2.2.1 Electron structure

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

That maximum is not the same as saying shells always fill completely before the next shell starts. Once we move into sub-shell energies, 4s fills 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.

When you draw an electrons-in-box diagram, one box represents one orbital and the arrows represent electrons. Two arrows in one box therefore point in opposite directions.

When you see an orbital diagram, I want you to recognise the 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.

For a p sub-shell, picture 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.

For bromine, Z = 35, so we place 35 electrons to get 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵. You can check yourself by adding the superscripts; they must total 35.

For an s- or p-block ion, start from the neutral atom and 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.