Periodicity - Ionisation Energy Trend

Across the period, another trend which we need to explain is how the first ionisation energy varies as atomic number increases. The diagram below shows the first ionisation energy trend.

1st IE trend across the period.

Generally, as we move across the period, the 1st ionisation energy increases. This is because despite an electron is added, the number of inner shells remains the same hence the shielding effect is relatively constant. While the nuclear charge increases. Therefore, the combination of these two reasons result in an overall increase in the 1st ionisation energy.

There are a couple of dips seen and these are exceptions to the trend. Generally, the dips are present because the causes for the dips are more significant than the effect due to increase in effective nuclear charge. The mind map below provides you with a structure which will explain the trend, including the exceptions.

Mindmap to explain trend across Period 3.

In addition, there is another explanation to why the ionisation energy differs between period. We move from Period 2 to Period 3, the size of the atoms get larger. The added inner shell of electrons results in the valence shell of elements in Period 3 to be further away from the nucleus. Hence, electrostatic attraction between nucleus and valence shell becomes weaker, therefore 1st ionisation energy is smaller.

Trends to notice across the period

Across the period, there are 4 main trends to take note. These trends usually appear even in the general trend for 2^nd and 3^rd I.E and their explanation used to explain is similar. However, do take note that sometimes we do not see the dip (e.g. when you compare the 2^nd I.E of S and Cl we don't see the expected drop) and that is because of the bold phrase of the article.

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Article written by Kwok YL 2009.}

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