Covalent Bonding and Dative Covalent Bonds

This lesson covers covalent bonding in detail, including single, double and triple bonds, dot-cross diagrams, dative (coordinate) bonds, and the relationship between bond length and bond energy. These concepts are central to AQA specification section 3.1.3.

---

What Is a Covalent Bond?

Key Definition: A covalent bond is a shared pair of electrons between two atoms, where the bonding pair of electrons is attracted to the nuclei of both atoms.

Covalent bonding typically occurs between non-metal atoms. Each atom contributes one electron to the shared pair (except in dative bonds — see below). The electrostatic attraction between the shared pair and both nuclei holds the atoms together.

---

Single Covalent Bonds

A single bond involves one shared pair of electrons.

Hydrogen (H₂)

Each hydrogen atom has 1 electron. They share one pair:

H • × H   →   H •× H   →   H—H

Hydrogen Chloride (HCl)

        ××
H• ×  ×Cl×   →   H—Cl (with 3 lone pairs on Cl)
        ××

Water (H₂O)

Oxygen has 6 outer electrons and needs 2 more. It shares one pair with each of two hydrogen atoms:

         ••
    H—O—H
         ••

Oxygen has 2 bonding pairs and 2 lone pairs (4 electron pairs total).

Ammonia (NH₃)

Nitrogen has 5 outer electrons and shares one pair with each of three hydrogen atoms:

         •
    H—N—H
       |
       H

Nitrogen has 3 bonding pairs and 1 lone pair.

---

Multiple Bonds

Double Bonds (Two Shared Pairs)

Oxygen (O₂): Each oxygen needs 2 more electrons. They share two pairs:

    ••       ••
    :O ═══ O:
    ••       ••

Each oxygen has 2 bonding pairs and 2 lone pairs.

Carbon dioxide (CO₂): Carbon shares two pairs with each oxygen:

    ••           ••
    :O ═══ C ═══ O:
    ••           ••

Carbon has 4 bonding pairs (in two double bonds) and no lone pairs.

Ethene (C₂H₄):

        H   H
         \ /
     C ═══ C
         / \
        H   H

The C═C double bond consists of one sigma (σ) bond and one pi (π) bond.

Triple Bonds (Three Shared Pairs)

Nitrogen (N₂): Each nitrogen needs 3 more electrons. They share three pairs:

    :N ≡≡≡ N:

Each nitrogen has 3 bonding pairs (in one triple bond) and 1 lone pair. The N≡N triple bond is very strong (bond enthalpy = 944 kJ mol⁻¹), which is why nitrogen gas is unreactive.

Hydrogen cyanide (HCN):

    H—C ≡≡≡ N:

Carbon forms a single bond with hydrogen and a triple bond with nitrogen.

---

Dative Covalent (Coordinate) Bonds

Key Definition: A dative covalent bond (also called a coordinate bond) is a covalent bond in which both electrons in the shared pair come from the same atom.

The atom that donates both electrons must have a lone pair. Once formed, a dative bond is identical in strength and length to an ordinary covalent bond.

Ammonium Ion (NH₄⁺)

Ammonia has a lone pair on nitrogen. When it reacts with H⁺ (which has an empty 1s orbital), nitrogen donates its lone pair:

         H
         |
    H—N: + H⁺  →  H—N→H    (overall charge +1)
         |              |
         H              H

The arrow → represents the dative bond. In NH₄⁺, all four N—H bonds are identical — you cannot distinguish which one is the dative bond. The bond angles are all 109.5° (tetrahedral).

Carbon Monoxide (CO)

Carbon has 4 outer electrons, oxygen has 6. If carbon and oxygen each contributed equally, carbon could only form 2 bonds. Instead, there is a triple bond: two ordinary covalent bonds plus one dative bond from oxygen to carbon.

    :C ←══ O:    or equivalently    :C≡O:

This gives carbon 3 bonding pairs and 1 lone pair, and oxygen 3 bonding pairs and 1 lone pair. CO is isoelectronic with N₂.

Oxonium Ion (H₃O⁺)

Water has 2 lone pairs on oxygen. When it reacts with H⁺:

              H
              |
    H—O: + H⁺  →  H—O→H    (overall charge +1)
              |          |
              H          H

The three O—H bonds in H₃O⁺ are identical. The shape is pyramidal with a bond angle of approximately 107°.

Aluminium Chloride Dimer (Al₂Cl₆)

In the gas phase, AlCl₃ dimerises. Each monomer is electron-deficient (Al has only 6 electrons in its outer shell). A lone pair from a Cl in one monomer is donated to the Al in the other:

    Cl    Cl→Al—Cl
     \   /
      Al
     /   \
    Cl    Cl←Al—Cl
              |
              Cl

Two dative bonds bridge the two aluminium atoms, giving each Al an octet.