, all other elements have less than 8 electrons in their
valence shell. To attain stability, these atoms lose, gain or share
electrons with other atoms to complete their octet.
The number of electrons gained, lost or shared by atom of an
element in order to complete its octet (or dublet) or to attain
stable configuration is known as the valency of the element
4.2CALCULATION OF VALENCY
To calculate the valency of an element, the electronic
configuration of the element must be written first and then the
valency is calculated. The valency of an element can be calculated
as follows:
(i)Elements having 1, 2, 3 and 4 electrons respectively in their
valency shell: For these elements valency is equal to the number of
electrons present in their valency shell.
(ii)Elements having more than 4 electrons in their valency
shell: For these elements having more than 4 electrons in their
valence shell, valency can be calculated as follows:
Valency = 8 Number of valency electrons
5. ATOMIC NUMBER AND MASS NUMB ER
5.1ATOMIC NUMBER
The number of protons present in the nucleus of an atom is known
as its atomic number So
Atomic number of an element (Z) = Number of protons in one atom
of the element.
Example:
(i)Nucleus of hydrogen atom contains one proton, its atomic
number = 1.
(ii)Nucleus of carbon atom contains 6 protons, its atomic number
Z = 6.
As we know that an atom is electrically neutral, i.e. the number
of protons is equal to the number of electrons. Hence, we can say
that
Atomic number (Z) = No. of protons = No. of electrons in one
neutral atom.
5.2MASS NUMBER
Mass number of an element is the sum of protons and neutrons
present in the atom of the element. i.e.
Mass number of an element (A) = Number of protons + number of
neutrons
For example, Nitrogen is written as =
N
14
7
, Oxygen is written as =
O
19
8
CALCULATION OF ELECTRONS, PROTONS AND NEUTRONS , ATOMIC NUMBER
(Z) AND MASS NUMBER (A)
Z = Number of protons (p) = No. of electrons (e) and
A = Number of protons (p) + number of neutrons (n)
But as we know that
p = Z
Thus,
A= Z+n
6. ISOTOPES
Isotopes are the atoms of the same element, having the same
atomic number but different mass number
Atomic number (Z) = Number of protons = Number of electrons
Mass number = Number of protons + Number of neutrons
But as we know that number of protons in them are equal so we
can conclude that isotopes of an element differ only in the number
of neutrons present the nucleus.
Example: Isotopes of Hydrogen: There are three isotopes of
hydrogen, namely protium, deuterium and friction.
Isotope
Atomic No.
Mass No.
No. of protons
No. of neutrons
No. of electrons
H
1
1
1
1
1
1 1 = 0
1
H
2
1
1
2
1
2 1 = 0
1
H
3
1
1
3
1
3 1 = 2
1
6.1GENERAL CHARACTERISTICS OF ISOPES
(i)Same atomic number: The isotopes of an element have the same
atomic number i.e. they have same number of protons and same number
of neutrons.
(ii)Different mass number: They have different mass number and
hence differ in the number of neutrons present in the nucleus.
(iii)Same chemical properties: They have same chemical
properties as they have same number of electrons and therefore same
electronic configuration and valence electrons.
(iv)Different physical properties: Since they have different
mass number hence they differ in their physical properties such as
melting point, boiling point, density etc.
(v)Different nuclear properties: Due to the difference in the
number of neutrons in their nucleus they show different nuclear
properties e.g. C14 isotope of carbon is radioactive whereas C12
isotope is non-radioactive. The radioactive isotope of an element
is known as radioisotope.
6.2FRACTIONAL ATOMIC MASSES AND CALCULATION OF AVERAGE ATOM
MASSES
if an elements occurs in isotopic form, then we have to know the
percentage of each isotopic form to calculate its average atomic
mass.
Example:In nature, the two isotopic forms of chlorine viz
Cl
35
17
and
Cl
37
17
are fond in the ratio of 3:1. Hence,
Average atomic mass =
+
100
25
37
100
75
35
=
=
+
4
142
4
37
4
105
= 35.4 u .
6.3APPLICATIONS OF ISOTOPES
Some isotopes have special properties which find them useful in
various fields. Some important and useful applications of the
isotopes are given below:
(i)As nuclear fuel: An isotope of uranium (U-235) is used as a
fuel in nuclear reactor.
(ii)In medial field:
(a)An isotope of cobalt (Co-60) is used in the treatment of
cancer.
(b)Phosphorus(P-32) isotope is used in the treatment of leukemia
(blood cancer).
(c)Iodine (I-131) isotope is used in the treatment of
goitre.
(d)Some radio isotopes are used tracers detect the presence of
tumours, blood clots etc.
(iii)In carbon dating.
(iv)In geological dating.
7. ISOBARS
Atoms of different elements which have different atomic number
but same mass number are called isobars. They have different number
of protons, electrons and neutron but the mass number, i.e. the sum
of protons and neutrons in the nucleus is same
Example:
Ar
40
18
,
Ca
40
20
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_1272285158.doc
5. HOW DO ATOMS EXIST ?
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+
N
M
4
3
2
1
E4
E3
E2
E1
Nucleus
L
K
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Energy absorbed by electron
E2
+
(a)
E1
Nucleus
Energy emitted by electron
E2
+
(b)
Nucleus
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7. ISOBARS
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6. ISOTOPES
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5. ATOMIC NUMBER AND MASS NUMBER
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2. THE STRUCTURE OF AN ATOM (ATOMIC MODELS)
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1. charged particles in atom or sub-atomic particles of atom
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6. WHAT IS A MOLECULE ?
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K
(NO3)
1(
1+
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9. MOLECULAR MASS AND MOLE - CONCEPT
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7. WHAT IS AN ION?
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3. DALTONS ATOMIC THEORY
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2. LAWS OF CHEMICAL COMBINATION
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