On the same topic about ionisation energy, i have this question
that i need some clarifications.
Element Z is the successive element from potassium across
the period. Compare and contrast the first ionisation energy of
potassium and element Z (calcium). Support your answers with
reasons.
At first, my answer was:
K - 1s2 2s2 2p6 3s2 3p6 4s1; Z (Calcium) - 1s2 2s2 2p6
3s2 3p6 4s2
Z < K as valence electron to be removed from Z is
paired while that from K is unpaired, thus less energy is required
due to inter-electronic repulsion.
However, after checking the Data Booklet, I realised that the
first ionisation energy of calcium is higher than potassium, so I
reconstructed my answer. My question is why, unlike sulfur and
phosphorus, there is no inter-electronic repulsion that causes the
first ionisation energy of calcium to be lower?
Chemistry is a microcosm of Real Life. When there are opposing
factors, in some cases the 1st factor wins, in some cases the 2nd
factor wins.
Considering K vs Ca, the factor of increased nuclear charge
outweighs the factor of inter-electron repulsion, consequently
ionization enthalpy for Ca is more endothermic than for K.
Considering P vs S, the factor of inter-electron repulsion
outweighs the factor of increased nuclear charge, consequently the
ionization enthalpy for S is less endothermic than for P.
In Chemistry, including for A levels, you can only show your
understanding of both (or all) opposing factors, then carry out
experiments to determine which factors outweigh which. To discuss
this deeper at higher levels (beyond A levels), yes it's possible
to show with more involved calculations why this factor outweighs
that factor at this point in the periodic table, but these would
not be relevant or useful for A level purposes, and would only
confuse the A level student and is therefore counterproductive.
