In a nuclear equation, each particle is written with its mass number at the top and its atomic number at the bottom. The mass number counts protons and neutrons. The atomic number gives the charge. An alpha particle is written 42He and a beta minus particle (an electron) is written 0-1e.
The equation is balanced when the total mass number on the left equals the total on the right, and the total atomic number on the left equals the total on the right. To find a missing particle, add up the numbers on each side and find the missing value by subtraction.
Example: radium-226 (atomic number 88) decays by emitting an alpha particle. The new nucleus has mass number 226 - 4 = 222 and atomic number 88 - 2 = 86, so it is radon-222. Example: carbon-14 (atomic number 6) emits a beta minus particle. The mass number stays 14. The atomic number is 6 - (-1) = 7, so the new nucleus is nitrogen-14.