Radioactive Decay
Unstable nuclei release energy by transforming into more stable configurations through radioactive decay. This process is spontaneous and random at the level of individual atoms -- you cannot predict when a specific nucleus will decay, only the statistical probability of decay over time. There are several distinct decay modes, each involving different particles and governed by different nuclear forces.
Alpha Decay
In alpha decay, a nucleus emits an alpha particle consisting of 2 protons and 2 neutrons -- essentially a helium-4 nucleus. The parent nucleus loses 4 units of mass number and 2 units of atomic number:
X(Z, A) -> Y(Z-2, A-4) + He(2, 4)
Alpha decay is common in heavy nuclei (Z > 82) where the strong force can no longer fully overcome proton-proton repulsion. For example, uranium-238 decays to thorium-234 by emitting an alpha particle. The emitted alpha particles are monoenergetic (typically 4-9 MeV) and highly ionizing but have very limited penetrating power -- a sheet of paper or a few centimeters of air will stop them.
The mechanism involves quantum tunneling: the alpha particle, preformed within the nucleus, has insufficient energy to classically escape the nuclear potential barrier, but quantum mechanics allows it a small probability of tunneling through. The relationship between decay energy and half-life is described by the Geiger-Nuttall law -- nuclei with higher decay energies have shorter half-lives.
Beta Decay
Beta decay comes in two varieties, both mediated by the weak nuclear force:
Beta-minus decay occurs in neutron-rich nuclei. A neutron converts into a proton, emitting an electron (beta particle) and an antineutrino:
n -> p + e- + antineutrino
The parent nucleus gains one proton and loses one neutron, so Z increases by 1 while A stays the same. Carbon-14 decaying to nitrogen-14 is a classic example, fundamental to radiocarbon dating.
Beta-plus decay (positron emission) occurs in proton-rich nuclei. A proton converts into a neutron, emitting a positron and a neutrino:
p -> n + e+ + neutrino
Here Z decreases by 1 while A remains unchanged. Fluorine-18 decaying to oxygen-18 is widely used in PET medical imaging. Unlike alpha particles, beta particles are emitted with a continuous spectrum of energies (from zero up to a maximum value), because the decay energy is shared between the beta particle and the neutrino.
Electron Capture
An alternative to positron emission, electron capture occurs when the nucleus absorbs an inner-shell orbital electron, converting a proton to a neutron:
p + e- -> n + neutrino
The result is the same nuclear transformation as positron emission (Z decreases by 1), but no positron is emitted. Instead, the atom emits characteristic X-rays as outer electrons fill the inner-shell vacancy. Electron capture is favored over positron emission when the energy difference between parent and daughter is less than 1.022 MeV (the energy needed to create an electron-positron pair).
Gamma Decay
After alpha or beta decay, the daughter nucleus is often left in an excited state. It releases this excess energy by emitting one or more gamma rays -- high-energy photons with energies typically ranging from 10 keV to several MeV. Gamma emission changes neither Z nor A; it is a purely electromagnetic process analogous to an excited atom emitting visible light, but at far higher energies.
Gamma rays are the most penetrating form of nuclear radiation, requiring dense shielding (lead, concrete, or water) to attenuate. They are also the most useful for medical imaging and industrial applications because they can pass through the body or materials and be detected externally.
Internal Conversion and Isomeric Transitions
In some cases, instead of emitting a gamma ray, the nucleus transfers its excitation energy directly to an inner-shell electron, which is ejected from the atom. This process is called internal conversion. Some excited nuclear states are metastable, meaning they have unusually long half-lives (milliseconds to years) before decaying by gamma emission. These are called nuclear isomers, and the decay process is an isomeric transition. Technetium-99m, the most widely used medical isotope, is a nuclear isomer with a 6-hour half-life.
Decay Chains
Many radioactive nuclei do not reach stability in a single decay step. Instead, they undergo a series of successive decays called a decay chain or decay series. The four natural decay series (thorium, neptunium, uranium, and actinium series) begin with long-lived parent isotopes and proceed through multiple alpha and beta decays until reaching a stable lead or bismuth isotope. Uranium-238, for instance, undergoes 14 decay steps before arriving at stable lead-206.