First Ionisation Energy Trend Down the Group

Let's take a look at the first ionisation energies of Group 2 elements.

230327 first ie down group 2

We notice there is a decrease in the first ionisation energy when we go down the Group.

In fact this trend is consistent down any Group.

So why is there a decrease in first ionisation energy down the Group?

We can use effective nuclear charge to explain the trend.

230327 explain why ie decrease down group

Down the Group, proton number increases so nuclear charge increases.

This will cause a stronger attraction on the valence electrons.

Electrons are added to the next principal quantum shell hence distance from the nucleus increases.

There is also an increase in the number of inner shells so shielding effect increases.

Both the increase in distance from nucleus and shielding effect will cause a weaker attraction on the valence electrons.

Overall the effective nuclear charge decreases and there is an overall weaker attraction on the valence electrons.

Less energy is required to remove the valence electrons hence ionisation energy decreases down the Group.

Topic: Atomic Structure, Physical Chemistry, A Level Chemistry, Singapore

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