Period 3: sodium to argon
Across period 3 the number of protons rises from 11 to 18 while electrons are added to the same shell, so the atoms get smaller and the ionisation energy and electronegativity generally rise. The bonding and structure change from metallic (Na, Mg, Al) through giant covalent (Si) to simple molecules (P₄, S₈, Cl₂) and single atoms (Ar).
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
|---|---|---|---|---|---|---|---|---|
| Structure | Metallic | Metallic | Metallic | Giant covalent | P₄ molecules | S₈ molecules | Cl₂ molecules | Atoms |
| Melting point | Low | Higher | Higher still | Highest | Low | Low | Very low | Very low |
| Conductivity | Good | Good | Good | Semi-conductor | None | None | None | None |
| Oxide | Na₂O | MgO | Al₂O₃ | SiO₂ | P₄O₁₀ | SO₂ / SO₃ | Cl₂O₇ |
|---|---|---|---|---|---|---|---|
| Type | Basic | Basic | Amphoteric | Acidic | Acidic | Acidic | Acidic |
| Bonding | Ionic | Ionic | Ionic with covalent character | Giant covalent | Covalent | Covalent | Covalent |
Choose the group number (1 to 7). The highest oxide has the element in the oxidation state equal to the group number.
Groups 1 and 2
Group 1 metals (Li, Na, K) have one outer electron and form 1+ ions. They react vigorously with water: 2Na + 2H₂O → 2NaOH + H₂. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded. Group 2 metals form 2+ ions and are less reactive. Their hydroxides become more soluble and their sulphates less soluble down the group.
Group 7: the halogens
| Halogen | State at 20 °C | Colour | Oxidising power |
|---|---|---|---|
| Fluorine | Gas | Pale yellow | Strongest |
| Chlorine | Gas | Green | Strong |
| Bromine | Liquid | Red-brown | Medium |
| Iodine | Solid | Grey-black (violet vapour) | Weakest |
Halogens are diatomic. They gain one electron to make halide ions. A halogen displaces a less reactive halogen from its halide. Chlorine reacts with cold dilute sodium hydroxide to make sodium chlorate(I) (bleach) and with hot concentrated alkali to make sodium chlorate(V): these are disproportionation reactions, in which the same element is both oxidised and reduced.
| Hydrogen halide | Thermal stability | Reducing power of the halide ion |
|---|---|---|
| HF | Most stable | Does not reduce sulphuric acid |
| HCl | Stable | Does not reduce concentrated H₂SO₄ |
| HBr | Less stable | Reduces H₂SO₄ to SO₂ |
| HI | Least stable | Reduces H₂SO₄ to S and H₂S |
Key terms
- Amphoteric
- Reacting with both acids and bases.
- Disproportionation
- A reaction in which one element is both oxidised and reduced.
- Diatomic
- Made of molecules containing two atoms.
- Shielding
- The repulsion by inner electrons that reduces the pull of the nucleus on outer electrons.
Summary
- Across period 3: structure changes from metallic to giant covalent to molecular; oxides change from basic to acidic.
- Group 1 and 2 reactivity increases down the group; group 7 reactivity decreases.
- Halogens displace less reactive halogens; chlorine disproportionates in alkali.
- Hydrogen halides become less stable and halide ions more reducing down group 7.