Know that a β-particle is a high-speed electron emitted from the nucleus, formed when a neutron changes into a proton and an electron and a reduction in the number of excess neutrons; the following change in the nucleus occurs during β-emission neutr

5.2.3 Radioactive Decay – Beta (β) Emission

What is a β‑particle?

A β‑particle is a high‑speed electron that is emitted from the nucleus of an unstable atom. It is produced when a neutron in the nucleus is transformed into a proton and an electron (plus an antineutrino). The electron escapes the nucleus and is observed as the β‑particle.

Fundamental Nuclear Change

The basic nuclear reaction for β‑decay can be written as:

\$n \;\rightarrow\; p^{+} + e^{-} + \bar{\nu}_{e}\$

where:

• \$n\$ – neutron (neutral particle)
• \$p^{+}\$ – proton (positively charged particle)
• \$e^{-}\$ – electron (the β‑particle)
• \$\bar{\nu}_{e}\$ – antineutrino (very low‑mass, neutral particle that carries away excess energy)

Effect on the Nucleus

Because a neutron (no charge) becomes a proton (positive charge) and an electron leaves the nucleus, the atomic number \$Z\$ of the element increases by one while the mass number \$A\$ remains unchanged.

The general nuclear equation for β‑emission is:

\$^{A}{Z}\!X \;\rightarrow\; ^{A}{Z+1}\!Y \;+\; e^{-} \;+\; \bar{\nu}_{e}\$

where \$^{A}{Z}\!X\$ is the parent nuclide and \$^{A}{Z+1}\!Y\$ is the daughter nuclide.

Key Points to Remember

1. The emitted particle is an electron, not a proton.
2. The neutron count in the nucleus decreases by one.
3. The proton count (atomic number) increases by one, changing the element.
4. The mass number \$A\$ stays the same because a neutron is simply converted into a proton.
5. β‑particles have a relatively low mass and a charge of \$-1e\$, giving them a higher penetrating ability than α‑particles but lower than γ‑rays.

Comparison with α‑Decay

Example: Decay of Carbon‑14

Carbon‑14 (\$^{14}_{6}\!C\$) undergoes β‑decay to become nitrogen‑14:

\$^{14}{6}\!C \;\rightarrow\; ^{14}{7}\!N \;+\; e^{-} \;+\; \bar{\nu}_{e}\$

Notice that the mass number stays at 14, while the atomic number increases from 6 (carbon) to 7 (nitrogen).

Common Misconceptions

• β‑particle = proton: The β‑particle is an electron, not a proton.
• Mass number changes: In β‑decay the mass number remains the same because a neutron is simply turned into a proton.
• Only electrons are emitted: An antineutrino is also emitted, but it is extremely difficult to detect and carries away some of the decay energy.

Summary

β‑decay is a process that reduces the number of excess neutrons in an unstable nucleus. A neutron transforms into a proton, emitting a high‑speed electron (β‑particle) and an antineutrino. The atomic number increases by one, the mass number stays constant, and the element changes to the next higher \$Z\$ in the periodic table.