atomic size

Meaning: Atomic size is the radius of the electron cloud, usually estimated as half the internuclear distance in a bond between identical atoms. It depends on the number of occupied shells and on how strongly the nucleus attracts the outer electrons. Down a group, outer electrons sit in higher shells, so size increases. Across a period, protons are added while electrons enter the same shell and shield each other poorly, so the pull on the outer electrons grows and the atom shrinks. Cations are smaller than their parent atoms, and anions are larger.

Equation: Zeff = Z − S, where Zeff is effective nuclear charge, Z is the atomic number (number of protons), and S is the shielding by inner electrons. Z and S are unitless counts, and Zeff is expressed in multiples of the elementary charge e (1.60 × 10⁻¹⁹ C).

On the MCAT: You may rank atoms or ions by size from their periodic table positions, or explain a trend in ionization energy or electronegativity through size. Isoelectronic series (O²⁻, F⁻, Na⁺, Mg²⁺) are common: the same electron count with more protons gives a smaller radius.

Common confusion: Students assume more electrons means a bigger atom. Across a period, size shrinks because the added electrons go into the same shell while Zeff rises. Across the transition metals, size changes only slightly.