For a neutral atom, electrons = atomic number
For a positive ion, electrons = atomic number – charge
For a negative ion, electrons = atomic number + charge
For a molecule, add the valence electrons of all atoms
For a polyatomic ion, add the valence electrons of all atoms and adjust for charge
For a chemical reaction, total electrons lost = total electrons gained
For an element in a specific shell, use the shell capacity formula 2n²
For a subshell, use its maximum electron capacity: s = 2, p = 6, d = 10, f = 14