Covalent and dative bonding

What holds atoms together

Both nuclei are attracted to the shared electron pair between them. Electron density maps of molecules show electron density concentrated between the bonded nuclei, which is evidence for covalent bonding. Multiple bonds share two (double) or three (triple) pairs: O=O in O2, N≡N in N2.

Dot-and-cross diagrams

Show the outer electrons of each atom with dots or crosses. Examples: H2O (two bonding pairs, two lone pairs on O), CO2 (two C=O double bonds), N2 (a triple bond and a lone pair on each N). Some atoms have fewer than 8 outer electrons (B in BF3 has 6) or more (S in SF6 has 12, P in PCl5 has 10).

Dative covalent bonds

In a dative (coordinate) bond both shared electrons come from one atom, shown as an arrow from the donor. Once formed it is identical to any other covalent bond.

• NH4+: NH3 donates its lone pair to H+, giving four identical N-H bonds.
• Al2Cl6: two AlCl3 units join when a Cl lone pair on each unit forms a dative bond to the Al of the other.

Bond length and strength

The more electron pairs shared between two atoms, the shorter and stronger the bond: C≡C is shorter and stronger than C=C, which is shorter and stronger than C-C.

Worked example

Explain how the ammonium ion forms from ammonia and a hydrogen ion.

1. NH3 has a lone pair on nitrogen.
2. H+ has no electrons.
3. N donates its lone pair to form a dative covalent bond to H+.

Answer: NH3 + H+ -> NH4+, with four identical N-H bonds and a tetrahedral shape.

Check yourself

1. What is a covalent bond?
   • The transfer of electrons from a metal to a non-metal
   • Attraction between oppositely charged ions
   • Attraction between positive ions and delocalised electrons
   • The electrostatic attraction between two nuclei and a shared pair of electrons
   Show answer

   The electrostatic attraction between two nuclei and a shared pair of electrons. That is the specification definition.

2. Which species contains a dative covalent bond?
   • CH4
   • NH4+
   • H2O
   • Cl2
   Show answer

   NH4+. In NH4+, nitrogen's lone pair is donated to H+.

3. How many lone pairs are on the oxygen atom in a water molecule?
   Show answer

   2. O has 6 outer electrons; 2 are used in bonds with H, leaving 4 electrons = 2 lone pairs.

4. In Al2Cl6, what donates the electrons for the dative bonds?
   • A lone pair on a chlorine atom
   • A lone pair on an aluminium atom
   • An empty orbital on chlorine
   • A delocalised electron
   Show answer

   A lone pair on a chlorine atom. Al in AlCl3 has only 6 outer electrons; a Cl from the other unit donates a lone pair.

5. Which carbon-carbon bond is the shortest?
   • C=C
   • C-C
   • C≡C
   • They are all the same length
   Show answer

   C≡C. Three shared pairs pull the nuclei closest together.

6. How many electrons are shared between the two atoms in N2?
   Show answer

   6. A triple bond is three shared pairs, 6 electrons.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page
• Pearson IAL Chemistry (2018) transition guide

Last updated 7 November 2026. Spotted a mistake? Tell us.