Two non-metal atoms cannot both win an electron, so they share. Sometimes one atom supplies both electrons of the shared pair, and sometimes an atom ends up with more, or fewer, than eight.
A covalent bond is the electrostatic attraction between two nuclei and a shared pair of electrons. Atoms can share one, two or three pairs to form single, double and triple bonds. In a dative covalent (coordinate) bond both electrons of the shared pair come from the same atom, which must have a lone pair, and the other atom must have an empty orbital to receive it; it is drawn as an arrow from donor to acceptor. Once formed, a dative bond is identical to any other covalent bond. Period 3 elements can use d orbitals to hold more than eight outer electrons (PCl5, SF6), while boron in BF3 has only six.
Build H2, O2 and N2 in the simulator, adding electrons one at a time. Count how many pairs each molecule needs before every atom has a full outer shell.
| Molecule | Bond | Shared pairs | Lone pairs |
|---|---|---|---|
| hydrogen, H2 | H–H | 1 | 0 |
| oxygen, O2 | O=O | 2 | 2 on each O |
| nitrogen, N2 | N≡N | 3 | 1 on each N |
| ethene, C2H4 | C=C | 2 between the carbons | 0 |
A dative bond needs a donor with a lone pair and an acceptor with an empty orbital. Look for a lone pair on N, O or a halide, and for H+, a metal ion or an electron-deficient atom such as Al or B.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
A covalent bond is:
How many pairs of electrons are shared between the two atoms in a nitrogen molecule, N2?
In a dative covalent bond:
Which species contains a dative covalent bond?
The octet rule works for most Period 2 molecules, but not all. Period 3 atoms have empty 3d orbitals, so they can expand their octet.
| Molecule | Electrons around central atom | Note |
|---|---|---|
| BF3 | 6 | electron deficient; accepts a lone pair, for example from NH3 |
| SO2 | 10 | two S=O double bonds and one lone pair on S |
| PCl5 | 10 | five P–Cl bonds |
| SF6 | 12 | six S–F bonds |
When you draw a dative bond, the arrow points from the atom that donates the lone pair to the atom that accepts it. In a dot-and-cross diagram, show both electrons of the dative pair with the donor's symbol. And remember the finished bond is not weaker or different: all four bonds in NH4+ have the same length and strength.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which molecule has a central atom with more than eight electrons in its outer shell?
Aluminium chloride exists as Al2Cl6 molecules in the vapour at lower temperatures. How many dative covalent bonds are there in one Al2Cl6 molecule?
BF3 reacts with NH3 to form F3B–NH3. Which statement is correct?
How many electrons are in the outer shell of sulfur in SO2, where each O is joined to S by a double bond and S has one lone pair?
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