A nuclear equation shows an unstable nucleus decaying. Each nucleus has a mass number (protons plus neutrons) and an atomic number (the number of protons, which gives its charge). In a balanced equation the total mass number is the same on both sides, and the total atomic number is the same on both sides.
In alpha decay the nucleus emits an alpha particle, which is two protons and two neutrons (the same as a helium nucleus, with mass number 4 and atomic number 2). The nucleus loses 4 from its mass number and 2 from its atomic number. For example, uranium-238 (atomic number 92) decays to thorium-234 (atomic number 90) plus an alpha particle. Check the totals: 238 = 234 + 4 and 92 = 90 + 2.
In beta decay a neutron in the nucleus changes into a proton and a fast electron is emitted, called a beta particle. The beta particle has mass number 0 and charge -1. The mass number of the nucleus stays the same but its atomic number goes up by 1. For example, carbon-14 (atomic number 6) decays to nitrogen-14 (atomic number 7) plus a beta particle. Check the totals: 14 = 14 + 0 and 6 = 7 + (-1).
Gamma rays have no mass and no charge, so emitting them changes neither number.