An atom of carbon has 6 protons and 8 neutrons. State its atomic number and mass number, and write its nuclear notation.
Atomic number \(= 6\); mass number \(= 14\). Notation: \(^{14}_{6}\mathrm{C}\).
Atomic Structure & Radioactivity
10 exam-style questions on atoms, isotopes, alpha, beta, gamma radiation and half-life. Tap Show answer for full working.
3 Easy4 Medium3 Hard
What is an isotope? Give one example.
Atoms of the same element (same number of protons) with different numbers of neutrons (different mass numbers). Example: \(^{12}\mathrm{C}\) and \(^{14}\mathrm{C}\).
State the nature of alpha, beta and gamma radiation, and rank them in order of increasing penetrating power.
Alpha: helium nucleus (\(^{4}_{2}\mathrm{He}\)); beta: high-speed electron; gamma: electromagnetic wave (photon).
Penetrating power: alpha < beta < gamma.
Complete the equation for the alpha decay of \(^{238}_{92}\mathrm{U}\).
\(^{238}_{92}\mathrm{U} \to \,?\, + \,^{4}_{2}\mathrm{He}\)
\(^{238}_{92}\mathrm{U} \to ^{234}_{90}\mathrm{Th} + ^{4}_{2}\mathrm{He}\).
A radioactive sample has a half-life of \(4.0\) hours. What fraction of the original activity remains after \(12\) hours?
Three half-lives: fraction \(= (1/2)^{3} = 1/8\).
Explain why alpha radiation is more ionising than gamma radiation, even though gamma is more penetrating.
Alpha particles have greater mass and charge (\(2+\)), so they interact strongly with atoms and cause many ionisations over a short distance. Gamma photons have no charge and interact less frequently, travelling further before being absorbed.
State two sources of background radiation.
Examples: radon gas, rocks/soil, cosmic rays, medical sources, nuclear industry, food (e.g. potassium-40).
The activity of a source falls from \(800\,\mathrm{Bq}\) to \(100\,\mathrm{Bq}\) in \(24\) days. Calculate the half-life.
\(800 \to 400 \to 200 \to 100\): three half-lives in \(24\) days.
Half-life \(= 24/3 = 8.0\) days.
Write the equation for the beta-minus decay of \(^{14}_{6}\mathrm{C}\) and explain what happens inside the nucleus.
\(^{14}_{6}\mathrm{C} \to ^{14}_{7}\mathrm{N} + ^{0}_{-1}\mathrm{e}\) (plus antineutrino).
A neutron changes into a proton, emitting an electron (beta particle).
Explain why radioactive waste with a long half-life is more difficult to manage than waste with a short half-life.
Long half-life means the material remains radioactive for a very long time (thousands of years or more), so it must be stored securely for generations. Short half-life waste decays to safe levels more quickly.