Covalent and Dative Covalent Bonding

Full Notes

Covalent and dative covalent bonding has been outlined with more background theory and detail here and here
This page is just what you need to know for Edexcel A-level :)

Covalent Bonding

Covalent bonding often occurs between non-metal atoms (this doesn’t have to be the case however - metal atoms can sometimes covalently bond to non-metal atoms).

Each atom shares a pair of electrons to achieve a full outer shell (usually 8 electrons, the octet rule).

• The shared pair of electrons forms a strong electrostatic attraction between the nuclei and the bonding electrons.
• The distance between the nuclei of each atom is called the bond length and the shorter the bond length, the stronger the bond as the shared electrons are closer to each nucleus.

Example Hydrogen (H₂)

The simplest example of a covalent bond is between two hydrogen atoms - each atom has one electron in its outer shell, meaning the two orbitals from each atom can combine, giving each atom a full outer shell.

Sometimes, two atoms will share more than one pair of electrons, forming double and triple bonds.

• Double bonds = two shared pairs of electrons (e.g. O=O, CO₂)
• Triple bonds = three shared pairs of electrons (e.g. N≡N)

These bonds are stronger and shorter than single bonds.

Example Oxygen molecule (O₂)

Two atoms of oxygen can share two electrons each, meaning four electrons (two pairs) shared in total. Each atom now has a full outer shell and a molecule of O₂ is formed with a double covalent bond.

Dative Covalent (Co-ordinate) Bonds

A dative covalent bond is a type of covalent bond where both bonding electrons come from the same atom.

Once formed, it is identical to a normal covalent bond in strength and length.

Dative covalent bonds are represented using an arrow (→) from the donor atom to the acceptor atom.

ExampleAmmonium ion (NH₄⁺)

NH₃ has a lone pair on nitrogen and it can donate that pair to a H⁺ ion, forming a co-ordinate (dative covalent bond) and making an ammonium ion, NH₄⁺.
NH₃ + HCl → [NH₄]⁺ + Cl⁻

ExampleAluminium Chloride (Al₂Cl₆)

Cl has a lone pair and it can donate that pair to Al, forming a co-ordinate (dative covalent bond) and making Al₂Cl₆.

Dot-and-Cross Diagrams for Covalent Compounds

Dot-and-cross diagrams represent covalent bonding by showing which electrons come from which atom.

Examples of molecules with Single bonds (1 shared pair):

Examples of molecules with Double bonds (2 shared pairs):

Examples of molecules with Triple bonds (3 shared pairs):

Examples of co-ordinate bonding:

Examples of Expanded Octets:
Some elements (usually in Period 3 or below) can hold more than 8 electrons.

Bond Length and Bond Strength

There is an inverse relationship between bond length and bond strength:

• Shorter bonds are generally stronger
• Multiple bonds (double or triple) are shorter and stronger than single bonds

Example C–C bonds

A C≡C triple bond is shorter and stronger than a C=C double bond, which is in turn shorter and stronger than a C–C single bond.

The more electrons shared between two atoms, the more tightly they are held together, and the closer their nuclei are to each other.
However, a double bond isn’t twice as strong as a single bond and a triple bond isn’t three times stronger than a single bond — for an explanation as to why, see sigma and pi bonding here.

Summary

• Covalent bonding involves sharing pairs of electrons between atoms.
• Double and triple bonds are stronger and shorter than single bonds.
• Dative covalent bonds occur when both electrons come from one atom (e.g. NH₄⁺, Al₂Cl₆).
• Dot-and-cross diagrams show the origin of electrons in covalent bonds.
• Shorter bonds are stronger; triple > double > single in bond strength.