Aqa Science - Blog Posts

Covalent and Dative Bonds

Covalent and dative (sometimes called co-ordinate) bonds occur between two or more non-metals,  e.g. carbon dioxide, water, methane and even diamond. But what actually are they?

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. They are found in molecular elements or compounds such as chlorine or sulfur, but also in macromolecular elements and compounds like SiO2 and graphite. Covalent bonds are also found in molecular ions such as NH4+ and HCO3-.

Single covalent bonds have just one shared pair of electrons. Regularly, each atom provides one unpaired electron (the amount of unpaired electrons is usually equal to the number of covalent bonds which can be made) in the bond. Double covalent bonds have two shared pairs of electrons, represented by a double line between atoms, for example, O=C=O (CO2). Triple covalent bonds can also occur such as those in N ≡ N.

Dot and cross diagrams represent the arrangement of electrons in covalently bonded molecules. A shared pair of electrons is represented by a dot and a cross to show that the electrons come from different atoms.

Unpaired electrons are used to form covalent bonds as previously mentioned. The unpaired electrons in orbitals of one atom can be shared with another unpaired electron in an orbital but sometimes atoms can promote electrons into  unoccupied orbitals in the same energy level to form more bonds. This does not always occur, however, meaning different compounds can be formed - PCl3 and PCl4 are examples of this.

An example where promotion is used is in sulfur hexafluoride (SF6). The regular configuration of sulfur atoms is 1s2 2s2 2p6 3s2 3p4. It promotes, as shown in the diagram (see excited state), two electrons: one from the 3s electrons to the 3d orbital and one from the 3p to the 3d. Therefore there are 6 unpaired electrons for fluorine atoms to join. It has an octahedral structure.

An atom which has a lone pair (a pair of electrons uninvolved in bonding) of electrons can form a coordinate bond with the empty orbital of another atom. It essentially donates an electron into this orbital which when formed, acts the same as a normal covalent bond. A coordinate bond therefore contains a shared pair of electrons that have come from one atom.

When ammonia reacts with a H+ ion, a coordinate bond is formed between the lone pair on the ammonia molecule and the empty 1s sub-shell in the H+ ion. An arrow represents the dative covalent bond (coordinate bond). Charges on the final ion must be showed.

Summary

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. They are found in molecular elements or compounds as well as in macromolecular elements and compounds. Also found in molecular ions.

Single covalent bonds have just one shared pair of electrons.  Double covalent bonds have two shared pairs of electrons, represented by a double line between atoms. Triple covalent bonds can also occur.

Dot and cross diagrams represent the arrangement of electrons in covalently bonded molecules. A shared pair of electrons is represented by a dot and a cross to show that the electrons come from different atoms.

Unpaired electrons are used to form covalent bonds - they can be shared with another unpaired electron in an orbital but sometimes atoms can promote electrons into unoccupied orbitals in the same energy level to form more bonds. This does not always occur, however, meaning different compounds can be formed.

An example where promotion is used is in sulfur hexafluoride (SF6). 

An atom which has a lone pair (a pair of electrons uninvolved in bonding) of electrons can form a coordinate bond with the empty orbital of another atom.

 It donates an electron into this orbital which when formed, acts the same as a normal covalent bond. A coordinate bond therefore contains a shared pair of electrons that have come from one atom.

When ammonia reacts with a H+ ion, a coordinate bond is formed between the lone pair on the ammonia molecule and the empty 1s sub-shell in the H+ ion. An arrow represents the dative covalent bond (coordinate bond). Charges on the final ion must be showed.