Nuclear chemistry deals with changes in the nucleus of an atom, rather than the rearrangement of electrons that drives ordinary chemical reactions. Some nuclei are unstable and break down, giving out radiation โ this is radioactivity. CSEC Chemistry expects you to know the structure of the atom, what isotopes are, the three main types of radiation, the idea of half-life, and the uses and dangers of radioactivity.
The nucleus and isotopes
An atom has a tiny central nucleus containing protons (charge +1) and neutrons (no charge), surrounded by electrons (charge โ1) in shells. The atomic (proton) number is the number of protons; the mass number is the number of protons plus neutrons.
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons โ so they have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are both carbon (6 protons) but have 6 and 8 neutrons respectively. Because they have the same electron arrangement, isotopes have identical chemical properties; only their masses and nuclear stability differ.
Radioactivity
An unstable nucleus (a radioisotope) becomes more stable by emitting radiation. This happens spontaneously and randomly โ it cannot be sped up or slowed down by temperature, pressure or chemical change, because it is a property of the nucleus.
There are three main types:
| Radiation | What it is | Charge | Penetrating power | Stopped by |
|---|---|---|---|---|
| Alpha (ฮฑ) | a helium nucleus (2 protons + 2 neutrons) | +2 | low (most ionising) | a sheet of paper / skin |
| Beta (ฮฒ) | a fast electron | โ1 | medium | a few mm of aluminium |
| Gamma (ฮณ) | high-energy electromagnetic wave | 0 | high (least ionising) | thick lead or concrete |
Note the pattern: the most ionising radiation (alpha) is the least penetrating, and the least ionising (gamma) is the most penetrating.
Half-life
The decay of a radioisotope is measured by its half-life โ the time taken for half of the radioactive nuclei in a sample to decay (or for the activity to fall to half its value). Each isotope has its own fixed half-life, ranging from fractions of a second to billions of years.
A simple calculation: if an isotope has a half-life of 2 days and you start with 80 g of it, then after 2 days 40 g remains, after 4 days 20 g, after 6 days 10 g, and so on โ halving each half-life.
What happens to the nucleus during decay
When a nucleus emits radiation its make-up changes, and you should be able to describe this:
- In alpha decay the nucleus loses 2 protons and 2 neutrons, so its mass number falls by 4 and its atomic number falls by 2 โ it becomes a different element.
- In beta decay a neutron in the nucleus changes into a proton (and the emitted electron is the beta particle), so the mass number is unchanged but the atomic number increases by 1 โ again a new element forms.
- In gamma emission no particles are lost; the nucleus simply releases surplus energy, so the element does not change.
This explains why radioactive decay can turn one element into another (transmutation) โ something ordinary chemical reactions can never do, because they only involve the outer electrons.
A worked half-life problem
Half-life questions often go beyond simple halving. Suppose a sample has an activity of 1600 counts per second and a half-life of 5 hours, and you are asked for the activity after 15 hours. Fifteen hours is three half-lives (15 รท 5 = 3), so the activity halves three times:
1600 โ 800 (after 5 h) โ 400 (after 10 h) โ 200 (after 15 h)
So the activity is 200 counts per second. The same method works for the mass of radioisotope remaining, or you can read values from a decay graph, where the half-life is the time taken for the curve to fall to half of any starting value.
Uses of radioactivity
- Medical: gamma rays are used to sterilise equipment and to treat cancer (radiotherapy); radioisotopes are used as tracers to follow processes in the body.
- Industrial: tracers detect leaks in underground pipes; radiation gauges measure the thickness of paper, metal or plastic sheet.
- Dating: carbon-14 is used to estimate the age of once-living material (radiocarbon dating); other isotopes date rocks.
- Energy: nuclear reactors use the energy released by decay/fission to generate electricity.
Dangers and safety
Radiation is ionising โ it can damage living cells, cause mutations and lead to cancer, and high doses can cause radiation sickness. Safety measures include:
- keeping sources in lead-lined containers;
- handling them with tongs, at a distance, and for the shortest possible time;
- wearing protective clothing and monitoring exposure with film badges;
- proper disposal of radioactive waste.
Common exam mistakes
- Saying isotopes have different chemical properties โ they have the same chemistry (same electrons); only mass/stability differ.
- Reversing the penetration order โ alpha is least penetrating but most ionising; gamma is the reverse.
- Thinking half-life is the time for all the sample to decay โ it is the time for half to decay.
- Saying radioactivity can be sped up by heat โ it cannot; it is spontaneous and random.
Nuclear reactions versus chemical reactions
A key idea is that nuclear changes are completely different from ordinary chemical reactions:
- Chemical reactions involve only the outer electrons; the nucleus is unchanged, so each element stays the same element.
- Nuclear reactions change the nucleus itself, so one element can turn into another (transmutation), and they release far more energy than any chemical reaction.
- Chemical reaction rates depend on temperature, concentration and catalysts; radioactive decay does not โ it is spontaneous and random, unaffected by such conditions.
Keeping this contrast clear explains why radioactivity behaves so differently from the reactions met elsewhere in the syllabus.
Key terms to remember
- Isotopes โ atoms of the same element with different numbers of neutrons.
- Radioisotope โ an unstable isotope that decays, emitting radiation.
- Alpha (ฮฑ) โ a helium nucleus; highly ionising, stopped by paper.
- Beta (ฮฒ) โ a fast electron; stopped by a few mm of aluminium.
- Gamma (ฮณ) โ high-energy electromagnetic radiation; stopped by thick lead.
- Half-life โ the time for half the radioactive nuclei (or the activity) to decay.
- Ionising radiation โ radiation that can damage cells, causing mutation or cancer.
- Transmutation โ the change of one element into another during nuclear decay.
Quick recap
- The nucleus holds protons and neutrons; isotopes are same-element atoms with different neutron numbers and identical chemistry.
- Radioactivity is the spontaneous, random emission of radiation from unstable nuclei.
- Alpha (paper, most ionising), beta (aluminium), gamma (lead/concrete, most penetrating).
- Half-life is the time for half the nuclei (or activity) to decay.
- Uses: medicine, tracers, thickness gauges, dating, power โ but radiation is ionising, so handle with shielding, distance and minimal time.