Radioactive decay

An unstable atomic nucleus can change on its own, releasing radiation. This change is called radioactive decay. It may produce an alpha particle, a beta particle or gamma radiation; some decays produce more than one type of radiation.

Decay is spontaneous: it happens without an external trigger. It is also random: the time at which an individual unstable nucleus will decay cannot be predicted.

How decay changes the nucleus

In alpha decay and beta decay, the number of protons in the nucleus changes. Because the number of protons determines the element, the nucleus becomes the nucleus of a different element.

Gamma radiation releases energy from the nucleus. It does not change the number of protons, so the nucleus remains the nucleus of the same element.

Alpha decay

A very heavy, unstable nucleus may emit a small group of particles and become more stable. This process is called alpha decay. The emitted group is an alpha particle, made of two protons and two neutrons.

When the alpha particle leaves, the original nucleus loses those two protons and two neutrons. Its proton number therefore decreases by 2, and its nucleon number—the total number of protons and neutrons—decreases by 4.

An element is identified by its proton number. Because that number changes, the remaining nucleus belongs to a different element. It is more stable than the original nucleus, although it may still be unstable.

Beta-minus decay

A radioactive isotope may have an unstable nucleus because it has an excess of neutrons. In beta-minus decay, a neutron in the nucleus changes into a proton and an electron. The electron is emitted from the nucleus as a beta particle.

The new proton remains in the nucleus. As a result:

• The proton number increases by 1.
• The neutron number decreases by 1.
• The nucleon number stays the same: one nucleon has changed from a neutron into a proton, but the total number of nucleons has not changed.

The proton number identifies the element, so the nucleus becomes that of a different element. The change reduces the neutron excess and makes the nucleus more stable, although it may still be radioactive.

Nuclear decay equations

A nuclear decay equation shows an original nucleus changing into a resulting nucleus and emitting radiation. The original nucleus appears on the left; the resulting nucleus and radiation appear on the right.

In nuclide notation, $$^{A}_{Z}X$$, the symbol $$X$$ identifies the element. The upper number, $$A$$, is the nucleon number: the total number of protons and neutrons. The lower number, $$Z$$, is the proton number.

To balance a nuclear decay equation, the total upper numbers must be equal on both sides, and the total lower numbers must also be equal. The proton number of the resulting nucleus determines its element symbol.

Alpha emission

An alpha particle is written $$^{4}_{2}\alpha$$. When a nucleus emits one, its nucleon number decreases by 4 and its proton number decreases by 2. For example:

The upper numbers balance because $$238 = 234 + 4$$, and the lower numbers balance because $$92 = 90 + 2$$. The resulting nucleus has proton number 90, so its element is thorium, $$\mathrm{Th}$$.

Beta-minus emission

A beta-minus particle is written $$^{0}_{-1}\beta$$. During beta-minus decay, a neutron in the nucleus changes into a proton. The resulting nucleus therefore has the same nucleon number but a proton number greater by 1. For example:

The upper numbers balance because $$14 = 14 + 0$$. The lower numbers balance because $$6 = 7 + (-1)$$. The resulting nucleus has proton number 7, so its element is nitrogen, $$\mathrm{N}$$.

Gamma emission

An excited nucleus has excess energy. It can release this energy as gamma radiation and become more stable. An asterisk marks the excited nucleus:

The gamma radiation has upper and lower numbers of zero. The nucleus therefore keeps the same nucleon number, proton number and element symbol.