1.3.1Physical Chemistry
Periodicity — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 42
Section 1 of 4Melting Points across Period 3
Periodicity is the repeating pattern of properties across successive periods of the Periodic Table.
Na, Mg, Al
- Metallic bonding is the attraction between positive ions and delocalised electrons,
- and it strengthens as the ion charge rises from 1+ to 3+.
Silicon
- A giant covalent lattice in which every atom is bonded to four others by strong covalent bonds,
- giving the highest melting point of the period.
P, S, Cl
- Simple molecules (P₄, S₈, Cl₂) with weak instantaneous dipole-induced dipole forces between molecules, so melting points are low.
Argon
- A single-atom gas with the weakest forces of all, so it has the lowest melting point;
- S₈ melts higher than P₄ because its molecules are larger.
Trend summary
- Across Period 3 melting point rises from sodium to silicon,
- then drops sharply for the simple molecular non-metals before reaching its lowest value at argon.
Section 2 of 4Atomic Radius
1
Moving across Period 3 each element has one more proton and one more electron than the one before.
2
The extra electrons enter the same shell, so shielding by inner electrons stays about the same.
3
The nuclear charge increases, so the outer electrons are pulled closer to the nucleus.
4
Atomic radius therefore decreases across the period.
5
Period 2 shows the same pattern, so the decrease in atomic radius across a period is a general trend and not a feature of Period 3 alone.
Section 3 of 4First Ionisation Energy across a Period
First Ionisation Energy across a Period
General increase
- Greater nuclear charge, smaller radius and similar shielding make the outer electron harder to remove across the period.
Mg to Al dip
- Aluminium's outer electron is in a 3p subshell, higher in energy and further out than magnesium's 3s electron,
- so IE falls from 738 to 578 kJ mol⁻¹.
P to S dip
- Sulfur's fourth 3p electron must pair in an orbital, and the repulsion eases its removal, so IE falls from 1012 to 1000 kJ mol⁻¹.
Period 2 pattern
- The same dips occur at Be to B (900 to 800 kJ mol⁻¹) and N to O (1400 to 1310 kJ mol⁻¹) for identical reasons.
| Element | Structure | Melting point trend |
|---|---|---|
| Na to Al | Giant metallic | Increases |
| Si | Giant covalent | Highest |
| P, S, Cl, Ar | Simple molecular or atomic | Low and variable |
Section 4 of 4Exam Checklist & Tips
1.3.1 – Key Points for the Exam
- Periodicity The repeating pattern of physical and chemical properties across periods.
- Atomic radius Distance from the nucleus to the outermost electrons, found by halving the distance between bonded nuclei.
- Spin-pair repulsion Repulsion between two electrons in the same orbital, which lowers the energy needed to remove one.
- Giant lattice A structure in which many atoms or ions are held together by bonds throughout the solid.
Common Mistakes
- Saying the melting point of simple molecules depends on strong covalent bonds breaking;
- only weak intermolecular forces are overcome.
- Explaining the Al dip by shielding from inner shells instead of the higher-energy 3p subshell.
- Claiming atomic radius decreases across a period because shielding increases.
- Forgetting that sulfur melts higher than phosphorus because S₈ is a larger molecule than P₄.
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