Part 1: Shells, orbitals and configurations

Build configurations through krypton and distinguish an orbital from a shell or a fixed electron orbit.

Shells contain sub-shells; sub-shells contain orbitals

An orbital is a region that can hold up to two electrons with opposite spins. It is not a circular path. Main shells n = 1, 2, 3 and 4 have maximum capacities 2, 8, 18 and 32. Their capacities do not imply each shell fills completely before the next begins.

An s sub-shell has one spherical orbital, p has three orbitals with dumbbell shapes in perpendicular orientations, and d has five orbitals. Capacities are 2, 6 and 10 electrons respectively; an f sub-shell has seven orbitals and capacity 14.

Decode every part of an electron configuration

In 3p⁴, the 3 is the principal shell, p identifies the sub-shell, and 4 counts electrons across that whole sub-shell. It does not mean four p orbitals. A p sub-shell always contains three orbitals, each capable of holding two electrons. Shell, sub-shell, orbital and electron are different levels of description.

An orbital describes a spatial distribution for an electron, not a circular track travelled by a particle. An s orbital is spherical about the nucleus; each p orbital has two lobes. The two lobes are parts of the same orbital, not two orbitals or one electron in each. The three p orbitals differ in orientation.

Fill lower energies first

For the atoms in this AS scope, use 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p, allowing up to two electrons per orbital. Orbitals of equal energy fill singly with parallel spins before pairing. Chromium and copper exceptions are outside the specified AS assessment of configurations.

Oxygen is 1s² 2s² 2p⁴. Its three 2p orbital boxes contain ↑↓, ↑, ↑, not two paired boxes and one empty box. Calcium is [Ar]4s²; bromine is [Ar]3d¹⁰4s²4p⁵; krypton ends 4p⁶. Count superscripts to verify Z.

In a box diagram, one box represents one orbital, not one entire p sub-shell. A double-headed arrow is not an electron; use separate up/down arrows to represent opposite spins.

Build configurations from an electron budget

For phosphorus, Z = 15, begin with 15 electrons. Fill 1s² (13 remain), 2s² (11 remain), 2p⁶ (5 remain), 3s² (3 remain), then 3p³. The completed configuration is 1s²2s²2p⁶3s²3p³. Adding the superscripts gives 15, which checks the bookkeeping.

For an atom with Z = 26, fill through [Ar] (18 electrons), then 4s² (20), then 3d⁶ (26). It is [Ar]3d⁶4s². Writing 3d before 4s in the final notation does not change which was filled first in this construction. Do not insist that the third shell must reach eighteen electrons before the fourth begins.

The sequence is a useful ground-state filling rule for the specified atoms, not a claim that orbital energy ordering can never change. Chromium and copper exceptions and transition-metal ion configurations are outside the configurations required in this OCR AS outcome; s- and p-block ions still need careful charge adjustment.

Use boxes to distinguish pairing from filling

Place one electron in each of the three equal-energy p orbitals before pairing. Nitrogen’s 2p³ therefore gives three unpaired electrons. Oxygen’s 2p⁴ gives one pair and two unpaired electrons. Fluorine’s 2p⁵ gives two pairs and one unpaired electron; neon’s 2p⁶ gives three pairs.

Use opposite spin arrows within each pair. Two parallel arrows in a single orbital violate the two-electron spin rule; putting two pairs into oxygen while leaving a p orbital empty fails the ground-state single-filling rule. Draw three boxes even if some are empty so the intended arrangement is unambiguous.

Swipe horizontally to view the whole diagram.

Adjust electron count for ion charge

For the s- and p-block ions in this scope, remove the outermost electrons to form cations and add electrons to form anions. Na is 1s²2s²2p⁶3s¹, so Na⁺ is 1s²2s²2p⁶. O²⁻ also has 1s²2s²2p⁶. Equal configurations do not mean equal nuclei or equal chemical identity.

Worked example: S²⁻ contains 16 + 2 = 18 electrons, so 1s²2s²2p⁶3s²3p⁶. The noble-gas shorthand [Ar] is useful, but write the full configuration when asked. The periodic table’s s, p and d blocks correspond to the sub-shell being filled; this helps check the final term.

Connect the configuration to period, group and block

For chlorine, [Ne]3s²3p⁵, the highest occupied principal shell is 3, so it is in Period 3. Its seven outer-shell electrons place it in Group 17; the sub-shell being filled is p, so it is a p-block element. “Seven electrons” alone is insufficient: the inner shells already contain ten more.

Elements in a main-group column repeat the same outer pattern with a different shell number: fluorine ends 2s²2p⁵ and chlorine ends 3s²3p⁵. Across Period 2, 2s fills and then 2p; across Period 3 the corresponding pattern is 3s then 3p. This repeated arrangement explains recurring chemical behaviour.

When asked for total p electrons, count every occupied p sub-shell. Chlorine has 2p⁶ plus 3p⁵, giving eleven p electrons, not five. Bromine has 2p⁶ + 3p⁶ + 4p⁵ = seventeen. Neither count includes s or d electrons.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources