Sodium and magnesium are adjacent in the third period
CHEMISTRY · Single correct · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
Sodium and magnesium are adjacent in the third period. Yet the second ionization energy of sodium (4560 kJ mol−1) is much larger than that of magnesium (1450 kJ mol−1).
Which statement most directly accounts for this difference?
(a) Sodium has a greater effective nuclear charge than magnesium, so its second electron is held more tightly.
(b) Because sodium atoms are smaller than magnesium atoms, their electrons are closer to the nucleus and require more energy to remove.
(c) The second electron removed from Na comes from a filled 2p subshell, whereas for Mg it comes from the partially filled 3s subshell.
(d) Magnesium attains a stable noble-gas configuration after losing its first electron, so less energy is needed to remove a second electron.