Why does the atomic radius of elements generally decrease across a period in the periodic table?

Across a period the elements are added to the same electron shell, so the shielding from inner electrons stays roughly the same. Each new element, however, has one more proton in the nucleus, which increases the effective nuclear charge that pulls electrons toward the nucleus. Because the added electrons do not significantly increase shielding, the stronger attraction pulls the electron cloud closer, shrinking the atomic radius. For example, sodium’s 3s¹ electron is pulled less tightly than chlorine’s 3p⁵ electron, so chlorine’s radius is smaller than sodium’s. Thus the radius steadily decreases as we move right across a period.