What are non-metals?
About 17 elements are non-metals. They sit on the right side of the periodic table, plus hydrogen. Some are gases (hydrogen, nitrogen, oxygen, chlorine), one is a liquid (bromine) and the rest are solids (carbon, sulfur, phosphorus, iodine).
Usual properties: dull, brittle when solid, poor conductors of heat and electricity (graphite and silicon are exceptions), low melting and boiling points. Their atoms have 4 to 7 outer electrons, so they like to gain or share electrons.
Hydrogen with non-metals
Hydrogen has one electron and needs one more, so it shares electrons with other non-metals. This gives molecules held by covalent bonds:
- H₂ + Cl₂ → 2HCl (in sunlight, or with a flame, it can be explosive)
- 2H₂ + O₂ → 2H₂O (a flame or spark starts it)
- N₂ + 3H₂ ⇌ 2NH₃ (iron catalyst, high pressure, about 450 °C)
- H₂ + S → H₂S (when hot)
- C + 2H₂ → CH₄ (needs a catalyst and high temperature, so methane is usually taken from natural gas)
Pattern: the number of H atoms equals the number of electrons the other atom is short of. Cl is short of 1 (HCl), O of 2 (H₂O), N of 3 (NH₃), C of 4 (CH₄). Molecular compounds like these have low melting points and do not conduct electricity as solids.
Chlorine with metals
Chlorine is a strong electron taker. A metal gives its outer electrons and becomes a positive ion; each chlorine atom becomes a Cl⁻ ion. The ions attract and pack in a ionic lattice.
- 2Na + Cl₂ → 2NaCl (sodium burns in chlorine with a bright yellow flame, white smoke of NaCl)
- Mg + Cl₂ → MgCl₂ (Mg gives 2 electrons, so 2 Cl⁻ are needed)
- 2Fe + 3Cl₂ → 2FeCl₃ (hot iron wool; chlorine is strong enough to make iron 3+)
Ionic chlorides are hard crystals with high melting points. They dissolve in water, and the solution (or the melt) conducts electricity.
Sulfur with metals
Sulfur needs 2 electrons, so it makes S²⁻ ions. It is a weaker electron taker than chlorine, so it needs heat.
- 2Na + S → Na₂S (2 Na give 2 electrons to one S)
- Fe + S → FeS (heat a mixture of iron filings and sulfur: it glows and a dark grey solid forms, which a magnet no longer picks up as iron)
- Zn + S → ZnS, Cu + S → CuS or Cu₂S
Notice: iron with chlorine gives FeCl₃ but iron with sulfur gives FeS. The stronger oxidiser (chlorine) takes more electrons from iron.
Try it: the iron and sulfur test
At school (with a teacher), mix iron filings and powdered sulfur. Bring a magnet near: the iron comes out. Now heat the mixture in a test tube until it glows. After it cools, the magnet does not pick up the grey lump. A new substance, FeS, formed. In the 3D, predict how many metal balls each sulfur needs before you click Na₂S, FeS and MgCl₂.
Key formulas and definitions
- H₂ + Cl₂ → 2HCl
- 2H₂ + O₂ → 2H₂O; N₂ + 3H₂ ⇌ 2NH₃; C + 2H₂ → CH₄
- 2Na + Cl₂ → 2NaCl; Mg + Cl₂ → MgCl₂
- 2Na + S → Na₂S; Fe + S → FeS
- H atoms joined = electrons the other atom is short of
Worked examples
1. How many hydrogen atoms bond to one sulfur atom, and why?
Sulfur has 6 outer electrons so it needs 2 more. It holds 2 H: H₂S.
2. Write the equation for magnesium burning in chlorine.
Mg + Cl₂ → MgCl₂. Mg gives 2 electrons, each Cl takes 1.
3. Why is the formula of sodium sulfide Na₂S and not NaS?
S²⁻ has a 2− charge; each Na⁺ has 1+. You need 2 Na⁺ to balance 1 S²⁻.
4. Is HCl ionic or covalent? Why?
Covalent. H and Cl are both non-metals and share an electron pair.
5. Iron filings and sulfur are mixed and then heated. How do you show a new substance formed?
Before heating a magnet picks out the iron. After heating the grey lump is not attracted like iron. It is FeS, a different compound with new properties.
6. Balance: Fe + Cl₂ → FeCl₃.
2Fe + 3Cl₂ → 2FeCl₃. Check: 2 Fe, 6 Cl on each side.
Common mistakes
- Writing the sulfur compound of sodium as NaS. Charges must balance: Na₂S.
- Calling HCl ionic because it dissolves in water to give ions. As a gas or pure liquid it is covalent.
- Thinking iron and sulfur only mix. When heated they react and make a new compound, FeS.
- Forgetting that conditions matter: H₂ + N₂ needs a catalyst, pressure and heat, so ammonia does not form just by mixing.