Nuclear reactions. The displacement law
A nuclear reaction is the interaction of a nucleus with another nucleus or particle that transforms it into other nuclei. In every reaction electric charge, the number of nucleons (mass number), total energy, momentum and angular momentum are conserved; so in an equation the sums of the lower indices (Z) and of the upper indices (A) must be equal on both sides. Displacement rules: in α-decay (the nucleus emits ⁴₂He) A decreases by 4 and Z by 2, so the element moves two places to the left in the periodic table; in β⁻-decay a neutron in the nucleus turns into a proton, emitting an electron and an antineutrino: A is unchanged, Z increases by 1 (one place to the right); in β⁺-decay a positron is emitted and Z decreases by 1; in γ-radiation an excited nucleus drops to its ground state and emits a photon, so neither A nor Z changes. The energy release is Q = (Σm_initial − Σm_final)·c²: if Q > 0 the reaction is exothermic (energy is released), if Q < 0 endothermic (energy is absorbed). With masses in u, Q (MeV) = Δm·931.5. Artificial radioactivity was discovered in 1934 by I. and F. Joliot-Curie by bombarding aluminium with α-particles.
In your notebook decide whether each step in the chain ²³⁸U → ²³⁴Th → ²³⁴Pa → ²³⁴U is α or β⁻ by checking the sums of A and Z (Th Z = 90, Pa Z = 91, U Z = 92). A paper exercise.