Lesson overview

Covalent molecules: How do atoms share electrons?

Hydrogen has one outer electron and chlorine has seven: how do both get a full outer shell? Predict, then share pairs of electrons between atoms on a board and watch each atom’s count, building hydrogen chloride, water and ammonia, and nitrogen in the Supplement.

Suggested sequence

Lesson 1 · Sharing electrons. Shared pairs and full outer shells; dot-and-cross diagrams for H₂, Cl₂, H₂O, CH₄, NH₃ and HCl; Supplement: CH₃OH, C₂H₄, O₂, CO₂ and N₂. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
Lesson 2 · Molecules that melt easily. Why simple molecular compounds melt and boil at low temperatures and don’t conduct; Supplement: the explanation, weak forces between molecules and no moving charges. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
One to two weeks later · return questions. 2 questions, 15 min, lesson closed.

Learning goals

Part 1: Sharing electrons

State that a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations
Describe the formation of covalent bonds in simple molecules, including H₂, Cl₂, H₂O, CH₄, NH₃ and HCl, using dot-and-cross diagrams
Describe the formation of covalent bonds in simple molecules, including CH₃OH, C₂H₄, O₂, CO₂ and N₂, using dot-and-cross diagrams

Part 2: Molecules that melt easily

Describe in terms of structure and bonding the properties of simple molecular compounds: low melting points and boiling points, poor electrical conductivity
Explain in terms of structure and bonding the properties of simple molecular compounds: low melting points and boiling points in terms of weak intermolecular forces, poor electrical conductivity

Misconceptions to listen for

“One atom gives its electron to the other, as in ionic bonding.” — Two non-metal atoms both need electrons, so neither gives one away. They share a pair, and no ions form.
“A covalent bond is the attraction between oppositely charged ions.” — That is the ionic bond. Covalent bonds join non-metal atoms, which share electrons instead.
“A covalent bond is a weak attraction between neighbouring molecules.” — That describes the forces between molecules. A covalent bond holds two atoms together inside one molecule, and it is strong.
“Row B.” — Each hydrogen atom counts both electrons of the shared pair, its own and its partner’s: 2, like helium.
“Row C.” — 6 is the number of unshared electrons on iodine. They sit in pairs: 6 ÷ 2 = 3 lone pairs.
“Row D.” — A covalent bond is a pair: each hydrogen atom gives one electron, so 2 electrons are shared, and each atom counts both.
“Phosphorus has 5 outer electrons, so it bonds to 5 hydrogen atoms (PH₅).” — The bonds equal the electrons it needs, 8 − 5 = 3, not the electrons it has: PH₃.
“Sulfur has 6 outer electrons, so it bonds to 6 hydrogen atoms (H₆S).” — Sulfur needs only 8 − 6 = 2 more electrons, so it shares 2 pairs: H₂S.
“The diagram is right, because each hydrogen has 2.” — Check every atom: the hydrogens are full, but oxygen counts only 6.
“A shared pair counts for only one of the two atoms.” — It counts for both atoms. Each atom adds both electrons of every pair it shares to its own count.

Scope

Simple molecules and covalent bonds (2.5 Core 1–3, Supplement 4–5): the covalent bond as a shared pair of electrons that gives noble-gas electronic configurations, dot-and-cross diagrams for H₂, Cl₂, H₂O, CH₄, NH₃ and HCl and, in the Supplement, CH₃OH, C₂H₄, O₂, CO₂ and N₂, and the low melting and boiling points and poor electrical conductivity of simple molecular compounds, explained in the Supplement by weak intermolecular forces and no moving charged particles. Giant covalent structures (2.6), metallic bonding (2.7) and formulae (3.1) have their own modules; the shapes of molecules are not in the syllabus.

Open the first part as pupils see it