1. Development of the Modern Atomic Model

2. I. Dalton (1808):
First explained the Law of Conservation of Mass.
He basically said that chemical reactions involve atoms of different elements recombining to form new compounds.
His theory can be summarized as:
1. All matter is composed of indivisible atoms. These atoms are like tiny, hard, marbles.
2. All atoms of the same element are identical.
Atoms of different elements have different properties.
3. In chemical reactions, atoms of various elements are rearranged to form new compounds. These new compounds are made up of the same type and number atoms that you began with, just rearranged in different combinations.
While his explanation of the Law of Conservation of Mass was basically correct, his model of the atom was wrong.

3. II. Thomson (1897):
Performed the famous “Cathode Ray Experiment”,
in which he discovered:
1. the electron,
2. the electron has a negative charge,
3. the electron has a very small mass compared to the rest of the atom.

4. Thomson then reasoned that:
1. because atoms generally have a neutral charge, there must something in the atom that has a positive charge that cancels the electrons negative charge.
2. because the negative electron has such a small mass compared the rest of the atom, the positive part of the atom must have a very large mass.
3. because atoms are made of different parts, they must be “divisible”.
His reasoning was correct but then he incorrectly hypothesized that an atom was like a “pudding” of positive charge with negative electrons sprinkled throughout the pudding.

5. “Plum Pudding” model of the atom proposed by Thomson.

6. III. Rutherford (1911):
Performed the famous “Gold Foil” Experiment”,
in which he discovered:
1. the nucleus of the atom
2. the nucleus had a positive charge
3. the atom was mostly empty space
Based on these discoveries, he reasoned that the electrons in an atom surround the central nucleus in a region called the “electron cloud”. The empty space is located between the cloud and the nucleus and in between the individual electrons.

7. Electrons in
“Electron Cloud”
Nucleus
Rutherford model of the atom

8. 4. Bohr (A few years later): Student of Rutherford
Came up with the idea of electrons orbiting the nucleus
in specific “orbits” or “shells” or “energy levels” similar to the way planets orbit the sun.
Electrons are found only in discrete energy levels.
Each energy level is a specific (“discrete”) distance from the nucleus.
Electrons are never found in between energy levels.
An electron may “jump” from one energy to the next,
but it doesn’t linger between energy levels.
All the different energy levels combined make up the electron “cloud”.

9. Bohr’s “Solar System” Model
If an atom absorbs just the right amount of energy, an electron can “jump” up to a higher energy level.
If an atom emits or loses just the right amount of energy, the electron can “fall” to a lower energy level.

10. 5. Schrodinger (1926):
Proposed that electron energy levels are 3 dimensional shapes, not simple circular orbits.
Different energy levels can hold different numbers of electrons.
(more on this later)
At any one instant in time, it is impossible to know exactly where in the 3 dimensional shape an electron is located.
Equations can be derived that tell you the likelihood of finding an electron at any particular place in the energy level at any
instant in time.

11. 2nd energy level
3rd energy level
1st (Lowest) energy level
4th energy level
Schrodinger’s model- Different electron energy levels are actually various 3 dimensional shapes, not circular “orbits”

12. Shapes of the different electron energy levels