Alpha decay is the emission of an alpha particle (two protons and two neutrons, equivalent to a helium nucleus) from an unstable atomic nucleus. This process decreases the atomic number by 2 and the mass number by 4, forming a new element.

• It decreases the mass number of the atom by 4 and the atomic number by 2.
• The emitted alpha particle has a charge of +2 and a mass of 4 units.
• It has low penetrating power (can be stopped by a sheet of paper or human skin).
• It has high ionizing power compared to beta and gamma radiation.
• It causes significant changes in the nucleus, leading to the formation of a new element.

$$^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$$ $$^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$$ $$^{210}_{84}\text{Po} \rightarrow ^{206}_{82}\text{Pb} + ^{4}_{2}\text{He}$$

Beta decay is the emission of a beta particle (an electron or positron) from the nucleus of a radioactive atom. In beta-minus (β⁻) decay, a neutron changes into a proton and an electron is emitted. In beta-plus (β⁺) decay, a proton changes into a neutron and a positron is emitted.

• In β⁻ decay, the atomic number increases by 1 while the mass number remains unchanged.
• In β⁺ decay, the atomic number decreases by 1 while the mass number remains unchanged.
• Beta particles are fast-moving and negatively or positively charged.
• They have medium penetrating power (can be stopped by aluminum sheet).
• They have moderate ionizing power compared to alpha and gamma radiation.

$$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$$ $$^{32}_{15}\text{P} \rightarrow ^{32}_{16}\text{S} + ^{0}_{-1}\text{e}$$ $$^{210}_{83}\text{Bi} \rightarrow ^{210}_{84}\text{Po} + ^{0}_{-1}\text{e}$$

Gamma decay is the emission of high-energy electromagnetic radiation (gamma rays) from the nucleus of an excited atom. It usually follows alpha or beta decay when the daughter nucleus is left in an excited state and releases energy to become stable. There is no change in mass number or atomic number during gamma emission.

• There is no change in atomic number or mass number during gamma emission.
• Gamma rays are neutral electromagnetic waves (no charge or mass).
• They have very high penetrating power (can pass through thick lead or concrete).
• They have low ionizing power compared to alpha and beta radiations.
• Gamma emission usually accompanies alpha or beta decay to release excess nuclear energy.

$$^{60}_{27}\text{Co}^{*} \rightarrow ^{60}_{27}\text{Co} + \gamma$$ $$^{137}_{56}\text{Ba}^{*} \rightarrow ^{137}_{56}\text{Ba} + \gamma$$ $$^{234}_{90}\text{Th}^{*} \rightarrow ^{234}_{90}\text{Th} + \gamma$$

Nuclear stability refers to the ability of an atomic nucleus to remain unchanged or resist disintegration over time. A stable nucleus does not spontaneously emit radiation, while an unstable nucleus tends to undergo radioactive decay to achieve a more stable state.

The stability of a nucleus depends mainly on the ratio of neutrons to protons (N/Z ratio). For lighter elements, stability is achieved when the number of neutrons and protons are nearly equal (N ≈ Z). However, for heavier elements, a higher number of neutrons is needed to offset the repulsive forces between the many protons.

Stable nuclei lie within a region called the band or belt of stability on a graph of neutrons versus protons. Nuclei outside this band are unstable and tend to undergo radioactive decay (such as alpha, beta, or gamma decay) to move into the stable region.

Factors affecting nuclear stability

• The neutron-to-proton (N/Z) ratio.
• The total binding energy per nucleon — higher binding energy indicates greater stability.
• The presence of magic numbers (2, 8, 20, 28, 50, 82, 126) of protons or neutrons, which correspond to extra stable configurations.
• The balance between nuclear attraction and electrostatic repulsion among protons.

Electron capture is the process by which an unstable nucleus absorbs one of its own inner orbital electrons (usually from the K-shell), causing a proton in the nucleus to convert into a neutron. This process helps the nucleus move toward greater stability by reducing the proton-to-neutron ratio.

During electron capture, the captured electron combines with a proton to form a neutron and a neutrino. The neutrino is then ejected from the nucleus with very high speed but almost no mass. This process decreases the atomic number by one while the mass number remains unchanged.

The general nuclear equation for electron capture is:

In symbolic form, for an element X:

Examples:

• Occurs in proton-rich nuclei that cannot stabilize by positron emission.
• An inner orbital electron (usually K-shell) is drawn into the nucleus.
• A proton is converted into a neutron, reducing the atomic number by one.
• The mass number remains the same because the total number of nucleons is unchanged.
• Emission of a neutrino (ν_e) occurs, carrying away excess energy.
• Often accompanied by emission of X-rays as the vacant electron shell is refilled.
• Electron capture competes with positron emission in certain isotopes.