IB Chemistry HLTopic 3 — Classifying the ElementsPaper 1 & 2Trends~11 min read
Ionisation Energy Trends Across a Period (HL)
Ionisation energy rises across a period. Except twice, where it drops instead. Those two dips are not annoying exceptions to be memorised — they are the experimental evidence that subshells exist, and an examiner who asks about them is really asking whether you understand where electrons actually live.
📘 What you need to know
First ionisation energy generally increases across a period: rising nuclear charge, roughly constant shielding, decreasing radius.
There is a sharp drop from the end of one period to the start of the next, because a new shell has opened.
Two dips break the trend in period 2: from beryllium to boron, and from nitrogen to oxygen.
The Be to B dip happens because boron’s outer electron is in a 2p orbital, which is higher in energy than the 2s and slightly more shielded.
The N to O dip happens because oxygen’s fourth 2p electron must pair up in an orbital that already holds one, and the two repel.
Both dips are evidence that sublevels exist. A model with only shells could not explain them.
The same pattern repeats in period 3, from Mg to Al and from P to S.
The graph you should be able to sketch
The big features are easy: a full shell is hard to break into, and a lone electron in a fresh shell is easy to remove. The two small dips are where the marks are.
Dip 1: beryllium to boron
Beryllium is 1s22s2 and boron is 1s22s22p1. Boron has one more proton, so the trend says its ionisation energy should be higher. It is lower — 801 against 899 kJ mol−1.
The reason is which orbital the electron comes from. Beryllium’s outermost electron is in a 2s orbital; boron’s is in a 2p. A 2p orbital is higher in energy than a 2s and sits slightly further from the nucleus on average, and it is also very slightly shielded by the 2s electrons underneath it. Both effects make boron’s outer electron easier to remove, and together they outweigh the extra proton.
Dip 2: nitrogen to oxygen
Here both electrons come from a 2p orbital, so the previous explanation cannot apply. Something else must be going on — and it is pairing.
Nitrogen’s half-filled subshell is often called “extra stable”. Be careful with that phrase in an answer — the mark is for the repulsion between the paired electrons in oxygen, which is the mechanism rather than the label.
What the dips prove
Imagine a model of the atom with shells but no subshells. Across period 2 you would be adding eight electrons to one shell while the nuclear charge climbed steadily, so the graph would rise smoothly with no interruptions at all.
It does not. It dips at exactly the two places where something changes about the type of orbital being filled — when the 2p opens, and when pairing starts within it. That pattern is only explicable if the shell is divided into subshells and orbitals, which is why these two dips are quoted as evidence for the existence of sublevels.
The same dips reappear in period 3, from magnesium to aluminium (3s to 3p) and from phosphorus to sulfur (pairing in 3p). If a question gives you period 3 data, apply exactly the same two arguments.
Worked examples
WORKED EXAMPLE
Explain why the first ionisation energy of aluminium is lower than that of magnesium. [3]
Mark 1: give both configurationsMg: [Ne] 3s² Al: [Ne] 3s² 3p¹Mark 2: identify the orbital involved
The electron removed from aluminium comes from a 3p orbital, which is higher in energy than the 3s.
Mark 3: complete the argument
It is further from the nucleus on average and slightly shielded by the 3s electrons, so it is held less strongly.
Less energy is needed, despite aluminium having one more protonthe phrase “despite the higher nuclear charge” shows you know why this is a surprise
WORKED EXAMPLE
Sulfur has a lower first ionisation energy than phosphorus. Explain, using orbital diagrams. [3]
Mark 1: phosphorusP: 3p³ — three orbitals, one electron in each, all unpairedMark 2: sulfurS: 3p⁴ — one orbital now contains a pairMark 3: the consequence
The two electrons sharing that orbital repel each other, so one of them is less tightly held.
Less energy is needed to remove it, so sulfur’s value is lowersay “electron–electron repulsion within the same orbital” — that exact idea is the mark
WORKED EXAMPLE
Why is there such a large drop in first ionisation energy from neon to sodium?
Step 1: compare the electron being removed
Neon’s comes from the 2nd shell; sodium’s comes from the 3rd.
Step 2: distance
Sodium’s outer electron is a whole shell further from the nucleus.
Step 3: shielding
It is also shielded by all ten inner electrons, which neon’s is not.
Distance and shielding both jump at once, so the value falls sharplythis drop is what defines the start of a new period on the graph
💡 Exam tip
Quote the electron configurations of both elements in a dip question. It usually earns the first mark by itself.
Name the orbital the electron leaves from. “A 2p electron” is much stronger than “the outer electron”.
For the second dip, the key phrase is repulsion between paired electrons in the same orbital.
Add “despite the increase in nuclear charge” to show you know the dip is against the general trend.
If asked what the dips are evidence for, answer the existence of sublevels.
Sketching the graph is a legitimate way to answer “describe the trend” — but label the axes and mark the elements.
⚠ Common mix-up
Using the pairing argument for the Be to B dip. Boron’s 2p electron is unpaired; the reason there is the orbital type.
Using the orbital-type argument for N to O. Both come from 2p, so only pairing can explain it.
Saying the half-filled subshell is stable, and stopping there. Explain the repulsion in oxygen, not just the label for nitrogen.
Claiming shielding increases a lot across a period. It is roughly constant; the 2s shielding of a 2p electron is a small effect, and only relevant at that one step.
Forgetting that ionisation energies are always positive. Energy must always be supplied to remove an electron.
Reading the graph as continuous. Only integer atomic numbers exist — the line is a guide for the eye.
Up next: Characteristic Properties of Transition Elements (HL) — we move into the d-block, where the 4s and 3d subshells sit so close in energy that a whole family of unusual behaviour falls out of it.
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