1. First 5 min of class
Identify the following as a molecule, compound, element, or/and an atom.
KCl
200 copper atoms O2
C2H4 He

2. Riddle
If a pen and a pencil cost a pound and ten pence and the pen is a pound more than the pencil, how much does the pen cost?
(A pound is a UK “dollar” and a pence is a UK “cent”)

3. History of the Atomic Model
Over 2000 years of science

4. Democritus, 400 BC He was the first to suggest the idea of the atom
He presumed there must be a “stopping point” when you’re cutting something, where it is “indivisible”…where it is “atomos”
Unfortunately he was laughed at by all the other mean philosophers who followed Aristotle. So atomic science became stagnant for 2000 years.

5. Aristotle, and therefore everyone, believed that the universe was made of Earth, Fire, Wind, Water, and Aether.

6. John Dalton
All matter is made up of tiny indivisible particles called atoms
Atoms cannot be created or destroyed
All atoms of a particular element are identical
Compounds are formed through the combination of elements
Atoms combine in whole number ratios to form compounds

7. J.J. Thomson
He shot cathode rays between two electrically charged plates. The cathode rays always bent towards the positive plate.
He concluded that
Cathode rays consist of negatively charged particles he called electrons
Electrons are much smaller than the smallest atom

8. Thomson’s Model of the Atom

9. Ernest Rutherford 1911

10. Rutherford’s Gold Foil Experiment

11. Rutherford’s Gold Foil Experiment

12. Results of Rutherford’s Experiment
Note: alpha particles are positive particles
1) He found that most of the alpha particles went straight through the foil unaffected
2) A very few particles were deflected slightly from their path
3) 1 in particles were strongly deflected from their path (90o or more)

13. Rutherford’s Results Explained

14. Thomson’s model vs Rutherford’s

15. Rutherford’s Conclusions
Most of the atom is empty space
The centre of the atom, the nucleus, is positively charged and contains most of the mass of the atom
Since only a very few alpha particles were affected by the nucleus, it must very small
His model is often called the “nuclear model of the atom”
atom

16. 1918
Rutherford discovers that atoms contain positively charged particles called
protons

17. Electromagnetic Radiation
Waves of energy travelling through space at the speed of light
The shorter the wavelength the higher the energy
More importantly: the bluer the colour of light, the more energetic it is.

18. Bohr’s Model of the Atom
Bohr’s model suggests that electrons orbit in rings around the nucleus
Electrons can exist in these orbits, and not in between
The lower the orbit, the lower the energy

19. Bohr’s Model of the Atom
Ground state = when an electron is in the lowest energy level that it can occupy
Excited state = when an electron is in any energy level higher than ground state
Electrons jump from ground state to excited state when they absorb energy from heat or electricity
Electrons release energy when they fall from the excited state back to ground state

20. Hydrogen’s Line Spectrum Explained