Radioactivity and Nuclear Changes

Objective: Relate protons, neutrons, and nuclide notation.

An atomic nucleus contains protons and neutrons; atomic number Z counts protons and determines element identity, while mass number A counts protons plus neutrons. Isotopes of an element share Z and differ in neutron count. Nuclide notation ¹⁴₆C indicates 6 protons and 8 neutrons; a neutral atom also has 6 electrons. Nuclear changes alter a nucleus and may change the element, unlike chemical changes that rearrange electrons and bonds without altering nuclear identity. Stability depends on nuclear structure and interactions, not merely whether a mass number is large. Mass number is an integer count and should not be confused with average periodic-table atomic mass.

Objective: Balance simple nuclear changes.

Alpha decay emits a helium-4 nucleus, decreasing parent mass number by 4 and atomic number by 2. In beta-minus decay, a neutron changes to a proton while an electron and antineutrino are emitted, so A stays constant and Z rises by 1. Gamma radiation is an energetic photon emitted as a nucleus moves to a lower-energy state; it changes neither A nor Z. Nuclear equations conserve nucleon number and electric charge when all emitted particles are counted. Radiation penetration and biological effects depend on particle, energy, shielding, route of exposure, and dose; no single simple ranking determines all risk. Decay is probabilistic for an individual nucleus.

Objective: Distinguish activity, absorbed dose, and risk.

Activity measures decay events per second and is reported in becquerels (Bq); it is not a complete measure of harm. Absorbed dose measures energy deposited per kilogram in grays (Gy), while sieverts (Sv) account for radiation weighting in specified dose quantities. Exposure effects also depend on time, tissue, distance, shielding, and whether material is external or internal. Background radiation arises from natural and human sources, but a measured count requires instrument calibration and background correction for interpretation. The concepts of irradiation and radioactive contamination differ: exposure to a source does not automatically deposit radioactive material onto a person. Evidence and context are essential to risk assessment.