Identify the species (atom, ion, molecule) and its charge/state
For a neutral atom: electrons = atomic number
For a positive ion (cation): electrons = atomic number − positive charge
For a negative ion (anion): electrons = atomic number + magnitude of negative charge
For a molecule/ionic compound: sum electrons over all atoms and add/subtract for total charge
For an element with isotope notation: electrons depend on charge only; isotope mass number does not change electron count
For a given electron configuration: total electrons = sum of electrons in all subshells (s, p, d, f)