1. Physical Science Mr. Willis
History of the Atom
Physical Science
Mr. Willis

2. Elements Element - substance made up of only one kind of atom
cannot be broken down by a chemical change
Represented by symbols such as N (nitrogen)

3. The Model of the Atom has Changed
Atom - smallest particle of an element w/the properties of that element
All matter is composed of atoms

4. Democritus – 420 B.C. Cheese thought experiment
Eventually, matter cannot be divided any further. This unit is called the atom

5. Aristotle – 360 B.C.
Matter is continuous; can always be divided

6. John Dalton – (1766-1844) (next slide)
Revived & added to Democritus’ theory
Solid spheres
Composed atomic theory
(next slide)

7. The Atomic Theory
1. All elements are composed of atoms, which are indivisible and indestructible.
2.The atoms of an element are identical in their masses. (This was proven false with the discovery of Isotopes)
3. Atoms of different elements have different masses
4. Compounds are formed by joining the atoms of two or more elements in definite whole-number ratios (e.g. 1:1, 1:2, 3:2, ...)

8. JJ Thomson
Plum-pudding model - Ball of + charge w/- charges stuck inside it
Used cathode-ray tube to discover electrons & create theory

9. Lord Ernest Rutherford - 1911
Gold foil experiment

10. Solar system model dense, positively charged center called nucleus
Most of the atom is empty space with electron particles orbiting the nucleus

11. Neils Bohr - 1913 Thought his theory solved light-emission problem
Electrons orbit nucleus in certain energy levels
Electricity excites electrons & they jump to higher energy level. They then return to original energy level, giving off excess energy as heat and/or light
Only works with Hydrogen

13. Quantum Mechanical Model – 1920s
Electrons behave not only like a particle, but also like a wave.
Electron energy levels are made up of sub-orbital levels
The speed and location of an electron in an atom cannot both be precisely known so electrons are in all places at all times
This is called the Heisenberg Uncertainty Principle

14. Today’s model:
positive nucleus made of protons & neutrons surrounded by cloud of negatively charged particles called electrons
nucleus contains nearly all of the mass
nucleus is extremely small
This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! % of an atom's volume is just empty space!