AP Chemistry Justifications

Which is larger: Mg or Cl?

Mg is larger. Mg and Cl are in the same period and both have 3 energy levels. However, Cl has a greater effective nuclear charge (ENC), leading to a stronger Coulombic attraction between the nucleus and its electrons. This pulls electrons closer, making Cl smaller than Mg.

Order by size: Ca²⁺, K⁺, Ar, Cl⁻ (smallest?)

Ca²⁺ is the smallest. All have 18 electrons and 3 energy levels, but Ca²⁺ has the highest ENC. The greater Coulombic attraction pulls electrons closer, decreasing ionic size.

Na vs K — larger electronegativity?

Na has the higher electronegativity. Na has 3 energy levels, K has 4. The extra energy level in K increases shielding and atomic radius, reducing the nucleus–electron attraction in bonds, and lowering K’s electronegativity.

Na vs K — larger ionization energy?

Na has the higher ionization energy. K’s additional energy level increases shielding and distance from the nucleus, reducing Coulombic attraction to valence electrons and making them easier to remove.

Na vs K — larger electron affinity (more negative value)?

Na has the higher (more exothermic) electron affinity. K’s extra energy level and shielding weaken the attraction for an added electron, so Na releases more energy when gaining one.

Paramagnetic

Element or ion with unpaired electrons; attracted to magnetic fields.

Diamagnetic

Element or ion with no unpaired electrons; slightly repelled by magnetic fields.

Effective Nuclear Charge (ENC)

Approximate formula: ENC = # of protons − # of inner (core) electrons. Represents the net positive charge experienced by valence electrons.

Cations

Smaller than their parent atom due to loss of electrons (and sometimes entire energy levels), increasing ENC per electron.

Anions

Larger than their parent atom due to gain of electrons and increased electron–electron repulsion.