Covalent bond: a shared pair of electrons between two atoms
A covalent bond is a shared pair of electrons between two atoms — the marking elements are a pair, and that it is shared. It forms between non-metal atoms, which reach a full outer shell by sharing (not transferring) electrons. A single bond is one shared pair, a double bond two (, ), a triple bond three (). In a dot-and-cross diagram one atom's electrons are dots and the other's crosses; show only outer shells.
Simple molecular: low melting points from weak forces between molecules
Simple molecular substances (, , ) are small molecules held to each other by weak intermolecular forces. They have low melting and boiling points because only these weak forces between molecules are overcome on melting — the strong covalent bonds inside each molecule are not broken. They do not conduct in any state: the molecules are neutral, with no free electrons or ions to carry charge.
Diamond, graphite and C60: same element, different structures
Three covalent forms of carbon. Diamond (giant covalent): each carbon bonded to four others — very hard, very high melting point, no conduction (no free electrons). Graphite (giant covalent): each carbon bonded to three others in layers that slide (soft), with one delocalised electron per atom that lets it conduct. C60 fullerene is simple molecular — balls held by weak intermolecular forces, so soft with a low melting point.
Drawn from real examiner reports.
Covalent bond is a shared PAIR of electrons
Writing that a covalent bond is atoms joined together, atoms sharing, or even sharing electrons does not score — the credited form is a shared PAIR of electrons between two atoms. Do not confuse it with ionic: covalent = sharing (non-metal + non-metal); ionic = transferring electrons. Never call a covalent bond an attraction between ions, or mention cations.
Covalent bonding described as sharing electrons without a shared PAIR, or referring to cations (implies metallic), not credited (Nov24 Q6cii; Jun24 1CR Q11b).
Weak forces are BETWEEN molecules
When explaining a simple molecular substance's low melting point, the bonds inside each molecule are strong and not broken on melting. What is overcome are the weak intermolecular forces between molecules. "The covalent bonds are weak" scores zero, and "weak forces between the bonds" is meaningless — the force acts between molecules.
"Weak forces between the bonds" scored 0; the marking point is weak intermolecular forces between molecules (Jun23 Q3b).
"Little/low energy", never "less energy"
When stating how much energy is needed to overcome the intermolecular forces, examiners want little/low energy, not less energy. "Less energy" is rejected because a small fraction of a large amount can still be large — it does not show the forces are weak. Say the weak forces need only a little (low) energy to overcome.
"Less energy" rejected (could still be large); "little/low energy" required (Jun23 Q3b, Nov24 Q11aiv).
"Giant covalent" alone is not enough
For diamond or graphite, writing only "giant covalent structure" is not creditworthy — it is true for both, so it does not distinguish them. Diamond: each carbon bonded to four others by strong covalent bonds needing much energy to break. Graphite: layers that slide. Do not mention intermolecular forces for diamond — it has none, and doing so loses marks.
"Giant covalent" alone not credited (true for both); citing intermolecular forces for diamond lost marks (Jun24 1CR Q11cii).
Graphite: delocalised electrons that move
Graphite conducts because it has delocalised electrons that are free to move and carry charge. "Free electrons" on its own — without the word delocalised, or without stating they move/flow — does not score. It is the one outer electron per carbon (not used in the three bonds) that becomes delocalised between the layers.
"Free electrons" without "delocalised" or without "move/flow" failed (Jun24 1CR Q11ci).
Dot-and-cross: H holds only 2 electrons
Two frequent dot-and-cross errors: drawing an extra electron on hydrogen — only 2 electrons fit hydrogen's first shell, so H forms just one single bond — and mis-drawing a double bond, which contains 4 electrons, two from each atom. A correct diagram shows the electrons in pairs, with each shared pair in the overlap between the two atoms.
Extra electron on hydrogen, and double bonds not shown as 4 electrons (2 from each atom), lost marks (Jun24 1CR Q9bi; Jun23 Q4aii).
C60 fullerene is simple molecular
C60 fullerene is made of carbon like diamond and graphite, but it is simple molecular, not giant covalent: separate molecules held to each other by weak intermolecular forces. So unlike diamond and graphite (very high melting points), fullerene is soft with a comparatively low melting point. Do not classify it as giant covalent.
Dot-and-cross: count electrons, then pair up
Find each atom's outer electrons from its group (H=1, C=4, O=6, Cl=7). Work out the shared pairs each needs for a full shell (2 for H, 8 for the rest). Draw one atom's electrons as dots, the other's as crosses, a pair in each overlap; add lone pairs; check each shell is full.
Explain low m.p. with the exact phrase
For a simple molecular melting point, name the weak intermolecular forces between molecules, say only these are overcome (the covalent bonds are not broken), and that they need only little/low energy. Avoid "covalent bonds are weak" and "less energy" — both are rejected.
Diamond vs graphite: state bonds per carbon
Give the structural numbers. Diamond: each carbon bonded to four others, no spare electrons hard, no conduction. Graphite: each carbon bonded to three, one delocalised electron per atom conducts; layers slide soft.
Outer-shell electron counts (for dot-and-cross):
| Atom | Group | Outer electrons | Bonds it usually forms |
|---|---|---|---|
| Hydrogen, H | 1 | 1 | 1 |
| Carbon, C | 4 | 4 | 4 |
| Nitrogen, N | 5 | 5 | 3 |
| Oxygen, O | 6 | 6 | 2 |
| Chlorine, Cl | 7 | 7 | 1 |
Each atom shares electrons until its outer shell is full (H reaches 2; the others reach 8).
Define a covalent bond.
Methane, , is a simple molecular substance in which carbon is covalently bonded to four hydrogen atoms.
Calculate the relative formula mass () of methane. (: H = 1, C = 12.)
