For a neutral atom, the number of electrons equals the atomic number
For a positive ion, subtract the charge from the atomic number
For a negative ion, add the charge to the atomic number
Use the element’s atomic number from the periodic table
If the mass number is given, use it only to find neutrons, not electrons
For a neutral molecule, add the electrons from all atoms
For a polyatomic ion, adjust the total electrons by the ion’s charge
For a cation, electrons = atomic number − positive charge
For an anion, electrons = atomic number + negative charge
For atoms with no charge shown, assume the atom is neutral