1. Atomic Structure
Ch. 3

2. (don’t write this page) Early Philosophers
Democritus – 400 BC
Thought matter was made of individual, indivisible particles – “atomus”
Aristotle
Disagreed with Democritus
Thought matter was continuous
His idea was accepted until late 1700s

3. 1790’s – Better analysis begins with improved equipment.

4. Dalton’s Theory Matter is made of atoms.
Atoms of the same element are identical.
Atoms cannot be divided.
Atoms combine in whole number ratios to form compounds.
Atoms are rearranged, separated, or combined in chemical reactions.
Wrong – Isotopes!!
Wrong – Nuclear Reactions!!

5. Modern Atomic Theory
Atoms of the same element can be different; called isotopes.
Atoms can be split.

6. Discovery of the Electron
JJ Thomson used cathode ray tubes to study electricity and matter

7. Observations Shadow was detected. The paddle wheel moved.
Cathode rays were attracted and deflected by a magnet.
Video on Cathode Ray Tube

8. Thomson’s ‘Plum Pudding’ model:
Conclusions
Electrons have mass, are negative, and are very small.
Thomson’s ‘Plum Pudding’ model:

9. Discovery of the Nucleus
Rutherford used the Gold Foil Experiment to study the atom
Alpha particles are fired at the thin gold sheet.

10. Observations Some particles were deflected (1 in 8000).
Most particles passed straight through.

11. Conclusions
The nucleus is very small, dense, positive. The atom is mostly empty space.
Rutherford’s model:

12. Neils Bohr
Studied light – Hydrogen spectra – made up of particles (atoms)
Conclusion:
Electrons have specific amounts of energy and exist in specific energy levels (Quantum jumps)
Bohr’s model:

13. Subatomic Particles Protons (p+) Neutrons (n0) Electrons (e-)
Positively charged, found in nucleus
Neutrons (n0)
Neutral, found in nucleus
Electrons (e-)
Negative, found in electron cloud surrounding nucleus
equals the number of protons in a neutral atom
Negligible mass

14. Atomic Number Whole number on periodic table
Indicates the number of protons
Also indicates number of electrons in the neutral atom.
EX: How many protons does Carbon have?
Calcium?
Uranium?

15. Mass Number Equals the number of protons plus neutrons (whole number)
NOT the mass found on the Periodic Table
Has to be provided to you by either
Hyphen Notation
Symbol (Nuclear) Notation
EX: What is the mass number of Carbon-14?
Uranium-235?

16. Summary Protons = Atomic number
Electrons (in neutral atom) = Atomic number
Neutrons = Mass number minus # of protons
Mass number = Protons plus neutrons

17. Nuclear Symbol Notation
Mass Number
(p+ + n0)
Atomic Number (number of protons)
Protons?
Neutrons?
Electrons?

18. Hyphen Notation Chlorine – 35 protons? neutrons? electrons?
Mass Number

19. Isotopes
Elements have the same number of protons but different number of neutrons.
Different mass numbers.
Ex: Carbon-12 and Carbon-14
How many protons, neutrons and electrons?
Which one is more common?

20. Average Atomic Mass
Weighted average of all isotopes found in nature based on % abundance (amu)
Found on PT – Helps identify most common isotope. (remember – mass number is not a fraction)
Example:
99.985% of hydrogen found in nature is hydrogen-1 and 0.015% is hydrogen-2.
This creates an average mass of amu.)

21. calculating Avg. Atomic Mass
(mass # x %)____+____(mass # x %)… = 100 In nature: Carbon-12 is 98.89% Carbon -13 is 1.10 % Carbon-14 is 0.01 %