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A short H2 Chemistry revision video on H2 Chemistry 1 - Atomic Structure: Why Period 3 IE1 Has Two Dips, built for quick recap before tutorial practice or exam revision.
Read through the explanation after watching, or jump straight to the step you want to replay.
Why two dips?
Across Period three, first ionisation energy mostly rises from sodium to argon.
Why two dips?
But the graph is not smooth.
Why two dips?
There are two dips: magnesium to aluminium, and phosphorus to sulfur.
Why two dips?
If you can explain those two dips, you understand the trend.
What IE1 measures
First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms.
What IE1 measures
The electron removed is the outermost electron.
What IE1 measures
So every explanation must say how strongly that outer electron is attracted to the nucleus.
The main pull
The main trend is upward because proton number increases across Period three.
The main pull
The outer electrons are still in the third shell, so shielding is roughly constant.
The main pull
Higher effective nuclear charge gives a stronger attraction, so more energy is needed.
Dip 1: Mg to Al
The first dip is magnesium to aluminium.
Dip 1: Mg to Al
Magnesium loses a three s electron.
Dip 1: Mg to Al
Aluminium loses a three p electron, which is higher in energy and shielded more by the three s pair.
Dip 1: Mg to Al
So aluminium's outer electron is easier to remove.
Dip 2: P to S
The second dip is phosphorus to sulfur.
Dip 2: P to S
Phosphorus has three unpaired three p electrons, one in each p orbital.
Dip 2: P to S
Sulfur has one paired three p orbital.
Dip 2: P to S
Pairing creates extra repulsion, so one sulfur electron is easier to remove.
The exam sentence
In an exam, do not just say that nuclear charge increases.
The exam sentence
Say that effective nuclear charge increases because proton number increases while shielding remains similar.
The exam sentence
Then add the subshell or electron-pairing exception for the dip being tested.
The trap
The common trap is an incomplete explanation.
The trap
"More protons" is true, but not enough.
The trap
For full credit, connect proton number, shielding, effective nuclear charge, and the subshell involved.