On moving from left to right across the third period (Na to Cl), the effective nuclear charge experienced by the valence electrons:

1. Aremains constant, because the number of shells is constant
2. Bfirst increases and then decreases, reaching a maximum at Mg
3. Cincreases steadily, because the nuclear charge rises while the added electrons enter the same shell and shield poorly
4. Ddecreases steadily, because the atomic size increases