Atomic structure, isotopes, alpha/beta/gamma and nuclear equations – IGCSE Physics exam questions with mark scheme

1Grade 1–34 marks

Alpha, beta and gamma are three types of nuclear radiation.

(a) State which type of radiation is the most ionising. [1]

(b) State which type of radiation is stopped by a sheet of paper. [1]

(d) Describe what a beta particle is. [1]

(a) alpha(1)
(b) alpha(1)
(c) (high-frequency / short-wavelength) electromagnetic radiation / wave (emitted from a nucleus)(1)
(d) a (fast-moving) electron emitted from the nucleus; 'electron' alone scores; reject an electron from an orbit / shell(1)
Answer: (a) alpha (b) alpha (c) electromagnetic wave (d) fast electron from the nucleus
Examiner tip: A gamma ray is not a particle with mass: call it electromagnetic radiation.

2Grade 4–54 marks

Complete the nuclear equations.

(a) Polonium-210 decays by alpha emission: $$^{210}_{84}\text{Po} \rightarrow\ ^{\;?}_{\;?}\text{Pb} +\ ^{4}_{2}\alpha$$ [2]

(b) Carbon-14 decays by beta emission to an isotope of nitrogen (N): $$^{14}_{6}\text{C} \rightarrow\ ^{\;?}_{\;?}\text{N} + \;?$$ [2]

(a) mass number of Pb = 206(1)
(a) atomic number of Pb = 82(1)
(b) $^{14}_{7}\text{N}$ (both numbers correct)(1)
(b) $^{\;0}_{-1}\beta$ or $^{\;0}_{-1}e$ (both numbers needed)(1)
Answer: (a) $^{206}_{82}$Pb (b) $^{14}_{7}$N + $^{\;0}_{-1}\beta$
Examiner tip: In beta decay the atomic number goes UP by 1 because a neutron changes into a proton.

3Grade 8–95 marks

Uranium-238 ($^{238}_{92}$U) decays through a series of alpha and beta-minus emissions to form the stable isotope lead-206 ($^{206}_{82}$Pb).

(a) Determine the number of alpha particles and the number of beta particles emitted in the series. [4]

(b) Explain why beta-minus decay does not change the mass number of a nucleus. [1]

(a) decrease in mass number = 238 − 206 = 32(1)
(a) number of alpha decays = 32 ÷ 4 = 8(1)
(a) atomic number after 8 alpha decays = 92 − 16 = 76(1)
(a) number of beta decays = 82 − 76 = 6(1)
(b) a neutron changes into a proton (and an electron), so the total number of nucleons (protons + neutrons) is unchanged; allow the beta particle (electron) has a mass number of 0 / is not a nucleon(1)
Answer: 8 alpha and 6 beta; nucleon number unchanged in beta decay
Examiner tip: Do the alphas first: only alpha decay changes the mass number, so it fixes how many alphas there are.

Book a free introductory session and we'll find exactly which step is costing you.