The Periodic Table: Chemical Periodicity

Inorganic Chemistry Weightage: AS · Papers 1, 2 and 3 (assumed in Papers 4 and 5) Topic 9

“Welcome! Period 3 is a story with one plot: as the nuclear charge rises, the structure changes from metallic to giant covalent to simple molecular, and everything else (melting point, bonding, acidity) follows that structure. Learn the plot, then attach the equations and pH values to it.”

— SCORECHEM ACADEMIC TEAM

0. What Examiners Want

How marks are earned in periodicity

Trends: describe the trend (one mark), then explain it with nuclear charge, shielding, structure or forces (the rest).
Equations: correct formulae, balanced, with state symbols when asked. P₄O₁₀ and Al₂Cl₆ are the usual traps.
pH: quote a value or range and say acidic, alkaline or neutral, with the equation for the species that produces the pH.
Bonding: link the type of oxide or chloride to the electronegativity difference using Data Booklet values.
Unknown element questions: use every clue (melting point, conduction, reaction with water, acid or alkali) and state the group and the reasoning.

1. Physical Trends Across Period 3

Periodicity is the recurrence of similar properties at regular intervals when the elements are arranged in order of atomic number. Period 3 is Na, Mg, Al, Si, P, S, Cl and Ar.

Atomic radius and ionic radius

Atomic radius decreases from Na to Cl: the nuclear charge increases, the extra electrons enter the same shell, so the shielding is about constant and the outer electrons are pulled in more strongly.
Cations are smaller than their atoms (one electron shell lost, and the same protons attract fewer electrons).
Anions are larger than their atoms (extra electrons repel each other while the nuclear charge is unchanged).
Across the cations Na⁺ to Si⁴⁺, and across the anions P³⁻ to Cl⁻, the radius decreases as the nuclear charge rises. There is a large jump from the cations to the anions, because the anions have an extra shell.

Atomic radius decreases across Period 3 (more protons, same shielding). Cations are smaller than their atoms and anions are larger; within each set the ions also get smaller as the nuclear charge increases.

	Na	Mg	Al	Si	P	S	Cl
Atomic radius / nm	0.157	0.136	0.125	0.117	0.110	0.104	0.099
Ion	Na⁺	Mg²⁺	Al³⁺	Si⁴⁺	P³⁻	S²⁻	Cl⁻
Ionic radius / nm	0.095	0.065	0.050	0.041	0.212	0.184	0.181

⚠️ Examiner Trap: Isoelectronic ions. Na⁺, Mg²⁺ and Al³⁺ have the same electron configuration, so the ion with the most protons is the smallest. Always compare the numbers of protons and shells. Do not say that “more electrons” makes an ion bigger without mentioning the shell.

First ionisation energy (recap from Topic 1)

The general increase across the period is due to the rising nuclear charge with similar shielding. Two dips show the structure of the atoms. The values below are from the Data Booklet.

	Na	Mg	Al	Si	P	S	Cl	Ar
1st IE / kJ mol⁻¹	494	736	577	786	1060	1000	1260	1520

Mg to Al: the Al electron is removed from a 3p sub-shell, which is higher in energy and shielded by the 3s electrons.
P to S: in S, two electrons share one 3p orbital and repel each other, so one is easier to remove.

Melting point and electrical conductivity

Melting points across Period 3 follow the structure: metallic bonding gets stronger from Na to Al, silicon has a giant covalent lattice (the peak), and P₄, S₈, Cl₂ and Ar have only weak instantaneous dipole–induced dipole forces.

Element	Structure and bonding	Why the melting point is as shown
Na, Mg, Al	giant metallic	strong attraction between the positive ions and the delocalised electrons; it increases with the charge on the ion (1+, 2+, 3+) and the number of delocalised electrons per atom (1, 2, 3)
Si	giant covalent	many strong covalent bonds must be broken: the highest melting point
P₄, S₈, Cl₂	simple molecular	only weak instantaneous dipole–induced dipole forces between molecules are overcome; S₈ is higher than P₄ and Cl₂ because larger molecules (more electrons) give stronger forces
Ar	simple atomic	very weak forces between single atoms: the lowest value

Conductivity: Na, Mg and Al conduct because of delocalised electrons, and the conductivity increases Na < Mg < Al as more electrons per atom are delocalised. Silicon is a semiconductor (it has no delocalised electrons, but a small number can be promoted). P, S, Cl₂ and Ar have no mobile charge carriers and do not conduct.

⚠️ Examiner Trap: Which bonds break? When a simple molecular substance melts, the intermolecular forces are overcome. The covalent bonds within the molecules stay. For silicon, the covalent bonds are the structure, so they all must be broken. Mixing these up loses both marks.

2. Reactions of the Elements

With oxygen

Element	Observation	Equation
Na	burns vigorously with a bright yellow flame, white solid	4Na(s) + O₂(g) → 2Na₂O(s)
Mg	burns with a bright white flame, white solid	2Mg(s) + O₂(g) → 2MgO(s)
Al	the powder burns with a white flame (the bulk metal is protected by its oxide layer)	4Al(s) + 3O₂(g) → 2Al₂O₃(s)
Si	reacts slowly when heated strongly	Si(s) + O₂(g) → SiO₂(s)
P	burns with a white or yellow flame, white clouds	P₄(s) + 5O₂(g) → P₄O₁₀(s)
S	burns gently with a blue flame, choking gas	S(s) + O₂(g) → SO₂(g)

Sulfur dioxide is oxidised further to sulfur trioxide with a V₂O₅ catalyst: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). Chlorine and argon do not react directly with oxygen.

With chlorine

Element	Equation	Notes
Na	2Na(s) + Cl₂(g) → 2NaCl(s)	vigorous, white solid
Mg	Mg(s) + Cl₂(g) → MgCl₂(s)	vigorous
Al	2Al(s) + 3Cl₂(g) → Al₂Cl₆(s)	allow AlCl₃ (2Al + 3Cl₂ → 2AlCl₃)
Si	Si(s) + 2Cl₂(g) → SiCl₄(l)	slower
P	2P(s) + 5Cl₂(g) → 2PCl₅(s)	excess chlorine; with limited chlorine, PCl₃ forms

With water (Na and Mg only)

Sodium: reacts vigorously with cold water, melts into a ball and moves on the surface; the gas is hydrogen; the solution is strongly alkaline (pH about 14). 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Magnesium: reacts extremely slowly with cold water, giving a weakly alkaline solution (pH about 9–10) because Mg(OH)₂ is only slightly soluble. Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)
Magnesium with steam: reacts vigorously to give the oxide: Mg(s) + H₂O(g) → MgO(s) + H₂(g)

⚠️ Examiner Trap: Why is NaOH more alkaline than Mg(OH)₂? Not because the reaction is faster, but because NaOH is much more soluble, so the solution contains a higher concentration of OH⁻. Say “a higher [OH⁻]”.

3. Oxidation Numbers and Electronegativity

In each oxide the element is positive (oxygen is more electronegative, oxidation number −2). The maximum oxidation number rises across the period because each element can use all its outer-shell electrons: Na₂O (+1), MgO (+2), Al₂O₃ (+3), SiO₂ (+4), P₄O₁₀ (+5), SO₂ (+4), SO₃ (+6) and Cl₂O₇ (+7). The chlorides follow the same rule to phosphorus: NaCl (+1), MgCl₂ (+2), Al₂Cl₆ (+3), SiCl₄ (+4), PCl₅ (+5).

Electronegativity is the power of an atom to attract the bonding electrons in a covalent bond. It increases across a period and decreases down a group. Pauling values (Data Booklet):

	Na	Mg	Al	Si	P	S	Cl	O
Electronegativity	0.9	1.3	1.5	1.9	2.2	2.6	3.0	3.5
Difference from O	2.6	2.2	2.0	1.6	1.3	0.9	0.5

A large difference (Na, Mg, and to a lesser extent Al) gives ionic bonding; a small difference (Si, P, S) gives covalent bonding. That is why the oxides change from ionic and basic to covalent and acidic.

4. The Oxides and Water

The Period 3 oxides change from giant ionic (basic) through giant covalent SiO₂ to simple molecular (acidic). Al₂O₃ is amphoteric. pH values are for the solution formed with water.

Oxide	Reaction with water	Solution
Na₂O	Na₂O(s) + H₂O(l) → 2NaOH(aq)	strongly alkaline, pH 13–14
MgO	MgO(s) + H₂O(l) → Mg(OH)₂(aq)	weakly alkaline, pH 9–10 (Mg(OH)₂ is only slightly soluble)
Al₂O₃	insoluble: the lattice is too strong	no reaction
SiO₂	insoluble: giant covalent	no reaction
P₄O₁₀	P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)	acidic, pH 1–2
SO₂	SO₂(g) + H₂O(l) → H₂SO₃(aq)	acidic, pH 1–3
SO₃	SO₃(g) + H₂O(l) → H₂SO₄(aq)	strongly acidic, pH 1–2

Why the oxide is basic or acidic

Ionic oxides (Na₂O, MgO) contain the O²⁻ ion, a proton acceptor (a base): O²⁻ + H₂O → 2OH⁻. They neutralise acids: MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l).
Covalent oxides of non-metals (P₄O₁₀, SO₂, SO₃) react with water to give acids, which donate H⁺.
SiO₂ does not dissolve, but it is acidic because it reacts with hot concentrated alkali: SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l). It does not react with acids.
P₄O₁₀ and SO₃ also react with alkali, e.g. SO₂ + 2NaOH → Na₂SO₃ + H₂O.

⚠️ Examiner Trap: “Dissolves” is not the same as “reacts”. Na₂O and P₄O₁₀ react with water to make a new substance (NaOH, H₃PO₄). Write the equation; never write “Na₂O dissolves to give Na₂O(aq)”. And SiO₂ and Al₂O₃ have no reaction with water, so do not give them a pH.

5. Amphoteric Behaviour

An amphoteric substance reacts with both acids and bases (alkalis). Aluminium oxide and aluminium hydroxide are the examples on the syllabus.

Amphoteric compounds react with both acids (as bases) and alkalis (as acids). Aluminium oxide and hydroxide are the examples to know; they show that aluminium oxide bonding is neither purely ionic nor purely covalent.

Reagent	Al₂O₃	Al(OH)₃
Acid (as a base)	Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)	Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l)
Alkali (as an acid)	Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq)	Al(OH)₃(s) + NaOH(aq) → NaAl(OH)₄(aq)

Mg(OH)₂ is a base only: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l), and it does not dissolve in excess NaOH. Sodium hydroxide is a strong base.

Because aluminium oxide behaves both like an ionic metal oxide (reacts with acids) and like a covalent oxide (reacts with alkalis), its bonding is neither purely ionic nor purely covalent: the small, highly charged Al³⁺ ion distorts the oxide ion, giving some covalent character.

⚠️ Examiner Trap: Amphoteric needs two reactions. Quote both reagents and both products. An answer with only the acid reaction earns half the marks. The alkali reaction needs hot, concentrated alkali for the oxide, and the product is the aluminate, not Al(OH)₃.

6. The Chlorides and Water

The Period 3 chlorides change from ionic (dissolve, about neutral) to covalent (hydrolysed by water, acidic). Aluminium chloride exists as the dimer Al₂Cl₆ without water.

Chloride	With water	pH
NaCl	dissolves: NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq); hydrated ions, no reaction	about 7
MgCl₂	dissolves: MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl⁻(aq); very slight hydrolysis	about 6.5
Al₂Cl₆	Al₂Cl₆(s) + 12H₂O(l) → 2[Al(H₂O)₆]³⁺(aq) + 6Cl⁻(aq), then [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺; white fumes of HCl	about 3
SiCl₄	SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(g); white fumes, white solid	about 2
PCl₅	PCl₅(s) + 4H₂O(l) → H₃PO₄(aq) + 5HCl(g); white fumes	about 2

Ionic chlorides (NaCl, MgCl₂) are dissolved by the polar water molecules, which attract the ions. No new substance forms.
Covalent chlorides are hydrolysed: water molecules attack the polar bonds, breaking them to give an acid (HCl, plus H₃PO₄ for P) and, for silicon, the oxide.
Aluminium chloride is covalent as the dimer Al₂Cl₆. In water, the small and highly charged Al³⁺ hexaaqua ion polarises the O–H bonds of its water ligands and releases H⁺, so the solution is acidic.

In the vapour (and as the anhydrous solid) aluminium chloride is the covalent dimer Al₂Cl₆. In water the small, highly charged Al³⁺ ion polarises its water molecules, releasing H⁺, so the solution is acidic (pH about 3).

⚠️ Examiner Trap: Hydrolysis products. SiCl₄ gives SiO₂ and HCl, while PCl₅ gives H₃PO₄ and HCl (and PCl₃ gives H₃PO₃ and HCl). The white fumes are HCl gas (not steam and not smoke). The acidic pH comes from the HCl and the acid formed.

7. Deducing an Unknown Element

Work through the clues in order, and write the conclusion each clue supports.

Clue	Conclusion
high melting point, conducts when solid	metal (Group 1, 2 or 13 in Period 3)
high melting point, does not conduct	giant covalent: Group 14 (Si or similar)
low melting point, does not conduct	simple molecular: Groups 15–18
chloride dissolves, pH about 7	ionic chloride: Group 1 or 2
chloride fumes in water, pH 1–2	covalent chloride: Group 14 or 15
oxide dissolves in both acid and alkali	amphoteric: Group 13 (Al)
oxide insoluble but dissolves in hot concentrated alkali	acidic giant oxide: Group 14 (Si)
oxide gives an acid with pH 1–2	non-metal oxide: Group 15 or 16

Predicting properties in a group: use the Period 3 element as the model. For example, selenium (Group 16, Period 4) should be a simple molecular non-metal, with a low melting point, no conduction, and a covalent chloride that is hydrolysed by water; germanium (Group 14) should have a giant covalent structure like silicon.

⚠️ Examiner Trap: Period 2 is different. Carbon dioxide is a gas (simple molecular), but silicon dioxide is a solid (giant covalent), although both elements are in Group 14. If a clue mentions a solid with a high melting point, the element is in Period 3 or lower.

8. Quick Sheet and Checklist

Property	Na	Mg	Al	Si	P	S	Cl
Structure	giant metallic	giant metallic	giant metallic	giant covalent	simple molecular	simple molecular	simple molecular
Oxide	Na₂O	MgO	Al₂O₃	SiO₂	P₄O₁₀	SO₂, SO₃	Cl₂O₇
Oxide nature	basic	basic	amphoteric	acidic	acidic	acidic	acidic
Chloride	NaCl	MgCl₂	Al₂Cl₆	SiCl₄	PCl₅
Chloride pH	7	6.5	3	2	2

Before you leave the question, check:

A trend answer has the description and the explanation (nuclear charge, shielding, structure, forces).
Equations are balanced, use P₄O₁₀ and Al₂Cl₆ correctly, and have state symbols if asked.
Each pH has a value or range, the word acidic or alkaline, and an equation.
Bonding answers quote Data Booklet electronegativity values and the difference.
Amphoteric answers give both the acid and the alkali reaction.