Questions on electron configuration: shells, subshells and orbitals

Question 1

Explain, in terms of subshells and their energies, why the third shell holds only eight electrons from argon to calcium although it can hold eighteen, and why the GCSE pattern of 2, 8, 8 fails from scandium onwards.

Accepted Answer

Indicative content

each shell divides into subshells: the third shell has 3s, 3p and 3d, holding 2, 6 and 10 electrons, 18 in all
from sodium to argon only the 3s and 3p subshells fill, so argon is 2, 8, 8 with its 3d subshell still empty
in potassium and calcium the 4s subshell is lower in energy than 3d, so the nineteenth and twentieth electrons go into 4s
a 4s electron spends part of its time close to the nucleus, inside the inner shells, where it is barely shielded, lowering its energy
from scandium the 3d subshell fills, so scandium is [Ar] 3d¹ 4s², with nine electrons in its third shell, not eight
by zinc the third shell holds eighteen electrons, 2n² for n = 3, so 2, 8, 8 fits only the first twenty elements

Levels

Level 3 (5–6 marks) the subshells of the third shell and their capacities, the filling of 4s before 3d in potassium and calcium with the reason in terms of energy and penetration, and the filling of 3d from scandium with the electron configuration are all present
Level 2 (3–4 marks) 4s is said to fill before 3d, so potassium and calcium add electrons to 4s and 3d fills from scandium; the capacities of the subshells, why 4s is lower in energy, or a configuration such as scandium's does not appear
Level 1 (1–2 marks) the third shell is said to be full at eight, or 3d to fill before 4s in potassium, with no energy order and no configurations

Question 2

State the number of orbitals in an s, a p and a d subshell, and describe the shapes of an s orbital and of a p orbital.

Accepted Answer

B1 an s subshell has one orbital and a p subshell three B1 a d subshell has five orbitals, holding up to ten electrons B1 an s orbital is spherical, centred on the nucleus B1 a p orbital is a dumbbell, two lobes on opposite sides of the nucleus, and the three lie at right angles

Question 3

Deduce the number of unpaired electrons in each of Ti²⁺, Cr³⁺, Ni²⁺ and Zn²⁺. Their atomic numbers are 22, 24, 28 and 30.

Accepted Answer

B1 Ti²⁺ is [Ar] 3d², with 2 unpaired electrons B1 Cr³⁺ is [Ar] 3d³, with 3 unpaired electrons B1 Ni²⁺ is [Ar] 3d⁸: five singles then three pairs, leaving 2 unpaired electrons B1 Zn²⁺ is [Ar] 3d¹⁰, with every orbital full and 0 unpaired electrons

Question 4

Give the full and the shortened electron configurations of a sulfur atom and of a sulfide ion, S²⁻. Sulfur is element 16.

Accepted Answer

B1 S is 1s² 2s² 2p⁶ 3s² 3p⁴ B1 the shortened form is [Ne] 3s² 3p⁴ B1 S²⁻ is 1s² 2s² 2p⁶ 3s² 3p⁶, with two electrons added to 3p B1 the shortened form of S²⁻ is [Ne] 3s² 3p⁶; allow [Ar]

Question 5

Explain, using the Pauli principle and Hund's rule, how the 2p electrons are arranged in a carbon atom and in an oxygen atom.

Accepted Answer

B1 Pauli: an orbital holds at most two electrons, and two in the same orbital have opposite spins B1 Hund: electrons occupy the orbitals of a subshell singly, with parallel spins, before any pair, because they repel less in separate orbitals B1 carbon, 2p², has one electron in each of two 2p orbitals, with parallel spins, and the third orbital empty B1 oxygen, 2p⁴, has one 2p orbital holding a pair of opposite spin and the other two singly occupied

Question 6

Suggest why the d block of the periodic table is ten elements wide, and why potassium is placed at the start of Period 4 rather than at the end of Period 3.

Accepted Answer

B1 a d subshell has five orbitals, each holding two electrons B1 ten electrons fill it, one more with each element from scandium to zinc B1 potassium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹, because 4s is lower in energy than 3d B1 its outer electron is in the fourth shell, and the period number is the number of the highest occupied shell

Question 7

Compare the electron configurations of a Mn²⁺ ion and an Fe³⁺ ion, and the way each forms from its atom. Manganese is element 25 and iron is element 26.

Accepted Answer

B1 both ions are [Ar] 3d⁵, with 23 electrons, so they are isoelectronic B1 both have one electron in each of the five 3d orbitals, so each has 5 unpaired electrons B1 manganese, [Ar] 3d⁵ 4s², loses only its two 4s electrons, while iron, [Ar] 3d⁶ 4s², loses both 4s electrons and one 3d electron B1 Fe³⁺ has 26 protons holding the 23 electrons against 25 in Mn²⁺, so its electrons are held more strongly

Question 8

Give the full electron configurations of a chromium atom (Z = 24), a gallium atom (Z = 31) and a Ga³⁺ ion.

Accepted Answer

B1 Cr is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹; reject 3d⁴ 4s² B1 Ga is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹ B1 Ga³⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰, the 4p and 4s electrons removed

Question 9

Explain why the electron configuration of chromium is [Ar] 3d⁵ 4s¹ rather than [Ar] 3d⁴ 4s².

Accepted Answer

B1 the 4s and 3d subshells are very close in energy B1 one 4s electron occupies 3d instead, giving a half-filled 3d subshell with one electron in each of the five orbitals B1 this arrangement is more stable, lower in energy, than 3d⁴ 4s²

Question 10

Explain why the 4s subshell fills before the 3d subshell, but the 4s electrons are removed first when a transition metal forms an ion.

Accepted Answer

B1 in potassium and calcium, with 3d empty, 4s is lower in energy than 3d, so it fills first B1 once 3d holds electrons, the 4s electrons are the outermost and highest in energy B1 so the 4s electrons are removed first: iron, [Ar] 3d⁶ 4s², forms Fe²⁺ as [Ar] 3d⁶

Question 11

Identify the element whose atoms have the configuration [Ar] 3d¹⁰ 4s² 4p⁵, state the block it is in, and give the number of unpaired electrons in one atom.

Accepted Answer

B1 the atom has 18 + 10 + 2 + 5 = 35 electrons, so it is bromine B1 it is in the p block, because its highest-energy electron is in 4p B1 five 4p electrons fill two orbitals and leave one single, so there is 1 unpaired electron

Question 12

Suggest the electron configuration of a Zn²⁺ ion, and suggest why zinc does not form a 3+ ion in its compounds. Zinc is element 30.

Accepted Answer

B1 a zinc atom is [Ar] 3d¹⁰ 4s² B1 Zn²⁺ has lost the two 4s electrons and is [Ar] 3d¹⁰ B1 a third electron would have to come from the full 3d subshell, closer to the nucleus, which needs much more energy

Question 13

Evaluate a student's answer that the configuration of Fe³⁺ is [Ar] 3d³ 4s².

Accepted Answer

B1 the total number of electrons, 23, is right, since iron has 26 and three are lost B1 but the three electrons have been taken from 3d, when the 4s electrons are lost first B1 the correct configuration is [Ar] 3d⁵

Question 14

Explain how a sodium street lamp produces its yellow light, using the terms ground state and excited state.

Accepted Answer

B1 in the ground state the outer electron of a sodium atom is in 3s, the lowest-energy arrangement B1 energy from the electric discharge promotes the electron to the higher-energy 3p subshell, an excited state B1 the electron falls back to 3s and the energy difference is given out as light, of one fixed wavelength because the gap is fixed

Question 15

Name one positive ion and one negative ion that have the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶.

Accepted Answer

B1 a positive ion: the potassium ion K⁺ or the calcium ion Ca²⁺ B1 a negative ion: the chloride ion Cl⁻ or the sulfide ion S²⁻

Question 16

Identify which of the configurations 1s² 2s² 2p² and 1s² 2s¹ 2p³ is the ground state of a carbon atom, and give a reason.

Accepted Answer

B1 1s² 2s² 2p² is the ground state B1 both hold six electrons, but in 1s² 2s¹ 2p³ an electron has moved from 2s to the higher-energy 2p while 2s has room, so it is an excited state