Not all reactions between particles are possible. Besides the well known laws of conservation of momentum, angular momentum and energy (all slightly different from the laws used for macroscopic processes), there are several laws specific to reactions between particles. These are
conservation of electric charge. Net electric charge is conserved in any particle interaction, so that positive – negative charge = constant. Individual charges may disappear, but an appearing or disappearing negative charge is always balanced by an appearing or disappearing positive charge.
Conservation of baryon number. If all baryons are given a baryon number of +1 and all anti – baryons are given a baryon number of – 1 then the total number of baryons (equal to the number of baryons minus number of anti – baryons) is constant.
Conservation of lepton number. If all leptons are given a lepton number of +1 and all anti – leptons are given a baryon number of – 1 then the total number of leptons (equal to the number of leptons minus number of anti – leptons) is constant.
Some particles possess quantity called strangeness. Strangeness is conserved in all reactions involving the strong nuclear force and electromagnetic for but not the weak force.
Some particles possess a quantity called charm. Strangeness is conserved in all reactions involving the strong nuclear force and electromagnetic for but not the weak force.
Mass is not conserved. Mass may be changed into energy and vice versa, and there is no such thing as antimass (even antiparticles have positive mass) to be given a mass number of -1.