Covalent bonding and dative bondsEdexcel International A Level Chemistry: Revision notes

A covalent bond is the strong electrostatic attraction between two nuclei and the shared pair of electrons between them. Both positive nuclei are attracted to the negative electron pair, and this holds the atoms together.

Evidence:

• Giant atomic structures such as diamond and silicon dioxide have very high melting points and are hard, because every atom is joined to its neighbours by strong covalent bonds, and a huge number of bonds must be broken to melt them.
• Electron density maps of simple molecules such as H₂ or HCl show a region of high electron density between the two nuclei. The electrons are shared in the region between the atoms.

In a dot-and-cross diagram the outer electrons of each atom are shown as dots or crosses, and shared pairs are drawn in the overlap between atoms. Most atoms achieve a full outer shell (eight electrons, or two for hydrogen).

• Single bond: one shared pair, as in H₂, HCl, CH₄
• Double bond: two shared pairs, as in O₂, CO₂ and C₂H₄
• Triple bond: three shared pairs, as in N₂ and HCN (C≡N)

Show lone pairs on atoms as well. In HCN the nitrogen has one lone pair, so both carbon and nitrogen have eight outer electrons. More bonds between two atoms means a shorter, stronger bond.

In a dative (coordinate) covalent bond both electrons in the shared pair come from the same atom. The donor atom needs a lone pair and the acceptor needs an empty orbital.

Once formed, a dative bond is the same as any other covalent bond: it is a shared pair between two nuclei. It is shown in diagrams as an arrow from the donor to the acceptor.

In ammonia, NH₃, nitrogen has three bonding pairs and one lone pair. An H⁺ ion has no electrons but has an empty 1s orbital. The lone pair on nitrogen is donated into this orbital:

NH₃ + H⁺ → NH₄⁺

The fourth N–H bond is dative, with both electrons coming from nitrogen. The ion carries a 1+ charge, and all four bonds are identical in length and strength.

In AlCl₃ the aluminium atom has only six electrons in its outer shell, so it is electron deficient. Two AlCl₃ molecules join to form a dimer, Al₂Cl₆, because a lone pair on a chlorine atom of one molecule is donated to the aluminium atom of the other.

Each aluminium atom in Al₂Cl₆ is bonded to four chlorine atoms: three ordinary covalent bonds and one dative bond. Each aluminium atom now has eight outer electrons. The relative formula mass measured in the vapour, 267, is twice that of AlCl₃ (133.5), which confirms the dimer.