Radioactive Decay

Nature of Radioactive Decay

Radioactive decay is a spontaneous process where unstable atomic nuclei lose energy by emitting radiation. This happens naturally and does not require any external trigger.

The nuclei that undergo decay are unstable because they have an imbalance of protons and neutrons, making them energetically unfavourable. To become more stable, these nuclei emit particles or electromagnetic waves.

Radioactive decay is random and unpredictable at the level of individual nuclei. It is impossible to know exactly when a particular nucleus will decay, but the overall behaviour of a large number of nuclei follows statistical patterns.

There are different types of radioactive decay, each involving different particles or radiation being emitted. These types cause changes in the nucleus and are important to understand the behaviour of radioactive materials.

Example: A nucleus of radon-222 undergoes alpha decay. It emits an alpha particle and transforms into polonium-218.

Types of Radioactive Decay

There are three main types of radioactive decay: alpha decay, beta decay, and gamma decay. Each type affects the nucleus differently and emits different radiation.

Alpha Decay

In alpha decay, the nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons (the same as a helium-4 nucleus). This reduces the atomic number by 2 and the mass number by 4.

Because alpha particles are relatively large and carry a +2 charge, they do not penetrate far and can be stopped by a sheet of paper or skin.

Beta Decay

Beta decay involves the emission of a beta particle, which is an electron (beta-minus decay) or a positron (beta-plus decay). In GCSE AQA, focus is on beta-minus decay where a neutron in the nucleus changes into a proton and an electron is emitted.

This increases the atomic number by 1 but leaves the mass number unchanged because a neutron becomes a proton.

Gamma Decay

Gamma decay occurs when the nucleus emits gamma rays, which are high-energy electromagnetic waves. Gamma radiation does not change the number of protons or neutrons; it only reduces the energy of the nucleus.

Gamma rays are very penetrating and require thick lead or concrete to be stopped.

Changes in the Nucleus

Each type of decay changes the nucleus in a specific way:

• Alpha decay: Atomic number decreases by 2, mass number decreases by 4.
• Beta decay: Atomic number increases by 1, mass number stays the same.
• Gamma decay: No change in atomic or mass number, only energy is lost.

These changes can be represented in nuclear equations, showing the parent nucleus, the emitted particle or radiation, and the daughter nucleus.

For example, in alpha decay:

$$_{92}^{238}\text{U} \rightarrow _{90}^{234}\text{Th} + _{2}^{4}\alpha$$

Random Nature of Decay

Radioactive decay is a random process. This means:

• It is impossible to predict exactly when a particular nucleus will decay.
• Each nucleus in a sample has the same probability of decaying at any moment.
• Decay events are independent of each other.

Because of this randomness, the number of decays detected in a given time varies, but over large numbers of nuclei, the behaviour is statistically predictable.

This randomness affects measurements of radioactive decay. For example, when using a Geiger counter, the count rate fluctuates even if the source is constant.

Understanding the random nature is important for interpreting results and recognising that decay follows a statistical pattern rather than a fixed schedule.

Decay Equations and Nuclear Changes

Radioactive decay can be represented by nuclear equations that show the changes in the nucleus during decay. These equations use symbols for elements and particles.

The general form for a nuclear symbol is:

$$_{Z}^{A}X$$

• $X$ is the chemical symbol of the element.
• $A$ is the mass number (total protons + neutrons).
• $Z$ is the atomic number (number of protons).

In decay equations, the sum of the mass numbers and atomic numbers on each side must be equal, showing conservation of nucleons and charge.

For example, in alpha decay:

$$_{92}^{238}\text{U} \rightarrow _{90}^{234}\text{Th} + _{2}^{4}\alpha$$

The uranium nucleus loses 2 protons and 2 neutrons (an alpha particle), so the daughter thorium nucleus has atomic number 90 and mass number 234.

In beta decay:

$$_{6}^{14}\text{C} \rightarrow _{7}^{14}\text{N} + \beta^-$$

A neutron in carbon-14 changes into a proton and emits a beta particle (electron). The atomic number increases by 1 (from 6 to 7), but the mass number remains 14.

In gamma decay:

$$_{90}^{234}\text{Th}^* \rightarrow _{90}^{234}\text{Th} + \gamma$$

The asterisk (*) indicates an excited nucleus that emits gamma radiation to lose energy but does not change its atomic or mass number.

These equations help track the changes in nuclei during radioactive decay and are essential for understanding nuclear reactions.

For instance, if a nucleus emits an alpha particle, subtract 2 from the atomic number and 4 from the mass number to find the daughter nucleus.

For example, if a nucleus of radon-222 undergoes alpha decay:

$$_{86}^{222}\text{Rn} \rightarrow _{84}^{218}\text{Po} + _{2}^{4}\alpha$$

The daughter nucleus is polonium-218, with atomic number 84 and mass number 218.