(a)(i)
For the correct answer:
86
In standard nuclide notation $^{A}_{Z}\text{X}$, the bottom number $Z$ represents the proton number (or atomic number). For radon-222, this lower value is explicitly given as 86. Therefore, the nucleus of a radon-222 atom contains exactly 86 protons.
(a)(ii)
For the correct answer:
136
The top number $A$ in the nuclide notation represents the nucleon number (mass number), which is the total sum of protons and neutrons in the nucleus. To determine the number of neutrons, you subtract the proton number from the nucleon number using the formula $A – Z$. Calculating $222 – 86$ yields 136 neutrons.
(b)
For the correct answer:
3.8 days
First, determine the number of half-lives required for the sample to decay from 60 mg down to 7.5 mg. The halving sequence is 60 → 30 → 15 → 7.5, which means 3 complete half-lives have elapsed. Since 3 half-lives take a total of 11.5 days, a single half-life is found by evaluating $11.5 \div 3$, which gives approximately 3.83 days (or 3.8 days when rounded).
(c)
For the correct answer (any two from):
Rocks, buildings, food, drink, cosmic (rays).
Background radiation is the continuous, low-level ionising radiation present in our environment. Beyond radon gas in the air, significant natural contributions include cosmic rays penetrating the atmosphere from space, as well as naturally occurring radioactive isotopes found in rocks, building materials, and trace amounts within the food and water we consume daily.
