Atomic and Nuclear Physics

Atomic Structure

An atom consists of a nucleus and the electrons around it. The nucleus is made up of protons and neutrons.

• Rutherford's experiment (1911) proved the nuclear model of the atom.
• Bohr's postulates: an electron does not radiate while it is in a stationary orbit. When it moves from a higher orbit to a lower one, it emits a photon: hν = E_m − E_n, where E_m > E_n.
• The nucleus: A = Z + N. Here A is the mass number, Z is the number of protons, and N is the number of neutrons.

For example, in the nucleus of oxygen-17 (Z = 8), the number of neutrons is N = 17 − 8 = 9.

Radioactivity

Radiation	What it is
α	helium nucleus (He-4)
β	electrons
γ	electromagnetic wave

• The number of nuclei remaining, in terms of the half-life T: N = N₀·2^(−t/T).
• Binding energy: E = Δm·c², where Δm is the mass defect.

Calculating Half-Life Problems

Convert the time into the number of half-lives: n = t/T. Then the remaining fraction is 1/2ⁿ.

1. Find the number of half-lives: n = t/T
2. Write the remaining fraction: 1/2ⁿ
3. If needed, multiply by N₀

After one half-life, half of the nuclei remain, not all of them decay. If the half-life is 6 hours, then in 30 hours n = 5 half-lives pass and 1/32 of the nuclei remain.