HookEvery microchip begins at the strangest melting point in Period 3
Inside every phone and laptop sits a sliver of the purest solid object humans routinely manufacture: electronic-grade silicon, refined to 99.9999999% — nine nines. To grow it, a seed crystal touches a crucible of molten silicon held just above 1,410 °C and is drawn upwards at a few millimetres a minute, pulling behind it a single flawless crystal the size of a fence post, later sliced into the wafers that carry every circuit you own. Now look one square to the left on the periodic table: aluminium melts at 660 °C. Two squares: magnesium, 650 °C. Three: sodium, 98 °C — it would melt in a mug of tea. Walk right from silicon instead and the collapse is steeper still: white phosphorus melts at 44 °C, sulfur at 115 °C, chlorine at −101 °C, and argon will not hold itself solid above −189 °C. One row of the table, a melting-point range of 1,600 degrees, with the peak sitting in the middle.
Periodicity is the claim that none of this is random — that properties repeat in a predictable rhythm as you read the table left to right, because electron structure repeats. Section 3.2.1 asks exactly two things of you: classify any element into the s, p or d block from its electron configuration, and explain the trends in atomic radius, first ionisation energy and melting point across Period 3, sodium to argon. The explanations are machines with three moving parts — nuclear charge, shielding, distance — plus, for melting points, structure and bonding. Learn the machines properly here, because AQA re-runs them relentlessly: down Group 2 in 3.2.2, down Group 7 in 3.2.3, and across the Period 3 oxides in 3.2.4.
ModelBlocks — an element's address is its electron configuration
An element's block is named after the sub-shell that holds its outermost, highest-energy electrons. Sodium is 1s²2s²2p⁶3s¹: the last electron sits in an s sub-shell, so sodium is s block. Chlorine is 1s²2s²2p⁶3s²3p⁵ — p block. Iron is 1s²2s²2p⁶3s²3p⁶3d⁶4s²: the electrons that define its chemistry are filling 3d, so iron is d block. Read across a period and you watch the sub-shells fill in order; read down the table and the blocks form solid rectangles — Groups 1 and 2 are the s block, the six right-hand groups are the p block, and the transition-metal slab between them is the d block.
The classification is done by configuration, not by where the printer happened to put the element. Helium is the standing example: it sits above the noble gases because it behaves like one, but its configuration is 1s² — by the rule AQA uses, helium belongs to the s block. Expect exactly that kind of question, because it tests whether you know the rule or just the picture.
Why examiners care: the block tells you the chemistry before you do any. s-block elements hold one or two easily-removed outer electrons and form ionic compounds as +1 and +2 metals; the p block runs the whole spectrum from metals through metalloids to non-metals and noble gases; the d block brings variable oxidation states, colour and catalysis (that story is 3.2.5). The two skills to drill: given a configuration or atomic number, name the block; given a period and group, write the configuration.
DataAtomic radius — seventeen protons beat eleven
Across Period 3 the atoms shrink: sodium's atomic radius is 0.191 nm, chlorine's is 0.099 nm — very nearly halved, in a row where every element gains electrons on the last one. The wrong instinct is that more electrons means more atom. What actually decides the size is the tug-of-war between the nucleus and the outer shell, and it has three parts, which is how AQA marks it.
First, nuclear charge climbs from +11 at sodium to +17 at chlorine. Second, each added electron joins the same third shell — no new shell opens until the next period. Third, because the inner screen is the same ten electrons (1s²2s²2p⁶) all the way across, shielding stays effectively constant. Rising pull, unchanging screen: every outer electron is held more tightly, and the whole shell is drawn inwards. That is the complete argument, and writing all three parts is what separates two marks from one.
Argon is usually missing from radius tables, and the reason is honest measurement, not chemistry: these values are covalent or metallic radii, deduced from bond lengths, and argon forms no bonds to measure. This little trend earns its keep later — the shrink across a period and the growth down a group between them organise half the data tables in inorganic chemistry.
DataFirst ionisation energy — a rising staircase with two broken steps
The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions: \(\text{Na(g)} \rightarrow \text{Na}^+\text{(g)} + \text{e}^-\). Every word is load-bearing — gaseous appears twice, and dropping either state symbol in the equation version costs the mark.
The Period 3 data, in kJ mol⁻¹: Na 496, Mg 738, Al 578, Si 789, P 1,012, S 1,000, Cl 1,251, Ar 1,521. The general rise is the same three-part machine as radius: nuclear charge up, shielding constant, radius shrinking, so the outer electron is progressively harder to pull off. But the staircase has two broken steps, and they are the most examined numbers in the section.
Aluminium (578) sits below magnesium (738). Magnesium's outer electron leaves 3s; aluminium's leaves 3p, a sub-shell higher in energy and slightly further out, and shielded a little by the 3s pair beneath it. The 3p electron is simply cheaper to remove, despite aluminium's extra proton. Sulfur (1,000) sits below phosphorus (1,012). Phosphorus is 3p³ — one electron in each p orbital. Sulfur is 3p⁴ — for the first time in the period, two electrons share one p orbital, and their mutual repulsion offsets the extra nuclear charge, making one of the pair easier to evict. The dips matter beyond the marks: a model of the atom with only whole shells predicts a smooth rise, so the kinks at Al and S are direct experimental evidence that sub-shells exist.
A standard four-marker: explain why aluminium has a lower first ionisation energy than magnesium, and sulfur lower than phosphorus. Model answer, mark by mark. Aluminium: the electron removed comes from the 3p sub-shell, which is higher in energy than magnesium's 3s (1); it is also shielded by the 3s² electrons, so less energy is needed despite the greater nuclear charge (1). Sulfur: in 3p⁴, two electrons are paired in the same p orbital, whereas phosphorus's 3p³ are all unpaired (1); repulsion between the paired electrons makes one easier to remove (1). Keep the two explanations in their own boxes — the aluminium dip is a sub-shell energy argument and the sulfur dip is a pairing repulsion argument. Writing 'sulfur's electron is further out' transplants the wrong mechanism and scores nothing.
DataMelting points — one row, four kinds of solid
The data first, in °C: Na 98, Mg 650, Al 660, Si 1,410, P 44, S 115, Cl −101, Ar −189. Four different kinds of structure are hiding in that list, and every melting-point mark comes from naming the structure and the force that actually breaks.
Sodium to aluminium — metallic lattices. Melting a metal means loosening the attraction between positive ions and the sea of delocalised electrons. Across these three, the ion charge steps up +1, +2, +3, each atom donates one more electron to the sea, and the ions get smaller — three reasons the electrostatic grip strengthens, so melting points climb. Be honest with the data, though: aluminium melts only 10 °C above magnesium. The model reliably predicts the direction of the trend; it does not promise the size of each step.
Silicon — macromolecular. A giant covalent lattice, every atom bonded to four others. There are no molecules and no weak forces to exploit: melting silicon means snapping strong covalent bonds throughout the structure, which is why it towers at 1,410 °C.
Phosphorus, sulfur, chlorine — molecular. P₄, S₈ and Cl₂ are small molecules held to each other by van der Waals forces, and only those inter-molecular forces break on melting; the covalent bonds inside each molecule survive intact. Strength tracks the number of electrons per molecule: S₈ carries 128 electrons, P₄ has 60, Cl₂ has 34 — which is exactly the order S (115 °C) > P (44 °C) > Cl (−101 °C), and why sulfur outmelts phosphorus despite coming later in the period. Argon ends the row as single atoms with the feeblest dispersion forces of all: −189 °C.
A five-mark classic: explain why silicon's melting point is far higher than sulfur's, and why sulfur's is higher than phosphorus's. Silicon is macromolecular (giant covalent) (1); melting requires breaking many strong covalent bonds, which needs a very large amount of energy (1). Sulfur and phosphorus are molecular — S₈ and P₄ — so melting breaks only weak van der Waals forces between molecules, not the covalent bonds within them (1). S₈ has more electrons than P₄ (1), so the induced-dipole attractions between its molecules are stronger and need more energy to overcome (1). The two answers that score zero every session: 'sulfur's covalent bonds are stronger than phosphorus's' (those bonds do not break on melting) and 'silicon has strong intermolecular forces' (a macromolecular lattice has no separate molecules at all).
CaseTwo machines answer every 3.2.1 question
Strip the section to its engine room and there are only two explanation machines. Machine one — charge, shielding, distance. It answers every atomic radius and ionisation energy question: state what happens to nuclear charge, state what happens to shielding, conclude what happens to the attraction on the outer electron. Across a period, charge wins because shielding is frozen; the anomalies at aluminium and sulfur are the only places the machine needs its sub-shell attachment. Machine two — structure, force broken, why that force varies. It answers every melting-point question: name the structure type (metallic, macromolecular, molecular, monatomic), name the force that breaks on melting, then explain why that force is stronger or weaker than the comparison element's.
The exam format is nearly always data-response: a table of values, a 'describe the trend' command worth one mark (quote the numbers, including units), then 'explain' commands worth two to four (run the right machine), and one anomaly to account for. Practise deciding which machine the question wants before writing — the most common wasted paragraph in 3.2.1 answers is a beautifully explained wrong mechanism.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Trend questions are marked as machines with named parts. For radius and ionisation energy, AQA's mark schemes credit three separate points — nuclear charge, shielding, and the resulting attraction on the outer electron — so write all three as their own sentences rather than one fused clause the examiner has to dissect. 'Describe the trend' is a data command: quote values with units (496 rising to 1,521 kJ mol⁻¹), because 'it increases' without numbers routinely drops the mark.
The ionisation energy definition is an AO1 gift with teeth: one mole of electrons, gaseous atoms, gaseous 1+ ions. If you give the equation instead, both state symbols must read (g) — \(\text{Na(g)} \rightarrow \text{Na}^+\text{(g)} + \text{e}^-\) — and examiners' reports single out the missing (g) on the ion as a perennial lost mark.
Melting-point explanations are marked on naming the structure AND the force broken: metallic lattice/attraction between ions and delocalised electrons; macromolecular/covalent bonds; molecular/van der Waals between molecules. Keep the anomaly explanations in their correct boxes — 3p sub-shell energy for aluminium, paired-electron repulsion for sulfur — and when a question hands you a table with an unfamiliar element in it, trust the machines: they are being tested, not your recall of that element.