From the history of the atom to the power of radioactive decay.
Atomic Number: The number of Protons in the nucleus.
Mass Number: The total number of Protons + Neutrons.
Neutral Atom: The number of Electrons always equals the number of Protons.
| Particle | Relative Mass | Relative Charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | Very Small | -1 |
Q1: An atom has atomic number 11 and mass number 23. How many neutrons does it have?
$$ 23 - 11 = 12 \, \text{neutrons} $$
Q2: How does an atom become a positive ion?
By losing one or more electrons from its outer shell.
Q1: What did alpha particles passing straight through gold foil prove?
That the atom is mostly empty space.
Beta Decay Note: A neutron turns into a proton and emits a fast-moving electron (beta particle).
| Property | Alpha ($\alpha$) | Beta ($\beta$) | Gamma ($\gamma$) |
|---|---|---|---|
| Ionising Power | Strong | Moderate | Weak |
| Penetration | Low (Paper) | Moderate (Aluminium) | High (Lead/Concrete) |
| Range in Air | ~5 cm | ~1 m | Infinite |
Q1: Which radiation is the most dangerous outside the body?
Gamma, because it can penetrate skin and reach organs.
Rule: The total mass number and atomic number must be the same on both sides.
$$ {}^{238}_{92}\text{U} \rightarrow {}^{234}_{90}\text{Th} + {}^{4}_{2}\alpha $$
$$ {}^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}e $$
Q: An isotope undergoes alpha decay. What happens to its mass number?
The mass number decreases by 4.
Q: A sample has 800 Bq. If its half-life is 10 days, what is the activity after 20 days?
After 2 half-lives: $$ 800 \rightarrow 400 \rightarrow 200 \, Bq $$
| Irradiation | Contamination |
|---|---|
| Exposure to radiation. | Radioactive atoms get inside/on you. |
| Object does not become radioactive. | Object remains radioactive until source is removed. |