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First ionisation energy across period 3

Learning outcomes

After studying this page, you should be able to:

  • describe and explain the trend in first ionisation energy across period 3

First ionisation energy

The table shows first ionisation energy values for the elements Na to Ar.

Element Symbol Atomic number First ionisation energy /kJ mol–1
sodium Na 11 496
magnesium Mg 12 738
aluminium Al 13 578
silicon Si 14 789
phosphorus P 15 1012
sulfur S 16 1000
chlorine Cl 17 1251
argon Ar 18 1521

First ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.

It is an endothermic process, i.e. ΔH is positive.

A general equation for this enthalpy change is:

X(g) → X+(g) + e

Description of trend

The graph shows how the first ionisation energy varies across period 3.

The first ionisation energy generally increases across period 3. However, the trend needs a more detailed consideration than the trend in group 2. This is because the first ionisation energy:

  • decreases from magnesium to aluminium then increases again, and
  • decreases from phosphorus to sulfur then increases again.
When you click on the download symbol, you will be able to download the graph as an image file or pdf file, save its data, annotate it, and print it. Note that graphs will be watermarked.

Explanation of this trend

General increase across the period

Going across Period 3:

  • there are more protons in each nucleus so the nuclear charge in each element increases …
  • therefore the force of attraction between the nucleus and outer electron is increased, and …
  • there is a negligible increase in shielding because each successive electron enters the same shell …
  • so more energy is needed to remove the outer electron.

Magnesium to aluminium

Look at their electronic configurations:

  • Magnesium: 1s2 2s2 2p6 3s2
  • Aluminium:  1s2 2s2 2p6 3s2 3p1

The outer electron in magnesium is in an s sub-shell. However, the outer electron in aluminium is in a p sub-shell, so it is higher in energy than the outer electron in magnesium. This means that less energy is needed to remove it.

Phosphorus to sulfur

Look at their electronic configurations:

  • Phosphorus: 1s2 2s2 2p6 3s2 3p3
  • Sulfur:             1s2 2s2 2p6 3s2 3p4

It's not immediately obvious what's going on until you look at the arrangements of the electrons:

The 3p electrons in phosphorus are all unpaired. In sulfur, two of the 3p electrons are paired. There is some repulsion between paired electrons in the same sub-shell, so the force of their attraction to the nucleus is reduced. This means that less energy is needed to remove one of these paired electrons than is needed to remove an unpaired electron from phosphorus.

It may help your understanding when you look at the diagrams below.