Atomic Radius, IE, EA, Zeff.

Atomic radius

Half the distance between nuclei of two identical bonded atoms; increases down a group, decreases across a period

Ionization energy (IE)

Energy needed to remove an electron from a gas-phase atom; decreases down a group, increases across a period

Electron affinity (EA)

Energy change when a gas-phase atom gains an electron; most negative for halogens (group 7A), ~zero for noble gases and alkaline earths

Effective nuclear charge (Z_eff)

Z_eff = Z - S; the net nuclear charge felt by valence electrons after subtracting core (shielding) electrons

Shielding

Core electrons block valence electrons from feeling the full nuclear charge; more inner shells = more shielding = lower Z_eff

Why does atomic radius decrease across a period?

Z_eff increases (more protons, same shell), pulling electrons closer to nucleus

Why does atomic radius increase down a group?

New electron shells are added, putting valence electrons farther from nucleus despite higher Z

Why does IE decrease down a group?

Valence electrons are farther from nucleus and more shielded, so less energy needed to remove them

Big jump in successive IEs means…

The next electron is being removed from an inner shell; the number of electrons before the jump = number of valence electrons = group number

Isoelectronic series

Ions with the same number of electrons; ranked by atomic number — more protons = smaller radius

Cation vs neutral atom size

Cations are SMALLER than the neutral atom (lost electrons, Z_eff increases on remaining electrons)

Anion vs neutral atom size

Anions are LARGER than the neutral atom (gained electrons, increased repulsion expands electron cloud)