1. Atomic Structure

2. SC Science Standards
Interpret Dalton’s atomic theory in terms of the Laws of Conservation of Mass, Constant Composition, and Multiple Proportions.
Compare and contrast the contributions of Dalton, Thomson, Rutherford, Bohr, Planck and Schodinger to the development of the current atomic model.
Based on the quantum theory, write electron configurations and orbital notation for the representative elements.
Use Bohr’s model of the atom to explain the bright line spectrum in terms of electrons moving between energy levels.
Describe and identify the regions of the electromagnetic spectrum in terms of frequency, wavelength and energy.

3. Atomic Theory Democritus…(>2000 years ago) Greek philosopher
First suggested the idea of atoms
Matter is composed of tiny indivisible particles
Named these particles “atomos”
Now called atoms
Ideas lacked experimental support

4. Atomic Theory John Dalton (1776 – 1844) English school teacher
Studied chemistry
Particularly interested in meteorology
Performed experiments
Studied the ratios that chemicals combine to form compounds
Formulated the Atomic Theory

5. Dalton’s Atomic Theory
All elements are composed of submicroscopic indivisible particles called atoms
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

6. Dalton’s Theory Cont…
Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

7. What is an atom?
The smallest particle of an element that retains the properties of that element
Individual atoms are visible with the proper instrument

8. Subatomic Particles Particles that are smaller than atoms
Three main subatomic particles
Protons
Neutrons
Electrons

9. Electrons Negatively charged Discovered by JJ Thomson in 1897
Experimented with the flow of electric current through gases in cathode ray tubes
Found that the cathode rays were attracted to the metal plates with a positive charge and repelled by metal plates with a negative charge

10. Electrons cont… Thomson
Concluded that cathode rays are composed of negatively charged particles
Called these negatively charged particles electrons
Concluded that electrons are a part of the atoms of every element
Electron has 1 unit of negative charge
Electron has mass of about 1/2000 of a hydrogen atom

11. Thomson’s Plum Pudding Model

12. Protons Positively charged subatomic particle
Discovered by E. Goldstein in 1886
Has one unit of positive charge

13. Neutrons Discovered by Sir James Chadwick in 1932
Subatomic particle with no electric charge
Mass is equal to the mass of a proton

14. Structure of the atom Ernest Rutherford (1871 – 1937)
Performed famous gold foil experiment
Tested popular theory that atoms were composed of evenly distributed protons and electrons
Experimented with alpha particles (+ charges) aimed at a thin sheet of gold foil
Most particles went straight through
Some (a very few) were bounced back

15. Rutherford’s experiment
Rutherford proposed
Most of the mass and all of the positive charge of the atom is concentrated in a small region at the center of the atom
Called the center region the nucleus
The nucleus is the center core of the atom and is composed of protons and neutrons

16. The Nucleus Very small and dense
If the nucleus were the size of a pea, its mass would be 250 tons!
Has a positive charge
Occupies a very small volume of the atom
Electrons occupy the largest volume of the atom outside of the nucleus

17. Atomic Number Different numbers of protons make atoms different
Protons determine the identity of an element
Atomic number is the number of protons in the nucleus of the atom
Each element has a unique atomic number
Reported on the periodic table

18. Atoms Atoms are electrically neutral
Number of protons must be equal to the number of electrons

19. Mass Number Most of an atom’s mass is concentrated within the nucleus
Protons and neutrons contribute to the mass
Mass number = # protons + # neutrons

20. Mass number Au
197 79
Atomic number

21. Isotopes
Atoms of the same element may have different nuclear structures
Number of neutrons may vary within atoms of the same element
Isotopes are atoms that have the same number of protons, but different numbers of neutrons

22. Average Atomic Mass
Masses of atoms are measured in units called atomic mass units (amu)
An atomic mass unit is defined as 1/12 the mass of a carbon-12 atom
The mass of Carbon-12 is amu
Mass of a single proton or neutron is approximately 1 amu

23. Atomic Mass
In nature, most elements exist as a mixture of 2 or more isotopes
Each isotope has a fixed mass and a natural percent abundance
Atomic mass is the weighted average mass of the atoms in a naturally occurring sample of the element

24. Calculating Atomic Masses
You need to know:
The number of stable isotopes of the element
The mass of each isotope
The natural percent abundance of each isotope
Masses and relative abundances are values that can be looked up in chemical reference books

25. Atomic Mass of Element X
Element X has two natural isotopes. The isotope with mass of amu has a relative abundance of 19.90%. The isotope with mass has a relative abundance of 80.10%. Calculate the atomic mass of this element and name it.

26. Nitrogen
mass number
exact weight
percent abundance 14
99.63 15
0.37

27. Chlorine
mass number
exact weight
percent abundance 35
75.77 37
24.23

28. Silicon
mass number
exact weight
percent abundance 28
92.23 29
4.67 30
3.10

29. Calculate average atomic mass:
mass number
exact weight
percent abundance 24
78.99 25
10.00 26
11.01

30. Calculate average atomic mass:
mass number
exact weight
percent abundance 92
14.84 94
9.25 95
15.92 96
16.68 97
9.55 98
24.13
100
9.63

31. Calculate average atomic mass:
mass number
exact weight
percent abundance
112
0.97
114
0.65
115
0.36
116
14.53
117
7.68
118
24.22
119
8.58
120
32.59
122
4.63
124
5.79