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Contents :
Laws of Chemical Combination
About Atoms
About Molecules
About Ions
Writing Chemical Formulae
Molecular Mass & Mole Concept
Multiple Choice Questions
Some Interesting Pictures
Laws of Chemical Composition
The following two laws of chemical combination were established after much experiments by Lavoisier and Joseph L Proust :
(i) Law of conservation of mass :
- Law of constant of mass states that mass can neither be created nor destroyed in a chemical reaction.
(ii) Law of Constant Proportions
- Lavoisier, along with other scientists,
noted that many compounds were
composed of two or more elements and
each such compound had the same
elements in the same proportions,
irrespective of where the compound came
from or who prepared it.
In a compound such as water, the ratio
of the mass of hydrogen to the mass of
oxygen is always 1:8, whatever the source
of water. Similarly in ammonia, nitrogen
and hydrogen are always present in the
ratio 14:3 by mass, whatever the method
or the source from which it is obtained.
This led to the law of constant
proportions which is also known as the
Law of Definite Proportions.
About Atoms
According to Dalton’s atomic theory, all
matter whether an element, a compound
or a mixture is composed of small particles
called Atoms.
The postulates of this theory may be
started as follows :
(i)All matter is made of very tiny particles
called Atoms.
(ii) Atoms are indivisible particles, which
cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical in
mass and chemical properties.
(iv)Atoms of different elements have
different masses and chemical properties.
(v) Atoms 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.
The building blocks of all matter are atoms.
Atoms are very small. They are smaller than
anything we can imagine or compare with.
►Atomic radius is measured in nanometers.
Relative Sizes :
Modern Day Symbols of Atoms of Different Elements
Symbols for Some Elements
Aluminum : Al
Argon : Ar
Barium : Ba
Boron : B
Bromine : Br
Calcium : Ca
Carbon : C
Chlorine : Cl
Cobalt : Co
• Copper : Cu
►Fluorine : F
►Gold : Au
Hydrogen : H
Iodine : I
Iron : Fe
Lead : Pb
Magnesium : Mg
• Neon : Ne
Nitrogen : N
Oxygen : O
Potassium : K
Silicon : Si
Silver : Ag
Sodium : Na
• Sulphur : S
►Uranium : U
Zinc : Zn
Phosphorus : P
Platinum : Pt
Mercury : Hg
• Nickel : Ni
Tin : Sn
Bismuth : Bi
Antimony : Sb
Arsenic : As
Manganese : Ma
Tungsten : W
Titanium : Ti
• Cerium : Ce
Strontium : Sr
Beryllium : Be
Zirconium :
In 1961 a university accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
One atomic mass unit is a mass unit equal to exactly one twelfth the mass of one atom of carbon-12.
Atomic Masses of Few Elements
Element Atomic Mass (u)
(i) Hydrogen 1
(ii) Carbon 12
(iii) Nitrogen 14
(iv) Oxygen 16
(v) Sodium 23
(vi) Magnesium 24
Element Atomic Mass (u)
(vii) Sulphur 32
(viii) Chlorine 35.5
(ix) Calcium 40
Atoms of most elements are not able to exist independently.
Atoms form molecules and ions.
These molecules or ions aggregate in
large numbers to form the matter that we
can see, feel or touch.
About Molecules
A molecule is a general group of two or
more atoms that are chemically bonded
together, that is, tightly held together by
attractive forces.
A molecule can be defined as the smallest
particle of an element or a compound & is
capable of an independent existence and
shows all the properties of that substance.
Atoms of the same element or of different
elements can join together to form
molecules.
Molecules of Elements
• The molecules of an element are
constituted by the same type of atoms.
►Molecules of an element are constituted by the same type of atoms.
►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 non-metals.
For example, a molecule of oxygen consists of
two atoms of oxygen and hence it is known as
a diatomic module O2.
If 3 atoms of oxygen unite into a molecule,
instead of the usual 2, we get ozone.
The number of atoms constituting a molecule is known as its Atomicity.
Atomicity of Some Elements
Type of Element : Non – Metal
Name Atomicity
(i) Argon - Monoatomic
(ii) Helium - Monoatomic
Type of Element : Non – Metal
Name Atomicity
(i) Oxygen Diatomic
(ii) Hydrogen Diatomic
(iii) Nitrogen Diatomic
(iv) Chlorine Diatomic
Type of Element : Non – Metal
Name Atomicity
(i) Phosphorus Tetra - atomic
Type of Element : Non – Metal
Name Atomicity
(i) Sulphur Poly-atomic
Atoms of different elements join together in definite proportions to form molecules of compounds.
Molecules of Some Compounds
Compound : Water
Combining Elements : Hydrogen, Oxygen
Ratio by Mass : 1:8
Compound : Ammonia
Combining Elements : Nitrogen, Hydrogen
Ratio by Mass : 14:8
Compound : Carbon Dioxide
Combining Elements : Carbon, Oxygen
Ratio by mass : 3:8
About Ions
Compounds composed of metals and non metals contain charged species. The charged species are known as Ions.
An ion is a charged particle and can be
negatively or positively charged.
A negatively charged ion is called an
‘Anion’ and the positively charged ion is
called a ‘Cation’.
Some Ionic Compounds• Ionic Compound : Calcium Oxide
Constituting Elements : Calcium &
Oxygen
Ratio by Mass : 5:2
►Ionic Compound : Magnesium Sulphide
Constituting Elements : Magnesium & Sulphur
Ratio by Mass : 3:4
►Ionic Solution : Sodium Chloride
Constituting Elements : Sodium & Chlorine
Ratio by Mass : 23:35.5
Molecular Mass & Mole Concept
The molecular mass of a substance is the sum
of the atomic masses of all the atoms in a
molecule of the substance.
It is therefore the relative mass of a molecule expressed in atomic mass units (u).
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.
Formula unit mass is calculated in the same manner as we calculate the molecular mass.
The only difference is that we use the word formula unit for those substances whose constituent particles are ions.
EXAMPLE : Reaction of hydrogen and oxygen to form water:
2H2 + O2 2H2O
The above reaction indicates that :
(i) Two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water. Or,
(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water molecules.
We can infer from the above equation that the quantity of a substance can be characterised by its mass or the number of molecules.
But, a chemical reaction equation indicates directly the number of atoms or molecules taking part in the reaction.
Therefore, it is more convenient to refer to the
quantity of a substance in terms of the number
of its molecules or atoms, rather than their
masses.
So, a new unit “Mole” was introduced.
Multiple Choice Questions
Q 1 : A molecule does not signify
(a) Atomic mass unit
(b) 22.4 litres of a gas at STP
(c) Gram molecular mass
(d) None of the above
Answer :
Option :
(a) Atomic mass unit
Q 2 : Which of the following does not change when a chemical reaction takes place ?
(a) Volume
(b) Mass
(c) Physical properties
(d) Chemical properties
Answer :
Option :
(b) Mass
Q 3 : Molecules of the following
element are made up of only one
atom of that element:
(a) Iron
(b) Sodium
(c) Helium
(d) Chlorine
Answer :
Option :
(c) Helium
Q 4 : Law of definite proportion was
given by
(a) John Dalton
(b) Lavoisier
(c) Joseph Proust
(d) Ritcher
Answer :
Option :
(b) Lavoisier
Q 5 : Argon is
(a) Monoatomic
(b) Diatomic
(c) Triatomic
(d) Polyatomic
Answer :
Option :
(a) Monoatomic
Q 6 : Which of the following supports Dalton's atomic theory of matter?
(a) Formation of isotopes
(b) Formation of isobars
(c) Law of multiple proportion
(d) Graphite and diamond though made from carbon, show different properties.
Answer :
Option :
(c) Law of Multiple Proportion
Q 7 : Choose the odd molecule
(a) Argon molecule
(b) Chlorine molecule
(c) Oxygen Molecule
(d) Fluorine Molecule
Answer :
Option :
(a) Argon Molecule
• Q 8 : choose the element which will
not have a charged valency
(a) Calcium
(b) Argon
(c) Silicon
(d) Aluminium
Answer :
Option :
(b) Argon
Q 9 : Take out the odd concept
(a) Atomic mass unit
(b) Mole
(c) Avogadro
(d) Gram atomic mass
Answer :
Option :
(a) Atomic mass unit
Q 10 : Which one of the following
elements give polyatomic molecules?
(a) Ne
(b) P
(c) Ni
(d) Si
Answer :
Option :
(b) P
Some Interesting Pictures
Thank You !!!
Compiled By :
Sudarshan.S.K.
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