A look at early models of the atom….

HISTORY OF ATOMIC THEORY

Historical Timeline

Democritus Model (403 BC)

1. All matter is composed of atoms, which are too small to be seen. These atoms CANNOT be further split into smaller portions.

2. There is no void, which is empty space between atoms.

3. Atoms are completely solid.

4. Atoms are homogenous, no internal structure.

5. Atoms are different in their size, their shapes, and weight.

DEMOCRITUS MODEL (403 BC)

Dalton Model (1803)

1.Each element was composed of the same kind of atom.

2. Elements of a given substance have atoms composed of the same mass, size, and shape.

3. Atoms form compounds in ratios (1:1, 1:2, etc.).

4. Chemical reactions are rearrangements of atoms.

DALTON MODEL OF 1803 (BILLIARD BALL MODEL)

Dalton prepared one of the first Periodic Tables of Elements using atomic symbol to represent certain elements.

Thomson Model of 1901 (Plum Pudding Model)

1. The atom was divisible.

2. The atom consisted of both positively and negatively charged material.

3. Thomson was responsible for the discovery of the idea an electron.

• Thomson had originally named them “corpuscles”

• Future research and experimentation by other scientists provided further evidence that this particle did in-fact exist.

Thomson Model of 1901 (Plum Pudding Model)

Nagaoka Model of 1904 (Saturnian Model)• Based around an analogy to the explanation of the stability of the Saturn’s rings the following predictions were made:

1. A very massive nucleus (in analogy to a very massive planet)

2. Electrons revolving around the nucleus, bound by electrostatic forces (in analogy to the rings revolving around Saturn, bound by gravitational forces).

Hantaro Nagaoka Model 1904 (Saturnian Model)

Rutherford’s Atom Model Conclusions

1. The atom contains a small dense center called the nucleus.

2. The nucleus contains most of the mass of the atom.

3. The nucleus is positively charged.

4. The atom is mostly empty space and the electrons move around in this empty space.

Nucleus(contains protons & neutrons)

Rutherford’s Atom Model

1. The area in which electrons are orbiting around the nucleus is called an electron shell or

orbit.

2. Each electron shell can only hold a certain number of electrons.

Once the shell is filled, the next outer shell begins to get filled.

Bohr’s Electron Shell Model of the Atom (1913)

Niels Bohr Model of 1913 (Electron Shell theory)

Electron Shell Theory1st shell holds: 2 electrons2nd shell holds: 8 electrons3rd shell holds: 8 electrons4th shell holds: 18 electrons5th shell holds: 18 electrons6th and 7th hold: 32 electrons

Wave Model – Schroedinger (1920s)

1. Electrons are in orbitals (energy levels) around the nucleus but don’t rotate around the nucleus like planets around the sun.

2. The orbitals are really energy levels

• Bigger energy level = higher energy

Lower energy

Higher energy

Highest energy of the three

Schroedinger - 1926

1. Electrons can change orbits (energy levels)

2. They give off or absorb electromagnetic radiation as a photon when they change levels

Chadwick - 1932

1. Discovered the existence of the neutron

2. The discovery of the neutron explained why an atoms mass is higher than the total mass of their protons and electrons

3. Named a neutron based on its lack of electrical charge

4. Side note: Chadwick’s discovery was a crucial step to understanding nuclear fission of Uranium 235. He was also member of the team who developed the first atomic bombs, including those dropped on Japan in WWII