Planetary model (Rutherford model) of atom

According to Rutherford's planetary model of the atom, electrons orbit the nucleus under the influence of the elctric force. For example, in a hydrogen atom, an electron with charge -e orbits a proton with charge +e.

Rutherford established his model of atom by doing an experiment with alpha particles. He fired alpha particles at an extremely thin sheet of gold foil. While most alpha particles sailed right through the target gold atoms, a small percentage of the alpha particles were deflected through very large angles (90° to 180°). These alpha particles hit the nucleus of the gold atoms and were thus deflected.

The problem with the planetary model of the atom is that the radiation emitted by an orbbiting electron, as predicted by Maxwell's theory, would result in the electron losing energy and spiralling into the nucleus.

Bohr model of atom

The main ideas of Bohr model are these:

• Electrons do not emit electromagnetic radiation while in a given orbit (energy state).
• Electrons cannot remain in any arbitrary energy state. The can remain only in discrete or quantized energy states.
• Electrons emit electronmagnetic energy when they make transitions from higher energy states to lower energy states. The lowest energy state is the ground state.
• The energy in a given state represents the amount of energy, called "ionization energy", needed to free an electron from the atom.
• The energy of a photon is equal to the energy difference between 2 energy states

The energy levels within an atom are given by the following formula:

E_n = (Z²/n²) x (-13.6 eV), where 1 eV = 1.6 x 10^-19 J, Z is number of protons in the atom's nucleus.

The Bohr model of atom can be used to explain the phenomenon known to physicists that atoms emit or absorb radiation only at certain discrete wavelengths.

The problem with the Bohr model of atom is that it works with hydrogen atom, but it does not work well with atoms with many electrons.

Nuclear structure and stability 

1. The basic structure of an atom consists of a nucleus with a certain number of protons and neutrons, which are together called nucleons.
2. The atomic number, Z, measures the relative charge on the nucleus, which is equal to the number of protons in the nucleus of an atom
3. The number of neutrons is usually denoted by letter N
4. The mass number (atomic mass), A, is a measure of the total number of nucleons (protons and neutrons) in the nucleus. A = Z + N
5. Isotopes are nuclei with the same number o fprotons but different numbers of neutrons.
6. Atomic mass units are used to measure nuclear masses. 1 atomic mass unit μ = 1.675 x 10^-27kg
7. The proton neutron plot: experiments show that lighter nuclei have about the same number of protons as neutrons. However, heavier stable nuclei have more neutrons than protons. See the diagram below.
8. Strong nuclear force vs electromagnetic force: In the nucleus, electric forces repel all protons away from each other, tearing apart the nucleus. However, both protons and neutrons are bound by the strong nuclear force, holding the nucleus together. This balancing act of forces gives us both stable and unstable nuclei.

Binding energy and mass defect

Refer to this video: https://m.youtube.com/watch?v=KgcqjILr97E

Mass of He nucleus < combined mass of 2 protons and 2 neutrons - the difference is the mass defect.

If we combine protons and neutrons into He, some mass will be converted into energy in accordance with E = mc²

According to the binding enery curve:

• Up to Fe⁵⁶, combining smaller nuclei into larger nuclei will convert mass into energy (nuclear fussion)
• After Fe⁵⁶, spliting nuclei into smaller nuclei will convert mass into enrgy (nuclear fission)

Radioactive decay

Watch this video for a detailed discussion: https://m.youtube.com/watch?v=fES21E0qebw

5 Types of Radiation

Half Life

Half life is the time required for half of a given sample to decay.

Nuclear fission and fusion

Here is 1 possible uranium fission reaction equation:

Here is 1 hydrogen fussion reaction, the one that powers the Sun:

 Note in above reactions equations, the energy component is called "disintegration energy", and is usually denoted by letter Q. Q can be positive (as in above nuclear fission and fusion reactions), or negative. If disintegration energy Q is positive, the reaction is "exothermic" and can occur spontaneously; If disintegration energy Q is negative, the reaction is "endothermic", and cannot occur spontaneously.