First Ionisation Energy Trend Across Period 3

In Atomic Structure we need to be familiar with a few ionisation energy trends.

In this video let's discuss the first ionisation energy trend across Period 3.

First ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms according to the following equation:

M(g) -> M+(g) + e-

The first ionisation energy (IE) trend across Period 3 is given in the sketch below.

1st IE trend across period 3 trend

There are 3 observations:
1. General increase in first IE
2. Slight decrease in IE from Mg to Al (first anomaly)
3. Slight decrease in IE from P to S (second anomaly)

1. General increase in first IE

The concept used to explain the general trend is involving effective nuclear charge.

Across the period proton number increases hence nuclear charge increases.

Electrons are added to the same principal quantum shell so shielding effect or screening effect stays constant.

Effective nuclear charge increases hence attraction between nucleus and valence electron is stronger.

More energy is required to remove the valence electron hence first IE increases across Period 3.

1st IE trend across period 3 general trend increases

2. Slight decrease in IE from Mg to Al

The first anomaly is the decrease in IE from Mg to Al.

We need to write down the electron configuration for Mg and Al and compare where the electrons are removed.

1st IE trend across period 3 first anomaly from Mg to Al

Electron is removed from 3s subshell for Mg while electron is removed from 3p subshell for Al, which is further away from the nucleus or has a higher energy level.

The attraction between nucleus and that electron is weaker hence less energy is required to remove it.

Therefore Al has a lower first IE.

3. Slight decrease in IE from P to S

The second anomaly between P and S requires us to draw the electron in box diagram as electrons are removed from 3p subshell for both elements.

1st IE trend across period 3 second anomaly from P to S

For sulfur, electron is removed from a fully filled 3p orbital hence it experiences interelectronic repulsion between the electron pair in the same 3p orbital.

This makes it easier to remove that electron, therefore S has a lower first IE.

For the detailed step-by-step discussion on the first ionisation energy trend for Period 3 elements, check out this video!

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

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