OxfordAQA A-Level Chemistry: Periodicity (9620) – Revision Notes

These notes boil down section 3.2.1 Periodicity of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: 3.2.1.1 (Classification) together with 3.2.1.2 (the Physical properties of Period 3 elements). The whole section is International AS content; none of it is International A-level only. Unit 1 (Inorganic 1 and Physical 1) examines it, and the synoptic Unit 5 may use it too.

New to the topic? Start with the periodicity study guide, which explains every idea with worked examples. When these notes feel secure, move on to the periodicity practice questions. The course hub links every other 9620 topic, and the printable checklist is there for marking off points as you go. A free 10-minute diagnostic is a quick way to find gaps.

Words you must use precisely

• Proton number (Z): sets an element’s place in the Periodic Table, and so its block.
• Block: named after the sub-shell (s, p, d or f) holding the highest-energy electron.
• Period: a row. In Period 3, outer electrons occupy the third shell.
• First ionisation energy: the energy for a mole of gaseous atoms to each lose one electron, making a mole of gaseous 1+ ions. Pattern: M(g) → M⁺(g) + e⁻
• Nuclear charge: the positive charge of the nucleus; it goes up by one with each proton added.
• Shielding: the reduction in attraction an outer electron feels because of the electrons in inner shells.
• Metallic bonding: attraction between delocalised electrons and positive ions in a lattice.
• Macromolecular (giant covalent): atoms linked by covalent bonds in a continuous network.
• Van der Waals forces: induced dipole–dipole forces, the only forces between non-polar molecules or noble gas atoms.

Background: atomic structure and bonding and structure.

3.2.1.1 Blocks

Block	Where in the table	Sub-shell being filled	Period 3 members
s	Groups 1 and 2, plus H and He	ns	Na, Mg
p	Groups 3 to 0 (13 to 18), except He	np	Al, Si, P, S, Cl, Ar
d	between Groups 2 and 3	(n − 1)d	none
f	lanthanide and actinide rows	(n − 2)f	none

Method: classifying any element

1. Use the proton number (in brackets in the table below) to place the element, or build its configuration in the usual sequence (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p).
2. Identify the sub-shell that receives the last electron in that order.
3. That sub-shell’s letter is the block.

Watch for: helium (s block although it sits in Group 0); elements Z = 31 to 36 (p block although 3d¹⁰ appears in the configuration); chromium and copper (d block despite ending 4s¹); any lanthanide or actinide (f block from its position alone).

3.2.1.2 Period 3 at a glance

	Na (11)	Mg (12)	Al (13)	Si (14)	P (15)	S (16)	Cl (17)	Ar (18)
Outer sub-shells	3s¹	3s²	3s² 3p¹	3s² 3p²	3s² 3p³	3s² 3p⁴	3s² 3p⁵	3s² 3p⁶
First IE / kJ mol⁻¹	496	738	578	787	1012	1000	1251	1521
Melting point / K	371	923	933	1687	317	388	172	84
Structure	metallic	metallic	metallic	macro.	P₄	S₈	Cl₂	atoms

(Rounded data values; phosphorus as white P₄, sulfur as S₈. macro. = macromolecular.)

Atomic radius: the three-link chain

Trend: decreases from Na to Cl.

Link	What to write
1	Proton number, and so nuclear charge, increases across the period
2	Outer electrons stay in the third shell, behind the same inner electrons, so shielding is similar
3	So the nucleus pulls the outer electrons in nearer

A comparison of two atoms needs all three links, with the proton numbers quoted.

First ionisation energy

Trend: general increase, Na lowest (496 kJ mol⁻¹), Ar highest (1521 kJ mol⁻¹).

General rise: the radius chain again, then one more link. The outer electron is gripped more tightly, so more energy is needed to remove it.

The two exceptions:

Drop	Reason in one line
Mg (738) → Al (578)	Al loses a 3p electron, higher in energy than Mg’s 3s electron and partly shielded by 3s²
P (1012) → S (1000)	In S, one 3p orbital is doubly occupied; repulsion within that pair makes one electron easier to take

Worked reminder

Why is the first ionisation energy of phosphorus higher than that of silicon? P has 15 protons and Si 14; both lose a 3p electron from the third shell with similar shielding; P is smaller. So P’s outer electron is held more strongly. Note this is a normal step in the trend, not one of the exceptions.

Melting point

Order, highest to lowest (data above): Si > Al > Mg > S > Na > P > Cl > Ar.

Elements	Structure	Melting overcomes	Why the values fall where they do
Na, Mg, Al	giant metallic	attraction between positive ions and delocalised electrons	ion charge 1+, 2+, 3+; delocalised electrons per atom 1, 2, 3; ions smaller; so bonding strengthens Na → Al
Si	macromolecular	many strong covalent bonds	highest in the period
P₄, S₈, Cl₂	molecular	van der Waals forces between molecules (covalent bonds stay intact)	more electrons per molecule, stronger forces: S₈ > P₄ > Cl₂
Ar	single atoms	van der Waals forces between atoms	fewest electrons per particle, lowest in the period

Method: any two-element melting point comparison

1. Name each element’s structure.
2. State what has to be overcome or broken in each.
3. Compare the strength of those forces or bonds, with a reason (charge, electrons, number of bonds).
4. Finish with energy: stronger attraction means more energy needed, so a higher melting point.

Worked reminder

Why does magnesium melt at a much higher temperature than argon? Magnesium is giant metallic: melting must overcome strong attraction between Mg²⁺ ions and delocalised electrons. Argon is made of single atoms: only weak van der Waals forces between atoms are overcome. Much less energy is needed for argon (84 K) than for magnesium (923 K).

Must-know distinctions

Keep apart	Because
Block and group	Block is a sub-shell type; group is a column (outer electron count for s and p blocks)
Aluminium’s block and its bonding	Al is p block but metallic; block does not tell you the structure
Covalent bonds in Si	Broken on melting
Covalent bonds in P₄, S₈, Cl₂	Not broken on melting; only forces between molecules are overcome
Trend in radius across a period	Falls (same shell, more protons)
Trend in radius down a group	Rises (more shells), covered in 3.2.2 for Group 2
Ionisation energy peak	Argon
Melting point peak	Silicon

Quick self-test

1. State the block of the element with Z = 13.
2. Copper (Z = 29) has the configuration [Ar] 3d¹⁰ 4s¹. State its block.
3. Krypton (Z = 36) is a noble gas. State its block.
4. Give the first ionisation equation for chlorine, including state symbols.
5. Which atom is larger, aluminium or sulfur? Give the main reason.
6. Name the only p-block element in Period 3 with metallic bonding.
7. Explain why the first ionisation energy falls from phosphorus to sulfur.
8. What is overcome when argon melts?
9. Arrange P₄, S₈ and Cl₂ in order of increasing melting point.
10. Why is silicon’s melting point higher than magnesium’s?
11. Using the data table, state by how much the melting point rises from sodium to magnesium.

Answers

1. p block (aluminium, 1s² 2s² 2p⁶ 3s² 3p¹: the last electron enters 3p).
2. d block. It sits in the d block, where 3d is being filled; the 4s¹ ending does not change that.
3. p block (4p⁶).
4. Cl(g) → Cl⁺(g) + e⁻
5. Aluminium. Sulfur has 16 protons against 13, with outer electrons in the same shell and similar shielding, so its outer electrons are pulled closer.
6. Aluminium.
7. In sulfur one 3p orbital holds a pair of electrons. They repel each other, so one is easier to remove.
8. Van der Waals forces between argon atoms.
9. Cl₂ < P₄ < S₈ (34, 60 and 128 electrons per molecule).
10. Silicon is macromolecular, so melting breaks many strong covalent bonds. Magnesium only needs the metallic bonding between Mg²⁺ ions and delocalised electrons to be overcome, which takes less energy.
11. 923 − 371 = 552 K.

Where marks are usually lost

• Saying atoms gain shells, or gain shielding, as you move from sodium to chlorine.
• Quoting “more protons” for a radius or ionisation energy comparison with no mention of the same shell.
• Calling the Mg → Al and P → S drops “anomalies” without giving the sub-shell or orbital reason.
• Saying the covalent bonds in P₄, S₈ or Cl₂ break on melting.
• Describing silicon as having intermolecular forces at all.
• Linking van der Waals strength to “bigger atoms” with no reference to the number of electrons.
• Writing metallic bonding as “atoms held together by electrons” without positive ions.
• Assuming aluminium is non-metallic because it is in the p block.
• Writing an ionisation equation with a solid on the left, or with two electrons removed.

Official syllabus

Source document: OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. Section 3.2.1 Periodicity (International AS): point 3.2.1.1, Classification; point 3.2.1.2, on the Physical properties of Period 3 elements.