Electron Structure

 
 

Electron shells

Electrons occupy specific energy levels around the nucleus, which are given numbers known as principal quantum numbers. Each energy level is able to hold a different number of electrons.

  • The first energy level holds a maximum of 2 electrons.

  • The second energy level holds a maximum of 8 electrons.

  • The third energy level holds a maximum of 18 electrons.

  • The fourth energy level holds a maximum of 32 electrons.

Each energy level contains sub-shells (s, p, d and f) which are further divided into orbitals. An orbital is a region around the nucleus that can hold a maximum of 2 electrons.

  • The s sub-shell only contains one orbital, which can hold a maximum of two electrons. So the total number of electrons within a s sub-shell is 2.

  • The p sub-shell contains three orbitals, which can hold a maximum of two electrons each. So the total number of electrons within a p sub-shell is 6.

  • The d sub-shell contains five orbitals, which can hold a maximum of two electrons each. So the total number of electrons within a d sub-shell is 10.

  • The f sub-shell contains seven orbitals, which can hold a maximum of two electrons each. So the total number of electrons within a f sub-shell is 14.

 
 

Electron configuration

When we want to show the electron configuration of a particular atom or ion, we list the quantum number, sub-shell and the electrons contained in each sub-shell. The electron configuration for magnesium would look like this:

 
 

From adding up all the little numbers after each letter, we can see that a magnesium atom has 12 electrons.

  • In the first energy level (quantum number = 1), there is only the s sub-shell which can hold a maximum of two electrons.

  • In the second energy level (quantum number = 2), there are both s and p sub-shells. Again, the s sub-shell can only hold a maximum of two electrons since there is only one orbital within the s sub-shell. The p sub-shell contains three orbitals, so there are six electrons here.

  • In the third energy level (quantum number = 3), only the s sub-shell is filled because we only have two electrons remaining.

Worked example - writing electron configurations

Write the electron configuration for phosphorous (atomic number = 15).

• The atomic number for a neutral atom will be the same as its total number of electrons, so P has 15 electrons.
• For quantum number 1, there is only one s-subshell so we write 1s2.
• For quantum number 2, there is an s-subshell (2s2 and a p sub-shell (2p6).
• For quantum number 3, there are s, p and d sub-shells. In this case, only the s an p sub-shells are occupied since we only have 15 electrons to work with. We've already used 10 in the first and second energy level, so there are five left. The electron configuration for the third energy level must be 3s2 3p3
• P = 1s2 2s2 2p6 3s2 3p3

Orbital shapes

An orbital is a region around the nucleus where a particular electron can be found. Orbitals from different sub-shells have different shapes. For instance, an s orbital is spherical and we can draw it as a simple circle. P orbitals have a dumbbell shape and look more like a figure of eight. Remember that there are three p orbitals within the p sub-shell, which are at right angles to one another.

All of the orbitals within the same sub-shell will have the same amount of energy and each orbital can only ever hold two electrons. These electrons have opposite spins, which we represent as two arrows facing opposite directions. This is called spin pairing.

Electron pairing

  • Electron shells further from the nucleus have a higher energy than shells closer to the nucleus — 1s electrons have a lower energy than the 2s electrons, which in turn have a lower energy than the 2p electrons.

  • Electrons will fill the lowest energy orbitals first before they fill up the higher energy ones.

  • Electrons will occupy orbitals singly before they start pairing up — if electrons are filling up a p sub-shell, which consists of three orbitals, each of the three orbitals will contain a single electron before they start pairing up.

We can represent electron pairing in boxes where each box represents an orbital. Have a look at the electron configuration below for phosphorous. See how the electrons start filling the lowest energy 1s orbital first before they go on to the higher ones and how those 3p3 electrons are occupying orbitals singly.