Composition of the atom and nuclear reactions

The nucleus is made of protons (charge +1) and neutrons (uncharged) — nucleons; the number of protons Z equals the atomic number, and the mass number is A = Z + N (N is the number of neutrons). Atoms with the same nuclear charge but different mass numbers are isotopes (for example protium, deuterium and tritium of hydrogen); atoms with equal mass numbers but different charges are isobars, atoms with equal numbers of neutrons are isotones, and particles with equal numbers of electrons are isoelectronic. The relative atomic mass of an element is the average of the masses of its natural isotopes weighted by their abundances: Aᵣ = w₁A₁ + w₂A₂ + … Unstable nuclei decay spontaneously: in α-decay (⁴He nuclei) the atomic number falls by 2 and the mass number by 4; in β⁻-decay a neutron turns into a proton, so the atomic number rises by 1 and the mass number is unchanged; γ-rays carry away energy without changing the mass number or charge. In every nuclear reaction the sum of the mass numbers and the sum of the charges are equal on the two sides. The fission of heavy nuclei by neutrons (for example uranium-235) releases a large amount of energy and is the basis of nuclear power; radioactive materials are handled only by specialists with proper shielding.