Covalent bond = a shared pair of electrons
A covalent bond is a shared pair of electrons between two atoms; both shared and pair are needed for the mark. A single bond is one shared pair, a double bond two, and a triple bond three (Extended). The molecules to know are mostly single-bonded — , , HCl, , , — while and have double bonds and a triple bond.
Weak intermolecular forces set the properties
Simple molecular substances have low melting and boiling points and do not conduct electricity. The low melting and boiling points arise because the forces between molecules are weak, so little energy separates them — the strong covalent bonds inside each molecule are not broken. They do not conduct because they contain no ions and no free (delocalised) electrons, so there are no mobile charged particles to carry a current.
(Extended) Dot-and-cross: bonding and lone pairs
Show only the outer electrons, using dots for one atom and crosses for the other. A bonding pair is one dot and one cross shared between the atoms; a lone pair is a non-bonding pair on one atom. Every atom should reach a full outer shell (8 electrons, or 2 for hydrogen). Counts: H forms 1 bond; C forms 4; N forms 3 bonds with 1 lone pair; O forms 2 bonds with 2 lone pairs; Cl forms 1 bond with 3 lone pairs.
Drawn from real examiner reports.
Shared pair — sharing electrons is too vague
Answers such as atoms share electrons or sharing of electrons are given no credit. The mark scheme needs the word pair: a covalent bond is a shared pair of electrons between two atoms. Pair is load-bearing because a single bond is one shared pair (2 electrons) and a double bond is two (4 electrons). Without pair, the definition is too vague to score.
Melting breaks forces, not covalent bonds
Writing that the covalent bonds are weak or break on heating is wrong — the covalent bonds within a molecule are strong. What is overcome on melting or boiling is the weak intermolecular forces between molecules. Correct: methane has a low boiling point because the forces between molecules are weak, so little energy separates them.
No conduction needs no ions and no electrons
When explaining why a simple molecular substance does not conduct, saying it has no ions alone is incomplete. It must also have no free (delocalised) electrons. With neither ions nor mobile electrons there are no charged particles to carry a current, in any state. State both parts — no ions and no free electrons — to secure the mark.
Covalent, ionic and metallic bonds differ
Keep the three bond definitions apart. Covalent is a shared pair of electrons between two atoms. Ionic is the electrostatic attraction between oppositely charged ions. Metallic is the attraction between positive ions and a sea of delocalised electrons. Describing a covalent bond as transfer (ionic) or delocalised electrons (metallic) loses the mark.
Dot-and-cross: keep lone pairs and octets
Common errors are drawing inner shells, leaving out lone pairs, and wrong outer-electron counts. Show only outer electrons. Include the lone pairs — one on N in , two on O in — as they are the most-omitted feature. Every atom must reach a full outer shell: 8, or 2 for H.
W22 Paper 31 Q2(b); S22 Paper 31 Q5(e)(iv) — outer-shell electron counts in bonding diagrams drawn wrongly.
Double bonds need two shared pairs
Molecules such as and contain double bonds — two shared pairs between the same two atoms — so a diagram must show both pairs. Drawing only one bonding pair, or forgetting that has a C=O double bond at each end, leaves the atoms short of a full outer shell and loses the mark.
Check each atom reaches a full shell
For each atom, count its lone-pair electrons plus both electrons of each shared pair. The total must be 8 for most atoms, or 2 for H. In , O has 2 lone pairs (4) plus 2 bonding pairs (4) = 8 and each H reaches 2. If an atom falls short, the diagram is wrong.
State both parts of each definition
Give each required element as its own clear point. For a covalent bond, both shared and pair of electrons. For a low melting point, weak intermolecular forces, not weak covalent bonds. For non-conduction, no ions and no free electrons.
Count bonds from the molecular formula
Count covalent bonds from the formula: has two O–H bonds, three, four, while has two C=O double bonds and one. Match each H or halogen to one bond and check the central atom fills its shell.
Covalent bond: A shared pair of electrons between two atoms. Both words are required in an exam answer — "pair" (exactly 2 electrons) and "shared" (between the two atoms). Writing "atoms share electrons" is too vague and does not score the definition mark.
Simple molecule: A small group of atoms held together by covalent bonds.
Single bond: One shared pair of electrons between two atoms. Shown as a single line (–) in a displayed formula.
Double bond: Two shared pairs of electrons between two atoms. Shown as a double line (=).
(Extended) Triple bond: Three shared pairs of electrons between two atoms (e.g. ). Shown as ≡.
Intermolecular forces: The weak forces of attraction between molecules (as opposed to the strong covalent bonds within molecules). These are the forces that must be overcome to melt or boil a simple molecular substance.
Define a covalent bond.
Calculate the relative formula mass () of each of the following simple molecules.
(a) Methane, (: C = 12, H = 1)
(b) Water, (: H = 1, O = 16)
