2 Variation across a period (left to right)

The atomic radius decreases from group 1A to group 7A. Reason: the effective nuclear charge (Z_eff) increases gradually.

• Z_eff is the actual nuclear charge felt by an electron in the atom. It is always less than the real nuclear charge (number of protons) because the inner (core) electrons shield part of the nuclear attraction from the outer electrons.
• Across a period the number of protons increases by one at each step while the electrons are added to the same main level, so the shielding by inner electrons does not change much. Z_eff rises, the valence electrons are pulled closer, and the atom contracts.
• Hence the largest atom in a period is the alkali metal (1A) and the smallest is the halogen (7A).

Example: period 3: Na (186) > Mg (160) > Al (143) > Si (117) > P (110) > S (104) > Cl (99) pm.