Covalent Bonding and Atomic Shapes

Covalent bond
A covalent bond is between two non-metals and involves the ‘sharing of electrons’.

Definition: A shared pair of electrons. There is a force of attraction between the bonding pair of electrons and the nuclei of the atoms.

Covalent properties:
· Low melting point due to the weak inter-molecular forces (except for diamond and silicon);
· Most dissolve better in non-polar solvents e.g. cyclohexane;
· Few dissolve in H2O (there are some exceptions e.g. Ammonia forms NH4OH);
· They don’t conduct electricity (except graphite which has a free electron).

Low melting/boiling points in simple covalent compounds:
This is because although the bonds between atoms is strong the inter-molecular forces are weak. It is these forces that break when the compound is heated.

Covalent compound are represented by either dot and cross diagrams or H – O – H

Examples: CO2


O = C = O

Dative covalent bond
A Dative covalent bond is when a lone pair of electrons is shared with an electron deficient compound (a compound that has not reached the nearest noble gas structure e.g. BF3).


Definition of Dative covalent bond
A covalent bond where both the bonding electrons are supplied by one atom.

Whilst the F atoms have reached a noble gas structure the B atoms still require another two electrons. This means BF3 could form a further bond.

E.g. NH3 bonds with BF3

5 Molecular Shapes
In the exam you may be asked to describe and draw the shapes of molecules. It is therefore vital you can draw the molecules and remember the bond angles and molecular names.

Electron repulsion theory
Pairs of electrons repel each other and therefore arrange themselves as far apart as possible. The repulsion between lone pairs is greater than that between bonding pairs.

For the Exam:
· You must mention the number of lone pairs of electrons in the molecules.
· You must also mention the number of bonding pairs of electrons.
· And you must mention that repulsion between lone pairs in greater than that of bonding pairs.


Molecular shapes

Note: When drawing molecular shapes in the exam remember to include any double or triple bonds!