1.5.1Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Periodicity – Ionisation Energy — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 15 of 36 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 6The General Trends
Periodicity means a pattern that repeats in each period of the Periodic Table.
Across a period
- Across a period First ionisation energy generally increases because nuclear charge rises while electrons are added to the same shell with similar shielding.
Atomic radius
- Atomic radius Stronger attraction pulls the outer shell closer, so the atoms get smaller and the outer electron is held more tightly.
Down a group
- Down a group Ionisation energy decreases because extra shells increase the distance and the shielding, outweighing the larger nuclear charge.
Group 1 and 18
- Group 1 and 18 Alkali metals have the lowest value in each period, while the noble gases have the highest because of their stable full shells.
Section 2 of 6Period 3 Data
Period 3 Data
Na and Mg
- Na and Mg First ionisation energies are about 496 and 738 kJ mol⁻¹, rising as the nuclear charge increases from 11 to 12.
Al dip
- Al dip Aluminium is about 578 kJ mol⁻¹, lower than Mg, because its outer electron is in a 3p orbital.
Si to P
- Si to P Values rise from about 789 to 1012 kJ mol⁻¹ as nuclear charge increases and the 3p electrons occupy separate orbitals.
S dip
- S dip Sulfur is about 1000 kJ mol⁻¹, lower than P, because one 3p orbital now holds a pair of electrons.
Cl and Ar
- Cl and Ar Values reach about 1251 and 1521 kJ mol⁻¹, with argon the highest in the period.
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
|---|---|---|---|---|---|---|---|---|
| IE₁ / kJ mol⁻¹ | 496 | 738 | 578 | 789 | 1012 | 1000 | 1251 | 1521 |
Section 3 of 6Explaining the Two Dips
1
From Mg to Al the outer electron is in a 3p orbital instead of 3s.
2
A 3p electron has slightly higher energy and is further from the nucleus, and it is also shielded by the 3s electrons.
3
It therefore needs less energy to remove, so Al has a lower value than Mg despite its greater nuclear charge.
4
From P to S the fourth 3p electron must pair with another electron in an orbital that already holds one.
5
The repulsion between the two paired electrons makes one easier to remove, so S is lower than P.
6
The same two dips occur in Period 2, between Be and B and between N and O.
Section 4 of 6Using Evidence in Answers
Using Evidence in Answers
Three factors
- Three factors Always refer to nuclear charge, distance from the nucleus and shielding when explaining any ionisation energy difference.
Comparing atoms
- Comparing atoms State which atom has the greater nuclear charge or more shells, then say how that changes the attraction on the outer electron.
Group 2 example
- Group 2 example Mg has a lower first ionisation energy than Be because its outer electrons are in a higher shell, further out and more shielded.
Sub-shell evidence
- Sub-shell evidence The dips are experimental proof that p orbitals sit at slightly higher energy than s and that electrons pair up only when they must.
Section 5 of 6Worked Example
Question Explain why the first ionisation energy of sulfur (1000 kJ mol⁻¹) is lower than that of phosphorus (1012 kJ mol⁻¹).
- Phosphorus has the configuration 3s² 3p³ with three single electrons in separate p orbitals.
- Sulfur is 3s² 3p⁴, so one 3p orbital must hold two electrons.
- The pair in the same orbital repel each other, so less energy is needed to remove one of them.
Answer Repulsion between paired 3p electrons in sulfur outweighs the extra nuclear charge, so IE₁ of S is lower than P.
Section 6 of 6Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- Periodicity A repeating pattern in properties across successive periods of the Periodic Table.
- First ionisation energy The energy to remove one mole of electrons from one mole of gaseous atoms.
- Nuclear charge The total positive charge of the protons in the nucleus.
- Shielding Repulsion of outer electrons by electrons in inner shells.
- Electron pairing Two electrons sharing an orbital, which repel each other and lower the energy needed to remove one.
Common Mistakes
- Saying the dip at aluminium is due to a smaller nuclear charge;
- it is because the electron is in a p subshell.
- Forgetting to mention shielding when explaining the decrease down a group.
- Explaining the sulfur dip by shielding rather than by the repulsion between paired electrons.
- Stating that ionisation energy always increases across a period without noting the two exceptions.
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