AQA GCSE Physics: Atomic Structure

In the early 20th century, scientists fired a beam of alpha particles at a very thin sheet of gold foil (the alpha-particle scattering experiment). Three things were observed:

• most alpha particles passed straight through the foil;
• some were deflected through small angles;
• a very small number bounced straight back towards the source.

Describe and explain what each of these three observations told scientists about the structure of the atom, and explain why the results meant the earlier plum pudding model had to be replaced. (6 marks)

A radioactive source has an initial count rate of 640 counts per second. The count rate is measured every hour (corrected for background radiation):

Time / hours	Count rate / counts per second
0	640
2	320
4	160
6	80
8	40

(a) Determine the half-life of this source from the data. (2 marks)

(b) Predict the count rate after a further 2 hours (at 10 hours). (1 mark)

(c) Define what is meant by the half-life of a radioactive isotope. (1 mark)

Three types of nuclear radiation are alpha, beta and gamma.

(a) State what an alpha particle is made of. (1 mark)

(b) Name the type of radiation that is stopped by a few millimetres of aluminium. (1 mark)

(c) State which of the three is the most ionising. (1 mark)

A radium nucleus undergoes alpha decay to become a radon (Rn) nucleus. The radium nucleus has a mass number of 226 and an atomic number of 88. (This is Higher-tier content.)

The decay can be written:

$^{226}_{\ 88}\text{Ra} \;\rightarrow\; ^{A}_{Z}\text{Rn} \;+\; ^{4}_{2}\text{He}$ 88226​Ra→ZA​Rn+24​He

(a) State the mass number $A$A and the atomic number $Z$Z of the radon nucleus. (2 marks)

(b) Explain why the atomic number decreases in alpha decay. (1 mark)

Carbon-12 and carbon-14 are isotopes of the element carbon.

Explain what is meant by saying that two atoms are isotopes of the same element, referring to the numbers of protons and neutrons. (2 marks)

In a nuclear reactor, energy is released when a large unstable nucleus, such as uranium-235, splits into two smaller nuclei.

Name this nuclear process. (1 mark)