Learning Objectives: a. The model of atomic theory was 1 st proposed by John Dalton in 1803. b. Atoms are composed of protons, neutrons, and electrons. c. All atoms of the same element contain the same number of protons (and electrons) but may vary in the number of neutrons (isotopes).
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Learning Objectives: a. The model of atomic theory was 1 st proposed by John Dalton in 1803. b. Atoms are composed of protons, neutrons, and electrons.
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Learning Objectives:
a. The model of atomic theory was 1st proposed by John Dalton in 1803.
b. Atoms are composed of protons, neutrons, and electrons.
c. All atoms of the same element contain the same number of protons (and electrons) but may vary in the number of neutrons (isotopes).
d. Protons and neutrons are found inside the tiny but dense nucleus, whereas electrons are found in orbitals outside the nucleus.
e. The arrangement of electrons in the orbitals is called the electronic configuration and determines the chemistry of an atom.
f. The different types of atoms are called elements, which are arranged systematically in the periodic table.
g. Having eight valence electrons is particularly desirable (“the octet rule”).
a. The Atomic Theory Dalton’s Atomic Theory
1. All matter is composed of indivisible atoms.2. All atoms of one element are identical to each
other but different than the atoms of other elements.
3. Compounds are formed when atoms of different elements combine in whole number ratios.
4. Atoms are rearranged during chemical reactions but atoms cannot be created or destroyed.
John Dalton (1766-1844)
Definitions
Law of conservation of matter states that matter is neither lost nor gained during a chemical reaction.
Law of definite proportions states that in a compound, the constituent elements are always present in a definite proportion by weight.
Ex. 2H2 (g) + O2 (g) 2H2O(g)
2C(s) + O2 (g) 2CO(g)
Ex. Pure water, a compound, is always made up of 11.2% hydrogen and 88.8% oxygen by weight or table sugar always contains 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen by weight.
b. Structure of the Atom Components
Positive protons, negative electrons, and neutral neutrons
Atomic Number The number of protons in an atom, which
determines what element it is Mass Number
Number of protons + the number of neutrons
Definitions
Protons are positively charged subatomic particles found in the nucleus.
Neutrons are electrically neutral subatomic particles found in the nucleus.
Electrons are negatively charged subatomic particles found in the space around the nucleus.
Nucleus is the small central core of the atom: contains the protons and neutrons.
Ernest Rutherford Gold foil experiment. A beam of positively charged alpha particles hits the gold foil. Most particles passed straight, some slightly deflected and some deflected back. The reason for deflection were the positions of the nucleus.
Separation of alpha, beta, and gamma particles by applying an electric field.
Ernest Rutherford (1871-1937)
Rutherford’s interpretation of the gold foil experiment done by Geiger and Marsden.
Modern View of the Atom
For an atom, which always has no net electrical charge, the number of negatively charged electrons around the nucleus equals the number of positively charged protons in the nucleus.
Scientists have been able to obtain computer-enhanced images of the outer surface of atoms using the scanning tunneling microscope (STM) and the atomic force microscope (AFM).
c. Structure of the Atom Isotopes
Isotopes of the same element have the same number of protons and electrons but differ in the number of neutrons.
Atomic Mass The atomic mass for each element on the
periodic table reflects the relative abundance of each isotope in nature.
DefinitionsAtomic number is the number of protons in the nuclei of the atoms of an element
Mass number or atomic mass is the number of neutrons plus number of protons in the nucleus of an atom
Mass number = #’s of protons + #’s of neutrons
Ex. How many protons, neutrons, and electrons are in atom of gold (Au) with a mass number of 197?
Isotopes are atoms of the same element having different mass numbers.
Definitions cont
Atomic mass unit (amu) is the unit for relative atomic masses of the elements; 1 amu =1/12 the mass of carbon-12 isotope. 1 amu = 1.6605x10-24 grams
Atomic weight is the number that represents the average atomic mass of the element’s isotopes weighted by percentage abundance.
Isotopes
H1
1 H1
2 H1
3
Write the nuclear symbols for atoms with the following subatomic particles.
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________
Learning Check
A. 8 p+, 8 n, 8 e- 16O
8
B. 17p+, 20 n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47
Solution
Learning Check
1. Which of the following pairs are isotopes of the same element?2. In which of the following pairs do both atoms have 8 neutrons? A. 15X 15X 8 7
B. 12X 14X 6 6
C. 15X 16X 7 8
Solution
B. 12X 14X
6 6
Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.
C. 15X 16X
7 8
An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.
Isotopes of Magnesium
In naturally occurring magnesium, there are three isotopes.
24Mg 25Mg 26Mg 12 12 12
Isotopes of Mg
Relative Masses of Atoms
Use atomic weights of the elements to calculate molecular weights (MW) of compounds
Relative masses provide a simple way of comparing the masses of atoms. Ex. The mass of neon atoms is 20.18 and the mass of calcium atoms is 40.08.
Ca atom massNe atom mass
= 40.0820.18
= 1.986 Mass of Calcium is 2x than Ne
He atom mass H atom mass
= 4.0031.008
= 3.971 Mass of Helium is 4x than Hydrogen
The exact relationship between two masses calculated
Calculating the atomic weight of compounds MW = CnHmOk
MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)
H2O the MW is
MW = 2(at. Wt. H) + 1(at. Wt. O)
MW = 2(1.008 u) + 1(15.996 u)
MW = 18.012 u or 18.01 u for water
Use atomic weighs from the periodic table inside the front cover of your book to determine the molecular weight of urea, CH4N2O, the compound by which much nitrogenous body waste is excreted in the urine.
Learning Check
a. 58.02 u
b. 62.25 u
c. 60.06 u
Solution
MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)
The chemical formula for urea is CH4N2O,
MW = 1(12.01 u) + 4(1.008) + 2(14.01) + 1(16.00)
MW = 60.062 u or rounded off 60.06 u
d. Models of the Atom The Plum Pudding Model
Electrons are embedded in a sphere of positive charge.
The Nuclear Model All of the positive charge is in a tiny central
nucleus with electrons outside the nucleus. This model was developed by Rutherford
after his landmark experiments.
The Rutherford Experiment
e. Models of the Atom (continued) Bohr Model or the Solar System Model
Niels Bohr in 1913 introduced his model of the hydrogen atom.
Electrons circle the nucleus in orbits, which are also called energy levels.
An electron can “jump” from a lower energy level to a higher one upon absorbing energy, creating an excited state.
The concept of energy levels accounts for the emission of distinct wavelengths of electromagnetic radiation during flame tests.
Niels Bohr (1885-1962)
Bohr’s Orbit ModelDefinitions
Quantum is the smallest increment of energy, for example, in an atom emitting or absorbing radiation.
Ground state is the condition of an atom in which all electrons are in their normal, lowest energy levels.
Excited state is an unstable, higher energy state of an atom.
A line spectrum for hydrogen
Fig. 3-6a, p. 49
Neon (Ne)
Fig. 3-6b, p. 49
Neon, a partially evacuated tube that contains neon gas gives a reddish-orange glow when high voltage is applied.
The line emission spectrum of neon is obtained when light from a neon source passes through a prism.
Potassiumburns with a violet flame
Credit: Photo Researchers, Inc.
Lithium burns with a red flame
Electromagnetic Radiation
c = c is the speed of light is wavelength is frequency
Fig. 3-7, p. 50
It is important to understand E (for energy), wavelength, and frequency relationship:
As increases, and E decreases
As decreases, and E increases
Models of the Atom (continued) The Orbital Model
Orbits are replaced with orbitals, volumes of space where the electrons can be found.
The arrangement of electrons in the orbitals is the electronic configuration of an atom, which determines the chemistry of the atom.
Definitions
Electrons in the highest occupied energy level are the greatest stable distance from the nucleus. These outermost electrons are known as valence electrons.
Shell is a principal energy level defined by a given value of n, where n can be 1,2,3,4 etc… and is capable of holding 2n2 electrons.
An orbital is a region of three-dimensional space around an atom within which there is a significant probability (usually shown as 90%) that a given electron will be found.
Subshells have different energy levels (orbitals) within a given shell
Valence Electrons
The valence electrons • determine the chemical properties of the elements.• are the electrons in the highest energy level.• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons. 5 valence electrons
P Group 5A(15) 2, 8, 5
All the elements in a group have the same number ofvalence electrons.
Example: Elements in group 2A(2) have two (2) valence electrons.
Be 2, 2Mg 2, 8, 2Ca 2, 8, 8, 2Sr 2, 8, 18, 8, 2
Groups and Valence Electrons
Periodic Table and Valence Electrons
Representative Elements Group Numbers
1 2 3 4 5 6 7 8
H He
1 2
Li Be Al C N O F Ne
2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
Li Mg Ge Si P S Cl Ar
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
State the number of valence electrons for each.A. O
1) 4 2) 6 3) 8
B. Al
1) 13 2) 3 3) 1
C. Cl
1) 2 2) 5 3) 7
Learning Check
State the number of valence electrons for each.A. O
2) 6
B. Al
2) 3
C. Cl
3) 7
Solution
State the number of valence electrons for each.
A. 2, 8, 5
B. 2, 8, 8, 2
C. 2, 7
Learning Check
State the number of valence electrons for each.
A. 2, 8, 5 5
B. 2, 8, 8, 2 2
C. 2, 7 7
Solution
Energy levels are spaced differently, like ladder rungs
Credit: Foto-Search.com
Atomic energylevels are like floors of a house
State transitions for hydrogen
Table 3-2, p. 52
Fig. 3-8, p. 54
Atomic Orbitals.
Fig. 3-9, p. 55
The Orbital Model:Electronic Configurations
Sample energy level diagram
Table 3-3, p. 55
Fig. 3-10a, p. 56
Fig. 3-10b, p. 56
f. The Periodic Table Used to organize the elements by
recurring chemical properties. Elements in the same vertical column of
the periodic table have similar chemical properties and are said to be in the same group or family.
The Periodic Table Dmitri Mendeleev (1834-1907)
Groups and Periods
On the periodic table,
• elements are arranged according to similar properties.
• groups contain elements with similar properties in vertical columns.