SWOT Revision

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

or 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).

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

H F

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H – N -> B – F

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H F

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.