Atomic and Nuclear Physics for Singapore N(A)-Level Science (Physics) 5105/5106: the nuclear atom with nuclide notation and isotopes, radioactive decay and the alpha, beta and gamma radiations, and half-life with the uses and dangers of radioactivity

What this module covers

Atomic and Nuclear Physics looks inside the atom in N(A)-Level Science (Physics) 5105/5106. It describes the structure of the atom, a tiny nucleus of protons and neutrons surrounded by electrons, and the notation that names each nuclide. It then explains radioactive decay and compares the three radiations that unstable nuclei emit, before finishing with half-life and the uses, dangers and safe handling of radioactivity.

These ideas connect the smallest building blocks of matter to real applications in medicine, industry and energy. Each dot point below has full worked answers and practice questions.

The nuclear atom

The nuclear atom describes the atom as a small, dense nucleus of protons and neutrons surrounded by orbiting electrons. The proton number names the element, and the nucleon number is the total of protons and neutrons. A nuclide is written as $^{A}_{Z}\text{X}$, where $A$ is the nucleon number and $Z$ is the proton number. Isotopes are atoms with the same proton number but different nucleon numbers.

Radioactivity and types of radiation

Radioactivity and types of radiation explains radioactive decay as a random process in which an unstable nucleus emits radiation. The three types are alpha (a helium nucleus, charge $+2$, stopped by paper), beta (a fast electron, charge $-1$, stopped by a few millimetres of aluminium) and gamma (an electromagnetic wave, no charge, reduced only by thick lead or concrete). Alpha is the most ionising; gamma the most penetrating.

Half-life and uses of radioactivity

Half-life and uses of radioactivity defines half-life as the time for half the radioactive nuclei (or the count rate) to fall by half. Radioactivity has many uses, from medical tracers and treating tumours to thickness gauges in industry, but it is dangerous to living cells, so sources must be handled with tongs, kept at a distance, and stored in lead-lined containers.

How this module is examined

• Read nuclide notation. Neutrons $= A - Z$; electrons equal protons in a neutral atom.
• Match radiation to its stopper. Paper stops alpha, a few millimetres of aluminium stop beta, and thick lead or concrete is needed for gamma.
• Count the half-lives. Divide the total time by the half-life, then halve the starting amount that many times.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

1. State what the proton number of an atom tells you. (1 mark)
2. An atom is written as $^{27}_{13}\text{Al}$. Find the number of neutrons. (2 marks)
3. Name the radiation that is stopped by a sheet of paper. (1 mark)
4. A source has a half-life of $6\ \text{hours}$. A sample starts at $640\ \text{counts per minute}$. Find the count rate after $18\ \text{hours}$. (2 marks)
5. State one safety precaution when handling a radioactive source. (1 mark)