Chemistry, Third edition

4 · BONDING BETWEEN ATOMS

2. Low solubility in water

Covalent compounds do not contain ions and therefore do not mix so readily with

water molecules. Covalent compounds tend to dissolve readily in non-polar organic

solvents, such as benzene, which also contain covalent molecules.

3. Do not conduct electricity

Covalent compounds cannot conduct electricity because they do not contain ions.

How to predict whether compounds are ionic or

covalent

Some general rules:

metal + non-metal ionic bonding

non-metal + non-metal covalent bonding

Remember that:

●In general, metals contain up to three electrons in the outer shells of their atoms

and non-metals contain more than three electrons.

●Hydrogen should be classified as a non-metal.

Coordinate bonding

Covalent bonds are sometimes formed between two atoms when one of the atoms

providesbothof the shared electrons. This type of covalent bonding is given the

namescoordinateordativebonding. The two electrons that one atom donates for

sharing are called a lone pairof electrons.

Boron atoms have the electron arrangement 2.3 and can only share three elec-

trons with other atoms. In the compound NH 3 BF 3 , the nitrogen atom provides both

of the shared electrons to boron, so that the boron atom is surrounded by an octet of

electrons:

To help you distinguish where the electrons come from:

●= electrons from the H atom

= electrons from the N atom

= electrons from the F atom

= electrons from the B atom.

The structural formula of the molecule may be written

HF

||

H—NB—F

||

HF

4.4

56

Formation of ionic

or covalent

compounds

What type of bonding do

you think would be present

in compounds formed by

the combination of the

following atoms?

(i) hydrogen and chlorine,

which form hydrogen

chloride

(ii) potassium and fluorine,

which form potassium

fluoride

(iii)silicon and hydrogen

which form silane, SiH 4.

Exercise 4F