Balancing nuclear equations is slightly different than balancing chemical equations. The major difference is that in nuclear reactions we must account for protons, neutrons, and electrons, as well as write out the symbols for various chemical elements. In a nuclear equation, the products and reactants are symbolized as AZX where X is the chemical symbol for the element, A is the mass number, and Z is the atomic number. There are two main rules to remember when balancing nuclear equations: The total of the superscripts (mass numbers, A) in the reactants and products must be the same.
The total of the subscripts (atomic numbers, Z) in the reactants and products must be the same

What is the value of A in the following nuclear reaction?

237Np93 →233 Pa91 +AZX