Home A Level Inorganic Chemistry - Core Periodicity First ionisation energy down group 2

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First ionisation energy down group 2

Learning outcomes

After studying this page, you should be able to:

  • describe and explain the trend in first ionisation energy down group 2

First ionisation energy

The table shows first ionisation energy values for the common elements in group 2.

Element Symbol Atomic number First ionisation energy /kJ mol–1
beryllium Be 4 900
magnesium Mg 12 738
calcium Ca 20 590
strontium Sr 38 550
barium Ba 56 503

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 down group 2:

  • as you go down group 2, the first ionisation energy decreases.
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

Going down group 2:

  • there are more filled shells between the nucleus and the outer electrons …
  • these shield the outer electrons from the attraction of the nucleus …
  • the radius of the atom increases, so the distance between the nucleus and the outer electrons increases …
  • therefore the force of attraction between the nucleus and outer electrons is reduced …
  • so less energy is needed to remove an outer electron.

You might expect the first ionisation energy to increase because the nuclear charge increases. This does not happen,  because the factors described in the Explanation have a greater influence on the value of the first ionisation energy.