# 2.4 Covalent Bonds

F.4 Chemistry · Topic II Microscopic World (I) · Lesson 2

How non-metal atoms share electrons: electron diagrams, lone pairs, naming, and the dative bond.

## Figure

Outermost shells only: carbon dioxide needs two double bonds; the ammonium ion has one dative N–H bond, with the charge outside the bracket.

## Key points

- **Molecule**: the smallest part of an element or a compound which can exist on its own. **Atomicity** = atoms per molecule: He 1 (monoatomic), Cl~2~ 2 (diatomic), O~3~ 3 (triatomic), P~4~ 4, S~8~ 8.
- **Covalent bond ／ 共價鍵**: the strong **directional** electrostatic attraction between the **shared electrons** and the two nuclei of the bonded atoms; usually between **non-metal** atoms.
- **Bond pair** = a pair shared between two atoms; **lone pair ／ 孤對電子** = an outermost-shell pair **not shared**. Show it on **every** atom that has one.
- The **octet ／ 八隅體** (H: a **duplet**) sets the number of bonds: H 1, O 2, N 3, C 4 (B in BF~3~ has only 6). **Single, double, triple** = 1, 2 or 3 shared pairs: H–Cl, O=C=O, N≡N.
- **Chemical formula**: criss-cross the numbers each atom shares (N 3, H 1 → NH~3~). Substances made of molecules do **not conduct** electricity in any state: no mobile ions or delocalized electrons.
- **Naming**: name first the element earlier in **B, Si, C, P, N, H, S, I, Br, Cl, O, F**; the second ends in **-ide**; prefixes **mono, di, tri, tetra** give the numbers (ClF~3~ chlorine trifluoride; N~2~O~4~ dinitrogen tetraoxide). Exceptions: methane, ammonia, water, hydrogen peroxide.
- **Dative covalent bond ／ 配位共價鍵** (coordinate bond): both shared electrons come from **one** atom, the **donor**, which shares its lone pair with an **acceptor** short of electrons; drawn →. NH~4~^+^, H~3~O^+^, NH~3~BF~3~; BF~3~ + F^−^ → BF~4~^−^.
- **NH~4~Cl** contains **ionic**, **covalent** and **dative covalent** bonds.

## Must know

| Molecule | Each atom shares | Bonds | Lone pairs | Structural formula |
|---|---|---|---|---|
| H~2~O | O 2, H 1 | 2 single | 2 on O | H–O–H |
| NH~3~ | N 3, H 1 | 3 single | 1 on N | three N–H |
| SiCl~4~ | Si 4, Cl 1 | 4 single | 3 on each Cl | four Si–Cl |
| ClF | Cl 1, F 1 | 1 single | 3 on each atom | Cl–F |
| O~2~ | O 2 | 1 double | 2 on each O | O=O |
| CO~2~ | C 4, O 2 | 2 double | 2 on each O | O=C=O |
| N~2~ | N 3 | 1 triple | 1 on each N | N≡N |

Electron diagram: outermost shells only, dots for one element and crosses for the other; in an ordinary covalent bond each atom gives **one** electron to the shared pair.

## Common mistakes

- ✗ "Helium has an octet, and it exists as He~2~." ✓ Helium has a **duplet** (2 electrons) in its only shell and is **monoatomic**: it is chemically inert.
- ✗ PCl~3~ or NCl~3~ drawn with a lone pair on the central atom only. ✓ Draw the lone pairs on the outer atoms too: each Cl atom keeps **three** lone pairs.
- ✗ "Each H atom in H~2~O contributes 2 bonding electrons." ✓ In an ordinary covalent bond each atom gives **one** electron to the shared pair: each H contributes 1, O contributes 2.
- ✗ CO~2~ drawn as O–C–O with single bonds. ✓ With single bonds C has only 6 outermost-shell electrons and each O only 7. C must share **two pairs** with each O: O=C=O gives every atom 8.
- ✗ "NH~4~Cl is a covalent compound; ozone O~3~ is a compound." ✓ Ammonium chloride contains ionic, covalent **and** dative covalent bonds; ozone is an **element** (one kind of atom).

## Quick check

1. Name ClF~3~ and N~2~O~4~, and write the formula of dinitrogen pentoxide.
2. Elements X and Y have atomic numbers 8 and 17. Write the formula of the compound they form and state its type of bonding.
3. How many lone pairs of electrons are there in one molecule of water, oxygen, carbon dioxide and chlorine?
4. Describe the formation of the dative covalent bond in H~3~O^+^.
5. A compound of sulphur and chlorine has a relative molecular mass of 135.2 and contains 52.5% of chlorine by mass. Deduce its molecular formula. (S = 32.1, Cl = 35.5)

## Answers

1. Chlorine trifluoride; dinitrogen tetraoxide; N~2~O~5~.
2. X (2,6) needs 2 more electrons, Y (2,8,7) needs 1, so XY~2~ (Cl~2~O); the atoms share electrons: covalent.
3. Water 2; oxygen 4; carbon dioxide 4; chlorine 6.
4. The O atom in H~2~O has lone pairs of electrons; H^+^ has no electrons. O shares one of its lone pairs with H^+^, so both electrons of the bond come from O.
5. Cl atoms per molecule $= 135.2 \times 0.525 / 35.5 = 2.0$; S atoms $= (135.2 − 70.98) / 32.1 = 2.0$; molecular formula S~2~Cl~2~.

## Did you know?

In 1916, the American chemist Gilbert N. Lewis suggested that two atoms can be held together by sharing a pair of electrons. Dot diagrams of molecules are still called Lewis structures after him.