AS-Level Atomic Structure

  • Electrons are attracted to the positive charge of a nucleus (electrostatic attraction).

  • The closer electrons are to a nucleus, the lower their energy.

  • Electrons want to get close to a nucleus but they repel each other.

    • They can only exist in set regions called orbitals.

  • Different orbitals have different shapes and therefore different energies.

  • Sub-shells describe how far away an electron (and its orbital) is from a nucleus.

  • Electron sub-shells are filled successively – lowest energy sub-shells and orbitals are filled first.


Electron Orbitals


Electrons travel around the nucleus of an atom because they are attracted to its positive charge.  Electrons are negatively charged, so when they move close to each other they repel. This repulsion causes the energy of these electrons to increase (opposite charges brought together LOWERS energy, similar charges brought together INCREASES energy).

As a result of this repulsion, electrons do not simply surround a nucleus, but must arrange themselves to keep the overall energy of an atom as low as possible. Nature is lazy – it will always take the option that keeps energy as low as possible.

Such specific arrangements of electrons are called orbitals. Don’t be confused by electron orbitals. Very simply – an orbital is just the space around a nucleus that a pair of electrons are ‘allowed’ to be in. They are called ‘pairs’ because one orbital can hold two electrons (one pair!). There are different shapes of orbitals to keep electron repulsion to a minimum.


There are only so many ways that electrons can arrange themselves without ‘bumping’ into each other. This means that some electrons have to move further away from the nucleus. The further an orbital is from the nucleus, the higher the energy of that orbital. Remember a nucleus is positive; bringing a positive and a negative charge closer together lowers the energy, so moving them further apart raises the energy.


When all orbitals at a particular distance from the nucleus are full, any more electrons must fill orbitals further away from the nucleus. We say that an electron then enters a new sub-shell.

A sub-shell just indicates how far an orbital is from the nucleus of an atom. It is completely pointless measuring this distance with a unit! Instead, we use basic integers (numbers!). The closest sub-shell to the nucleus is assigned the number 1, the second closest sub-shell is assigned the number 2, the third closest sub-shell the number 3….etc.


The sub-shells are always filled in order. Electrons will fill the 1 sub-shell before the 2   and the 2   before the 3  , etc.  Think of a multi-story car park – lower levels fill first as the people in the cars want to be as close to the ground as possible. But, if the lower levels are already taken, they must park at a higher level.


Orbitals can have different shapes and some are larger than others. This means some orbitals have to be further away from the nucleus than others.


An s-orbital is a nice, simple sphere that is close to the nucleus. Therefore, it is easy to fit in the first sub-shell.





A p-orbital is a dumb-bell shape, so it extends out further than an s-orbital (this explains why the p-orbital is a higher energy orbital than the s-orbital). Therefore, it’s not possible to fit a p-orbital shape in the first sub-shell (n=1). If a p-orbital is in sub-shell one it will actually end up going over the distance occupied by the s-orbital in sub-shell two. The first sub-shell that can have a p-orbital is the second, n = 2. There are three p-orbitals and they all have the same energies when full, they just ‘point’ in different directions.

D-orbitals are actually different shapes that occupy more space than the p-orbitals. In the same way a p-orbital can’t fit in the first sub-shell, d-orbitals cannot fit in the second sub-shell. This means there aren’t d-orbitals until the third sub-shell, n = 3.


Remember that all orbitals are filled successively (one after the other)? There is an exception, and it is all concerned with energy. A 4s orbital is filled before a 3d orbital. This is simply because the 4s orbital is actually a lower energy than the 3d orbitals, so it is filled first.

A bit extra: electrons move very fast continually, so we cannot ever say exactly where an electron will be at any one time. As soon as an electron is in one place, it will then move to another.  These orbital diagrams and shapes are probability densities. They show where an electron is most likely to be found at a given time. It is a bit like saying someone is at home. You can say that the person is within their house, but you have no real way to know where the person is exactly – all you can say is ‘they are likely to exist in this space’.