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Discovering chemistry
Discovering chemistry

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1.10.1 Filling atomic orbitals with electrons

At this point you should have some feel for how electrons arranged in atoms, and current descriptions draw upon the rather intangible field of quantum mechanics.

What is meant by an atomic orbital?


An atomic orbital describes an allowed distribution in space, about the nucleus where there is a high probability of finding an electron

But how are electrons arranged within each subshell; how many electrons will each orbital contain?

Consider the formula for the sub-shell electron capacities, which is 2(2l + 1), l being the orbital quantum number.

The factor (2l + 1) tells you the number of atomic orbitals in the sub-shell.

How many atomic orbitals are there in an s sub-shell, and how many in a p subshell?


For an s sub-shell l=0, so (2l + 1) = 1. An s sub-shell contains one orbital.For a p sub-shell l=1, so (2l + 1) = 3. A p sub-shell contains three orbitals.

So looking at the atom as a whole, the lowest energy level (n=1) contains one s-orbital only. However, moving to the next highest shell (n=2) you will find one s-orbital (the 2s) and a set of three 2p orbitals.

What orbitals would you expect to find in the n=3 shell?


A 3s orbital, a set of three 3p orbitals and a set of five 3d orbitals.

But how many electrons can occupy an individual orbital?

It turns out that each orbital can contain up to two electrons.

So now Figure 6, section 1.7.1 starts to become clear. The bottom row (labelled ‘electrons’) , is simply showing you the total number of electrons in each subshell assuming each orbital will hold a maximum of two.

How many electrons are required to completely fill a p-sub-shell?


As l =1 for a sub-shell, the factor (2l + 1) tells you that it contains three atomic orbitals. Each of the three orbitals can accommodate up to two electrons with opposed spins. So a p sub-shell can contain a maximum of 2 × 3 = 6 electrons.

The final piece of the jigsaw, when building up a picture of how electrons occupy the energy levels of atoms is connected to a property of the electron called spin. You will be looking at this in the next section.