Questions on ionisation energies and the evidence for subshells

Question 1

Explain the pattern in the first ionisation energies of the Period 3 elements given in the table.

Accepted Answer

Indicative content

the general rise from sodium to argon follows the nuclear charge, which increases by one proton from each element to the next
the electron removed is in the third shell each time, at a similar distance and shielded by the same inner electrons
the outer electrons are therefore attracted more strongly, the atomic radius falls, and more energy is needed to remove one
aluminium is lower than magnesium because its electron is removed from 3p, a subshell higher in energy than 3s
the 3p electron is on average further from the nucleus and partly shielded by the 3s electrons, outweighing the extra proton
sulfur is lower than phosphorus because two of its 3p electrons are paired in one orbital and repel, so one is removed more easily

Levels

Level 3 (5–6 marks) the general rise is explained through nuclear charge with similar shielding and the same shell, and both exceptions are explained correctly, aluminium through the higher-energy 3p subshell and sulfur through the paired electrons repelling; all three parts of the pattern are addressed with the table's values used
Level 2 (3–4 marks) the general rise is explained along with one of the two dips, or both dips are named and one explanation stops short
Level 1 (1–2 marks) the rise across the period is put down to nuclear charge; the dips are ignored, or the sulfur dip is put down to more shielding rather than to the repulsion between paired 3p electrons

Question 2

State what is meant by the first ionisation energy of an element, and give the equations, with state symbols, for the first and second ionisations of calcium.

Accepted Answer

B1 the energy required to remove one electron from each atom in one mole of atoms B1 the atoms are gaseous, and one mole of gaseous 1+ ions is formed B1 Ca(g) → Ca⁺(g) + e⁻ B1 Ca⁺(g) → Ca²⁺(g) + e⁻; reject Ca(g) → Ca²⁺(g) + 2e⁻

Question 3

Deduce the group of an element from its first five ionisation energies, given in the table, and identify the element if it is in Period 2.

Accepted Answer

B1 the largest jump is between the fourth and fifth values: the fifth is about six times the fourth B1 four electrons are removed before the jump, so there are four in the outer shell: Group 4 B1 the fifth electron comes from the first shell, closer to the nucleus and unshielded B1 the Group 4 element in Period 2 is carbon

Question 4

Compare the first ionisation energies of calcium, 590 kJ mol⁻¹, and gallium, 579 kJ mol⁻¹, and explain the difference.

Accepted Answer

B1 gallium's value is lower than calcium's, although gallium has eleven more protons B1 calcium's electron is removed from 4s, gallium's from 4p B1 4p is higher in energy than 4s, so its electron is on average further out and partly shielded by the 4s and 3d electrons B1 the extra protons are almost cancelled by the ten 3d electrons added between them, so less energy removes the 4p electron

Question 5

Describe the pattern in the graph of log₁₀(successive ionisation energy) against the number of the electron removed for sodium, and state what the pattern shows.

Accepted Answer

B1 the first value is low, and there is a large jump to the second B1 the next eight values rise gradually, followed by a second large jump B1 the last two values are the highest B1 each large jump marks removal from a shell closer to the nucleus, so sodium has 1 electron in the third shell, 8 in the second and 2 in the first

Question 6

Evaluate the claim that the first ionisation energies of the Period 3 elements are evidence that electrons are arranged in subshells.

Accepted Answer

B1 if the third shell were a single level, the values would rise smoothly with nuclear charge from sodium to argon B1 aluminium is lower than magnesium because its electron is removed from 3p, a subshell of higher energy than 3s B1 sulfur is lower than phosphorus because of pairing in a 3p orbital, consistent with orbitals each holding two electrons B1 the same dips appear in Period 2 at boron and oxygen, so the data support the claim

Question 7

Suggest why scandium forms Sc³⁺ ions in its compounds but no Sc⁴⁺ ions, using its first four ionisation energies given in the table.

Accepted Answer

B1 the largest jump is between the third and fourth values, the fourth about three times the third B1 the first three electrons are the 4s² and 3d¹ electrons; the fourth would come from 3p, part of the argon core B1 a 3p electron is much lower in energy and closer to the nucleus, and it would be removed from a 3+ ion B1 the energy released when Sc³⁺ forms bonds or a lattice repays the first three, but cannot repay the much larger fourth, so Sc⁴⁺ does not form

Question 8

Justify the conclusion that the element whose first four ionisation energies are given in the table is in Group 3, and explain why the same pattern would be seen for every element in that group.

Accepted Answer

B1 the first three values rise by 1239 and 928 kJ mol⁻¹, steps of similar size, so the first three electrons come from the same shell B1 the fourth jumps by 8832 kJ mol⁻¹, more than four times the third, a jump that shows the fourth electron is removed from an inner shell, closer to the nucleus and less shielded B1 three electrons in the outer shell place the element in Group 3; allow aluminium or boron as the example B1 every element in the group has three outer electrons and a full shell beneath them, so each shows a large jump between its third and fourth ionisation energies, whatever the period

Question 9

Explain why the first ionisation energy decreases down Group 2.

Accepted Answer

B1 the outer electron is in a shell further from the nucleus, so the atomic radius is larger B1 there are more inner shells of electrons, so there is more shielding B1 these outweigh the increase in nuclear charge, so the outer electron is attracted less strongly and less energy removes it

Question 10

Explain why the first ionisation energy of aluminium is lower than that of magnesium.

Accepted Answer

B1 the electron removed from aluminium is in a 3p orbital, while magnesium's is in 3s B1 3p is higher in energy than 3s, so its electron is further from the nucleus on average and partly shielded by the 3s electrons B1 less energy is needed to remove it, so the first ionisation energy falls despite the greater nuclear charge

Question 11

Explain why the first ionisation energy of sulfur is lower than that of phosphorus.

Accepted Answer

B1 phosphorus, 3p³, has one electron in each 3p orbital B1 sulfur, 3p⁴, has two electrons paired in one 3p orbital B1 the paired electrons repel each other, so one of them is easier to remove, which outweighs sulfur's extra proton; reject a stable half-filled subshell argument

Question 12

Explain why the second ionisation energy of sodium is much larger than its first.

Accepted Answer

B1 the first electron is removed from 3s, and the second from 2p B1 the 2p electron is in a shell closer to the nucleus and is shielded only by the 1s electrons B1 it is also removed from a positive ion, so it is attracted far more strongly and much more energy is needed

Question 13

Calculate the energy, in kJ, needed to ionise all the atoms in 1.00 g of gaseous sodium. The first ionisation energy of sodium is 496 kJ mol⁻¹, and Ar of sodium is 23.0.

Accepted Answer

M1 amount of sodium = 1.00 ÷ 23.0 = 0.0435 mol M1 energy = 0.0435 mol × 496 kJ mol⁻¹ A1 21.6 kJ, to three significant figures

Question 14

Calculate the energy needed to convert one mole of gaseous calcium atoms into one mole of gaseous Ca²⁺ ions. The first and second ionisation energies of calcium are 590 and 1145 kJ mol⁻¹.

Accepted Answer

M1 the change Ca(g) → Ca²⁺(g) + 2e⁻ is the first ionisation followed by the second M1 energy = 590 + 1145 A1 +1735 kJ mol⁻¹, to four significant figures

Question 15

Calculate the energy, in joules, needed to remove one electron from a single gaseous magnesium atom. The first ionisation energy of magnesium is 738 kJ mol⁻¹, and L = 6.022 × 10²³ mol⁻¹.

Accepted Answer

M1 convert to joules: 738 kJ mol⁻¹ = 7.38 × 10⁵ J mol⁻¹ M1 energy per atom = 7.38 × 10⁵ ÷ 6.022 × 10²³ A1 1.23 × 10⁻¹⁸ J, to three significant figures

Question 16

Suggest why the second ionisation energy of sodium, 4562 kJ mol⁻¹, is more than twice the first ionisation energy of neon, 2081 kJ mol⁻¹, although both remove a 2p electron from the arrangement 1s² 2s² 2p⁶.

Accepted Answer

B1 Na⁺ and Ne are isoelectronic, with the same arrangement of ten electrons B1 Na⁺ has 11 protons against neon's 10, so the same electrons are attracted more strongly B1 the electron is removed from an ion that is already positive, so more energy is needed

Question 17

Calculate the energy needed to form one mole of gaseous Sc³⁺ ions from one mole of gaseous scandium atoms. The first three ionisation energies of scandium are 633, 1235 and 2389 kJ mol⁻¹.

Accepted Answer

M1 Sc(g) → Sc³⁺(g) + 3e⁻ is the first three ionisations in turn, so their energies add M1 energy = 633 + 1235 + 2389 A1 +4257 kJ mol⁻¹, to four significant figures

Question 18

Explain why the fifth ionisation energy of silicon, 16091 kJ mol⁻¹, is more than three times its fourth, 4356 kJ mol⁻¹.

Accepted Answer

B1 the fourth electron is removed from 3s, and the fifth from 2p B1 the 2p electron is in a shell closer to the nucleus, shielded only by the 1s electrons B1 it is also removed from a 4+ ion, so it is attracted far more strongly and much more energy is needed

Question 19

Explain why the successive ionisation energies of an element always increase.

Accepted Answer

B1 each electron is removed from an ion that is more positive than the one before B1 the same nuclear charge holds fewer electrons, which repel each other less and are pulled closer, so more energy is needed

Question 20

Suggest, in terms of ionisation energy, why potassium reacts with water more vigorously than sodium does.

Accepted Answer

B1 potassium has a lower first ionisation energy, 419 against 496 kJ mol⁻¹, because its outer electron is in 4s, further out and more shielded B1 the reaction begins with the loss of the outer electron, which therefore happens more readily for potassium

Ionisation energy	Value / kJ mol⁻¹
First	578
Second	1817
Third	2745
Fourth	11577