History of the Atom
1. Dalton – Thought the atom was a
solid indivisible sphere
2. Thompson - Discovered the electron
using a cathode ray tube and thought that
the atom looked like a chocolate chip cookie.
3. Rutherford’s Gold Foil Experiment - Fired alpha (positively charged) particles through gold
foil and discovered that:
a. Atom was mostly empty space
b. Contained a small, positive, dense nucleus
4. Bohr Model- Electrons moved around
nucleus in fixed paths
5. Modern model/Wave-mechanical modelElectrons moved around the nucleus
in probable locations called orbitals
Atomic structure notes
**6. There are three subatomic particles found within the atom.
7. Protons are the ONLY subatomic particle that NEVER changes for an element.
8. ***Protons identify who the element is.
9. An ELEMENT has an EQUAL amount of protons and electrons.
10.
Protons - electrons = Charge of the atom
Protons + neutrons = Mass of the atom
11. Isotope – An element that has the same atomic number, but DIFFERENT atomic mass.
12. When writing in isotopic notation, the atomic mass goes on TOP, and the atomic number
goes on the BOTTOM.
13. ***The atomic mass of an element on the periodic table is the AVERAGE of all the atomic
masses of its isotope***
Quick think: Carbon occurs naturally as Carbon – 12 and Carbon – 14. Using the average mass
found on the periodic table determine which isotope is more abundant?
Can you estimate a percentage?
_____% Carbon – 12
_____ % Carbon – 14
Choose the best answer: Chlorine occurs naturally as Chlorine-35 and Chlorine-37
A. 100% Chlorine - 35 and 0% Chlorine - 37
B. 50% Chlorine -35 and 50% Chlorine - 37
C. 75% Chlorine 35 and 25% Chlorine -37
D. 25% Chlorine -35 and 75% Chlorine - 37
14. Ground state electron configuration. When an atom’s electrons are occupying the lowest
possible energy states. ALL the configurations on the reference tables are ground state
i.e. (Chlorine is 2-8-7)
15. Excited state electron configuration A condition where an atom’s electrons occupy higher
energy levels than they normally would.
i.e. (Chlorine: 2-8-6-1)
16. *The excited-state configurations will have the same amount of total electrons as the
ground state
Additions….
17. Protons = Atomic number
18. When the atomic mass changes, neutrons change
19. When the charge changes, electrons change
20. When the element changes, protons change.