Which one of the following arrangements represents the correct order of least negative to most negative electron gain enthalpy for C, Ca, Al, F and O?
- Al < Ca < O < C < F
- Al < O < C < Ca < F
- C < F < O < Al < Ca
- Ca < Al < C < O < F
Answer
(4) Ca < Al < C < O < F
Solution
Ca has a filled 4s subshell, so the added electron enters a higher-energy 4p orbital ⇒ its electron gain enthalpy is nearly zero (least negative).
Al ( -44 kJ mol-1) < C (-122) < O (-141) < F (-328 kJ mol-1)
∵ Electron gain enthalpy becomes more negative across a period as the effective nuclear charge increases.
∴ Order: Ca < Al < C < O < F
Answer: (4)