1. Unit 2 – Atomic Theory
Chapter 4 & 11.2
Unit Test:

2. Democritus
Lived around 400 B.C.
Came up with the concept of the atom

3. John Dalton - 1807 Known for Billiard Ball Model
Called the father of Atomic Theory

4. Dalton’s Atomic Theory - 1807
All matter is composed of atoms which are indivisible
All atoms of the same element are identical
Atoms of different elements are different
Atoms bond in different proportions to form compounds (Law of Definite Proportions)

5. J. J. Thomson
Discovered the electron (1st subatomic particle) through experiments with cathode ray tube
Plum Pudding model (or Chocolate Chip Cookie model)

6. Thomson’s Model
“Pudding” or “Cookie” is the positive charge and most of the mass of the atom
“Plums” or “Chocolate Chips” are the scattered electrons
POSITIVE CHARGE
ELECTRONS

7. Ernest Rutherford - 1911 Nuclear Model (atom contains a nucleus)
Gold foil Experiment

8. Rutherford’s Experiment

9. Rutherford’s Model Atoms have: A nucleus
Protons (positive charge) in nucleus
Mostly open space
Electrons found somewhere around the nucleus

10. Niels Bohr - 1913 Planetary Model
Electrons (e-) have definite path around the nucleus (orbit)
e- arranged around the nucleus according to energy level
e- with lowest energy level are closest to nucleus

11. Bohr’s Model

12. Quantum Mechanical Model - 1923
Electron Cloud (modern theory)
Calculates the probability of finding the electron within a given space
Electrons, instead of traveling in defined orbits, travel in diffuse clouds around the nucleus

13. Quantum Mechanical Model
Present Model of the Hydrogen Atom

14. Stepwise Timeline of Atomic Theory
Dalton
1803
Rutherford
1909
Modern Theory
Thomson
1897
Bohr
1913

15. Subatomic Particles Name Symbol Relative Mass Charge Position Proton
1H or p+
1 amu 1
Nucleus
Electron e-
0 amu -1
Outside
Neutron 1n
amu – atomic mass unit; based on carbon-12
1 amu = 1/12 mass of C-12 = mass H
Impractical to use actual mass of subatomic particles

16. Elements of the Periodic Table
Atomic Number
Identifies element
# protons
# electrons in neutral atom
Atomic Mass
a.k.a. mass number
# protons + # neutrons
Atomic Number
Atomic Symbol
Atomic Mass

17. X Isotope Notation Element- Mass
Element Symbol with mass number and atomic number
Can also be the element name dash mass number
Mass Number X
Element- Mass or
Atomic Number

18. Practice 9 10 19 28 59 150 94 30 65 Symbol # Protons # neutrons
# electrons
Atomic Number
Mass Number 9 10 19 28 59
150 94 30 65

19. Isotopes Atoms of the same element with different numbers of neutrons
Think of it as different sized shirts!
6 neutrons Carbon-12
7 neutrons Carbon-13
8 neutrons Carbon-14

20. Isotopes

21. Average Atomic Mass Mass listed on the periodic table
Determined by averaging the masses of all naturally occurring isotopes of that element
Explains why the atomic mass is usually not a whole number

22. Finding Average Atomic Mass
You are told there are 3 isotopes of Cycreekium and their percent abundance:
Cycreekium-20; 10%
Cycreekium-22; 20%
Cycreekium-23; 70%
Find their average atomic mass!

23. Finding Average Atomic Mass
Formula: % abundance as decimal × atomic mass for each isotope; sum results for avg. atomic mass.
10% × 20amu = 0.10 × 20amu = 2.0amu
20% × 22amu = 0.20 × 22amu = 4.4amu
70% × 23amu = 0.70 × 23amu = 16.1amu
Total: amu

24. Percent Abundance
If you are given information about an elements isotope you can estimate the most abundant isotope!
Example: Carbon-12, Carbon-13, Carbon-14. Look at the atomic mass on the periodic table. Which isotope is the mass closest to?

25. End of Unit 2 Notes!
Study for Unit 2 Test on: