1. Development of Atomic Theory

2. Gold Foil Experiment
In the early 1900’s, Hans Geiger sent radioactive particles (alpha particles “”) through thin gold foil
He expected these relatively heavy particles to go through the atoms with a small deflection (bouncing back)

3. What happened in the experiment?
Gold foil

4. What did he see?
Most of the alpha particles passed straight through with no deflection (they did not bounce back)
These particles did not run into anything
Some did deflect slightly
These particles ran into something much smaller than themselves
A few were reflected back the direction they came from
These particles ran into something very dense

5. What did that mean? Atoms are mostly empty space
Electrons (the smaller particles) were the cause of the small deflections
There must be a small area of the atom with most of its mass (the protons) that caused the reflections.
He called this small, dense area the nucleus

6. A third particle
The protons and electrons could explain the charges of the various parts of the atom
They could not explain the total mass of the atoms
Neutrons were proposed in 1920’s but not confirmed until 1932 by James Chadwick
Neutrons had mass similar to protons and no charge. They were located in the nucleus

7. More changes to the theory
Niels Bohr performed experiments with hydrogen atoms & light
He determined that electrons are in levels according to how much energy they have and that only certain energy amounts were allowed.

8. The Bohr Model
It consists of the nucleus with protons & neutrons and electrons in circular orbits outside the nucleus
The circle closest to the nucleus contains the lowest energy electrons
The first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18 electrons.

9. Pictures of the Bohr Models
Electron
Proton
Neutron
Hydrogen-1
Helium-4
Lithium-6

10. Use of the Bohr Model now
We now believe that electrons do not really travel around the nucleus in perfect circular patterns
But imagining a circular pattern helps explain where electrons are located

11. Atomic Structure

12. What are atoms?
Atom - smallest piece of matter that has the chemical properties of the element.

13. What’s in an atom? An atom is made of three sub-atomic particles
Location
Mass
Charge
Proton
Nucleus
1 amu +1
Neutron
Nucleus
1 amu
Electron
Outside the nucleus
amu -1
1 amu (“atomic mass unit”) = 1.66  kg

14. What gives an atom its identity?
Every atom has a different number of protons.
The number of protons determines the identity of the atom
The atomic number shows the number of protons.
Atomic number = protons

15. The Nucleus & Mass
The mass of the nucleus (in amu’s) is the number of protons + neutrons
Since electrons have relatively no mass, we ignore them for mass
Mass # = protons + neutrons

16. Charges Protons have a positive charge
Electrons have a negative charge
Neutrons have no charge
Overall charge of an atom =
protons - electrons
Charge = protons - electrons

17. How do we show information about an element?

18. X A C Z # Element symbols Element Symbol Charge Mass number
1 or 2 letters, found on the periodic table X A C Z #
Charge
# protons - # electrons
(assumed to be “0” if blank)
Mass number
# protons + # neutrons
Atomic number # of protons
Number
How many atoms do you have?

19. Example: Element symbols
O = Oxygen O 16 -2 8
Charge -2
Mass number 16
Atomic number 8
Number
Assumed to be “1” if blank