2.2.1 Electron structure

The first four shells have maximum electron capacities of 2, 8, 18 and 32. I’d learn the sequence and notice the pattern: shell n can hold up to 2n² electrons.

Those values are maximum capacities. Sub-shell energies determine the filling order, which puts 4s before 3d.

• shell 1: 2 electrons
• shell 2: 8 electrons
• shell 3: 18 electrons
• shell 4: 32 electrons

An atomic orbital is a region of space around the nucleus that can hold up to two electrons. If two electrons occupy the same orbital, they have opposite spins.

In an electrons-in-box diagram, one box represents one orbital and each arrow represents an electron. Two arrows in one box therefore point in opposite directions.

Recognise the orbital shape before worrying about its orientation. An s orbital is spherical; a p orbital has two lobes, often described as dumbbell-shaped, with the nucleus at the centre.

A p sub-shell contains three p orbitals with the same basic shape pointing in different directions in space.

• s orbital: spherical
• p orbital: two-lobed / dumbbell-shaped
• three p orbitals per p sub-shell

Once you remember that every orbital holds at most two electrons, the sub-shell capacities follow from the number of orbitals. An s sub-shell has 1 orbital, p has 3 and d has 5.

So the maximum electron counts are 2, 6 and 10 respectively. Learn the orbital count and let the electron count follow from it.

• s: 1 orbital → 2 electrons
• p: 3 orbitals → 6 electrons
• d: 5 orbitals → 10 electrons

Fill the available sub-shells from lower to higher energy. For the range OCR expects here, the order you need is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

The slightly non-obvious bit is 4s before 3d. If you keep the order visible while you build a configuration, you are much less likely to put the electrons into the wrong sub-shell.

• 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p

When several orbitals have the same energy, put one electron into each orbital before you start pairing them. For a p sub-shell, that means three separate boxes each receive one electron first.

Then pair electrons with opposite spins. So p⁴ is drawn as one paired orbital and two singly occupied orbitals, not two pairs with an empty third box.

Use the atomic number to get the total number of electrons in a neutral atom, then fill the sub-shells in energy order. Keep the sub-shell capacities in mind: s holds 2, p holds 6 and d holds 10.

The periodic table gives you another check. Count across the block: Br is fifth across the Period 4 p block, so the final p occupancy should be 4p⁵. In the d block, use one shell number lower than the period.

For bromine, Z = 35, so we place 35 electrons to get 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵. Add the superscripts at the end; they must total 35.

For an s- or p-block ion, start with the neutral atom, then add or remove electrons to match the charge. Positive ions have fewer electrons; negative ions have more.

Take S²⁻: sulfur has 16 electrons as an atom, so S²⁻ has 18. Its configuration is therefore 1s² 2s² 2p⁶ 3s² 3p⁶. OCR keeps this line to s- and p-block ions, so you do not need transition-metal ion exceptions here.