Contents: 6 sections
Syllabus points
- Describe and explain the trends across Period 3 in atomic radius, ionic radius, melting point and electrical conductivity.
- Describe and explain the trend in first ionisation energy across Period 3.
- Describe the reactions of the Period 3 elements with oxygen, chlorine and water.
- Describe and explain the acid and base behaviour of the Period 3 oxides.
- Describe the reactions of the Period 3 chlorides with water.
Physical trends across Period 3
Atomic radius decreases from sodium to chlorine. Each step adds a proton to the nucleus and an electron to the same shell, so shielding is essentially unchanged while the nuclear charge rises. The outer electrons are pulled in more tightly.
Ionic radius shows a break rather than a smooth trend, and the break is where the ion changes sign. Na⁺, Mg²⁺ and Al³⁺ have all lost their outer shell, so they are much smaller than their atoms and shrink across the three as the charge rises. P³⁻, S²⁻ and Cl⁻ have gained electrons into the outer shell, so they are larger than their atoms, and they shrink across the three for the same reason. The step up between Al³⁺ and P³⁻ is the feature to know.
First ionisation energy rises across the period, for the same reason the radius falls, with the two dips covered in topic 1: Mg to Al, where the electron comes from a higher-energy 3p orbital, and P to S, where a p electron must pair up.
Melting point and conductivity
These follow structure, not a simple trend, and the period divides into three blocks:
| Elements | Structure | Melting point | Conductivity |
|---|---|---|---|
| Na, Mg, Al | Giant metallic | Rises across the three | High, rising across the three |
| Si | Giant covalent | Very high, the maximum | Semiconductor |
| P, S, Cl, Ar | Simple molecular | Low | None |
Sodium to aluminium rise in melting point because each ion carries a larger charge and contributes more delocalised electrons, so the metallic bonding strengthens. Conductivity rises for the same reason.
Silicon is the peak. It is giant covalent, and melting it means breaking strong covalent bonds throughout the structure.
After silicon the melting points collapse, because only weak intermolecular forces hold the molecules together. Within that block the order follows molecular size: S₈ melts higher than P₄, which melts higher than Cl₂, and argon, a single atom, is lowest of all.
Reactions of the elements
With oxygen, the elements form oxides, and the vigour falls across the period:
- Sodium burns with a yellow flame to Na₂O
- Magnesium burns with a brilliant white flame to MgO
- Aluminium burns to Al₂O₃, though the oxide layer slows it
- Silicon needs strong heating to SiO₂
- Phosphorus burns readily to P₄O₁₀
- Sulfur burns with a blue flame to SO₂
With chlorine, the same pattern gives NaCl, MgCl₂, Al₂Cl₆, SiCl₄, PCl₅ and S₂Cl₂.
With water, only the first two react appreciably:
- Sodium reacts vigorously with cold water, floating and fizzing, giving NaOH and hydrogen. The solution is strongly alkaline, about pH 13.
- Magnesium reacts very slowly with cold water, giving Mg(OH)₂, which is only slightly soluble, so the pH is about 10. With steam it reacts rapidly to give MgO and hydrogen.
The oxides
The acid and base behaviour of the oxides changes across the period, and this is the heart of the topic.
| Oxide | Bonding | Behaviour | pH of the solution |
|---|---|---|---|
| Na₂O | Ionic | Basic | 13 to 14 |
| MgO | Ionic | Basic | 9 (only slightly soluble) |
| Al₂O₃ | Ionic with covalent character | Amphoteric | 7, insoluble |
| SiO₂ | Giant covalent | Weakly acidic | 7, insoluble |
| P₄O₁₀ | Simple molecular | Acidic | 2 |
| SO₂ and SO₃ | Simple molecular | Acidic | 1 to 3 |
The trend is basic to amphoteric to acidic, and it tracks the bonding: metal oxides are ionic and basic, non-metal oxides are covalent and acidic, and aluminium sits on the boundary.
Amphoteric means reacting with both acids and bases, and Al₂O₃ is the example to know:
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
Silicon dioxide is a useful special case: it is insoluble in water, so it does not produce an acidic solution, but it does react with hot concentrated alkali, which is what makes it acidic in character.
The chlorides in water
The chlorides split into two groups, and the difference is bonding again.
Ionic chlorides dissolve. NaCl and MgCl₂ simply dissociate into ions. The solution of NaCl is neutral; MgCl₂ is very slightly acidic, around pH 6.5, because the small highly charged Mg²⁺ ion polarises the water around it.
Covalent chlorides hydrolyse. They react with water rather than dissolving in it, releasing hydrogen chloride fumes and giving strongly acidic solutions:
SiCl₄ + 2H₂O → SiO₂ + 4HCl
PCl₅ + 4H₂O → H₃PO₄ + 5HCl
Aluminium chloride sits between the two, hydrolysing to give an acidic solution of about pH 3.
Steamy fumes of HCl when a chloride meets water is the observation that identifies a covalent chloride, and it is worth remembering as an observation rather than only as an equation.
Common mistakes
- Saying atomic radius increases across a period. It decreases.
- Explaining the melting point trend with one mechanism. It changes with structure, and silicon is the peak.
- Saying Al₂O₃ is basic or acidic. It is amphoteric, and the mark is for saying it reacts with both.
- Saying SiO₂ is basic because it is insoluble. It is weakly acidic and reacts with hot concentrated alkali.
- Saying magnesium reacts vigorously with cold water. It is very slow with cold water and fast only with steam.
- Forgetting that covalent chlorides hydrolyse rather than dissolve, and that the fumes are HCl.