Alpha Decay

Definition of Alpha Decay

Alpha decay is a type of radioactive decay where an unstable, heavy nucleus emits an alpha particle. An alpha particle consists of 2 protons and 2 neutrons, which is the same as a helium-4 nucleus. This process happens mainly in heavy elements such as uranium or radium, where the nucleus is too large to be stable. Heavy nuclei are unstable because of the imbalance of forces inside the nucleus, so they emit alpha particles to become more stable.

Process of Alpha Decay

During alpha decay, the parent nucleus loses 2 protons and 2 neutrons by emitting an alpha particle. This changes the original atom into a new element called the daughter nucleus because the number of protons (atomic number) decreases by 2, and the total number of nucleons (mass number) decreases by 4.

For example, if a uranium-238 nucleus undergoes alpha decay, it loses 2 protons and 2 neutrons, becoming thorium-234:

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

Here, the uranium nucleus (atomic number 92, mass number 238) emits an alpha particle (mass 4, atomic number 2), leaving thorium with atomic number 90 and mass number 234.

The general equation for alpha decay is:

$$^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}\alpha$$

Where:

• $^{A}_{Z}X$ is the parent nucleus
• $^{A-4}_{Z-2}Y$ is the daughter nucleus
• $^{4}_{2}\alpha$ is the alpha particle

For instance, if a nucleus with atomic number 88 and mass number 226 emits an alpha particle, the new nucleus will have atomic number 86 and mass number 222.

This process reduces both the atomic number and mass number, changing the element into a different one.

Quick example: A nucleus with atomic number 90 and mass number 230 undergoes alpha decay. What are the atomic number and mass number of the daughter nucleus?

Atomic number: 90 - 2 = 88
Mass number: 230 - 4 = 226

The daughter nucleus has atomic number 88 and mass number 226.

Example: If radium-226 undergoes alpha decay, what is the daughter nucleus?

Radium-226 has atomic number 88 and mass number 226. After alpha decay:

$$^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\alpha$$

The daughter nucleus is radon-222 (Rn), with atomic number 86 and mass number 222.

Properties of Alpha Particles

Alpha particles have several important properties:

• Highly ionising: Alpha particles can easily knock electrons off atoms, creating ions. This makes them very damaging to materials and living cells if they come into contact.
• Low penetration power: Alpha particles cannot penetrate far. They are stopped by a few centimetres of air, a sheet of paper, or even the outer layer of human skin.
• Positive charge: Since alpha particles contain 2 protons, they have a +2 charge.

Because of their low penetration, alpha particles are only hazardous if the radioactive source is inside the body (e.g., through inhalation or ingestion), where they can cause serious ionisation damage to cells.

Effects and Uses of Alpha Decay

Alpha decay causes the original atom to change into a different element because the atomic number decreases by 2. This is a nuclear change, not a chemical one.

One practical use of alpha decay is in smoke detectors. Some smoke detectors contain a small amount of an alpha-emitting radioactive source (usually americium-241). The alpha particles ionise the air inside the detector, allowing a small electric current to flow. When smoke enters, it disrupts this current, triggering the alarm.

However, alpha radiation is hazardous due to its strong ionising power. It can damage living tissue if alpha-emitting materials enter the body, so safety precautions are important when handling alpha sources.

Example: Calculating Daughter Nucleus After Alpha Decay

A polonium-210 nucleus undergoes alpha decay. Calculate the atomic number and mass number of the daughter nucleus.

Polonium-210 has atomic number 84 and mass number 210. After alpha decay:

$$\text{Mass number} = 210 - 4 = 206$$

$$\text{Atomic number} = 84 - 2 = 82$$

The daughter nucleus has atomic number 82 and mass number 206, which is lead-206:

$$^{210}_{84}\text{Po} \rightarrow ^{206}_{82}\text{Pb} + ^{4}_{2}\alpha$$