Ionic Bonding

You have already met covalent bonds, these involve the sharing of a pair of electrons to allow each atom to obtain a favourable conformation. They are very strong.

Ionic bonds usually form between metal and nonmetal atoms, and involve the transfer of an electron from one atom to the other. This forms two ions which are associated with each other, ionic bonds are not as strong as covalent bonds. The electron is normally transfered from the metal to the nonmetal because generally metals are electron donating and nonmetals are electron accepting.

For example:


Sodium atom

11 electons

(11 protons)


Chlorine atom

17 electons

(17 protons)


Sodium ion

10 electons

(11 protons)


Chlorine ion

18 electons

(17 protons)

Electron transfer

This produces two oppositely charged ions which are strongly attracted to each other. This is called electrostatic attraction. An ionic compound consists of lots of oppositely charged ions which are held together in a regular network called a crystal lattice within which each atoms is fixed in place. Ionic compounds are usually solid crystals at room temperature and have characteristic high melting points. If a soluble ionic compound is mixed with water the water will dissolve the crystal lattice, releasing the ions into solution. These ions have become solvated and are free to move around the solution.

Compound Name mp(oC) bp(oC)
Barium chloride 963 1560
Calcium oxide 2580 2850
Lithium bromide 547 1265
Magnesium chloride 708 1412
Potassium iodide 686 1330
Sodium chloride 801 1413

Coloured Ions

Compound Colour Compound Colour Compound Colour Compound Colour Compound Colour
sodium sulfate colourless sodium chloride colourless sodium nitrate colourless sodium dichromate orange sodium permanganate purple
calcium sulfate colourless calcium chloride colourless calcium nitrate colourless calcium dichromate orange calcium permanganate purple
copper (II) sulfate blue copper (II) chloride blue copper (II) nitrate blue
nickel sulfate green nickel chloride green nickel nitrate green
cobalt sulfate pink cobalt chloride pink cobalt nitrate pink cobalt permanganate purple
lithium dichromate orange

From the above data on compound's colors we can predict ion's colours:

Ion Colour
sodium, calcium, sulfur, chloride, nitrate colourless
copper blue
cobalt pink
nickel green
dichromate [O3CrOCrO3]2- orange
permanganate[MnO4]- purple