1. Chapter 3: Discovering the Atom and Subatomic Particles

2. Aristotle (384 – 322 BC)
Matter is composed of 4 qualities:
hot, cold, wet, and dry
Matter is able to change by one quality
replacing another.

3. Democritus (460-370 BC) Matter is composed of a specific number of
small units called atoms – meaning “indivisible”.
Matter is able to change by atoms
rearranging themselves.

4. Lavoisier Element - substance made from one type of atom
Compound - substance made of 2 or more elements bonded together

5. H2O (s)  H2O (l) Law of Mass Conservation :
There is no change in mass when substances go through a physical or chemical change.
“mass in = mass out” 
H2O (s)  H2O (l)
18.0 g = g

6. Proust Law of Definite Proportions:
Elements combine in definite mass ratios to form compounds.
C O  CO2
12 g g  g
How much C and O2 are needed to make 88 g of CO2?
How much C and O2 are needed to make 22 g of CO2?

7. C O  CO2
12 g g  g
If you combined 12 g C with 36 g O2, how much CO2 will you make? Will you have any reactants left over?

8. Dalton’s Theory (1800’s)
1. All matter is composed of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties.
3. Atoms cannot be broken apart, created, or destroyed.
4. Compounds are formed by elements combining in small whole number ratios.
5. In chemical reactions, atoms can be put together, separated, or rearranged.

9. #2 and #3 of Dalton’s theory have been disproved.
We now know:
2. Atoms of a given element are NOT identical.
3. Atoms CAN be divided. They are made of even smaller particles.

10. Evolution of the Atomic Model
Dalton
Solid, dense, indivisible sphere

11. Thomson
“Plum Pudding Model”
Positive pudding with negative plums

12. Rutherford “Nuclear Model” Nucleus with protons (+)
Electrons (-) surround
Mostly EMPTY SPACE

13. Chadwick “Neutron Model” Nucleus contains Protons (+) and Neutrons (0)
Electrons (-) still surround nucleus

14. Electrons (-) exist in energy levels that surround nucleus
Bohr
“Planetary Model”
Electrons (-) exist in energy levels that surround nucleus

15. Electron location is not defined.
Electron Cloud Model
Electron location is not defined.
Electron position is constantly changing.

16. How electrons were discovered………. Cathode Ray Tube Experiment
Video clip: JJ Thomson and the electron
Cathode Ray Tube Experiment
a) Cathode ray travels in straight-line path.
b) Ray is deflected by negative pole of magnet, proving that ray is made of negative particles.

17. How the nucleus was discovered…… Gold-Foil Experiment
video clip: Ernest Rutherford
Gold-Foil Experiment
Observations: While most particles traveled straight through the foil, a small number bounced back.

18. Conclusions: There must be a dense cluster of particles in the center of the atom. The particles have the same charge as the alpha particles (+).

19. Structure of the Atom
The number of protons (and the identity of an element) is indicated by its ATOMIC NUMBER.
Hydrogen is atomic number 1.
It has 1 proton in its nucleus.
Magnesium is atomic number 12.
It has 12 protons in its nucleus.

20. The MASS NUMBER of an atom is the total number of protons and neutrons that are present in the nucleus.
An atom that has 10 protons and 11 neutrons will have a mass number of 21.
An atom that has atomic number 20 and 22 neutrons will have a mass number of 42.

21. A NUCLEAR SYMBOL used to indicate both the atomic number and mass number of an element.
The mass number, 39, is written in the upper left corner.
The atomic number, 19, is written in the bottom left corner.
Since atoms are electrically neutral, the number of ELECTRONS equals the number of protons.

22. Sample Problems:
1. Write a nuclear symbol to represent an atom that has 4 protons, 5 neutrons, and 4 electrons.
2. How many protons, neutrons, and electrons are in an atom of Aluminum, ?

23. Isotopes
Isotopes are varieties of an element that differ in their number of neutrons. As a result, their masses are different, too.
Example:
Carbon Carbon-13 Carbon-14
6 p+ 6 n0 6 p+ 7 n0 6 p+ 8 n0

24. Isotopes are the reason that the atomic masses on the periodic table are NOT whole numbers.
Isotopes of the same element have the same chemical properties – even though they have different masses.