The Periodic Table · Grade 9

How do you predict a compound's formula from two positions in the table?

Check whether each element gives electrons away or takes them in. A metal with a non-metal makes an ionic bond: the metal gives and the non-metal takes. Two non-metals share electrons in a covalent bond. The group number gives you the charge on each ion, and then you balance the charges to zero — that is the formula.

Learning objectives

Two kinds of bond

A metal atom tends to give electrons away and a non-metal atom tends to take them in. When the two meet the electrons actually transfer: a positive ion and a negative ion form, and they attract each other electrically. That is an ionic bond.

When two non-metals meet there is nobody to give. Both want to take, so they share pairs of electrons and both count them as their own. That is a covalent bond.

So two positions in the table are enough to know what will happen: metal against non-metal means ionic, non-metal against non-metal means covalent.

From the group to the formula

Group 1 gives away one electron and its charge is 1+. Group 2 gives away two. Group 17 — 7 in the older numbering — takes one and its charge is 1−, and group 16 takes two.

The compound must be neutral, so you balance: the number of positive ions times their charge must equal the number of negative ions times theirs. Sodium at 1+ and chlorine at 1− meet one to one.

Magnesium at 2+ and chlorine at 1− need two chloride ions for every magnesium ion. Magnesium and oxygen, both at 2, meet one to one again. There is nothing to memorise here — there is a balance.

Why an ionic compound conducts only when molten

Electrical conduction needs charges that can move. In a solid ionic compound the ions are fixed in an ordered lattice and do not travel, so it is an insulator even though it is made entirely of charged particles.

On melting, the lattice breaks up and the ions are free to move, and then it conducts. Exactly the same explanation applies to a solution in water, where the ions are separated and surrounded by water.

This is a good example of a property depending on structure and not only on composition. The very same ions, two opposite electrical behaviours, according to the state they are in.

Worked examples

  1. What is the formula of the compound of magnesium and chlorine?

    1. Magnesium is in group 2 and gives away two, so its charge is 2+
    2. Chlorine takes one, so its charge is 1−
    3. Two chloride ions are needed to balance

    Answer: MgCl2 — magnesium chloride

  2. Magnesium and oxygen. What is the formula, and why is it different from the previous case?

    1. Magnesium has a charge of 2+
    2. Oxygen is in group 16 and takes two, so its charge is 2−
    3. The charges balance one to one

    Answer: MgO. This time one ion from each side is enough

  3. Solid table salt does not conduct electricity. Does that contradict its being ionic?

    1. Conduction needs mobile charges
    2. In the solid the ions are fixed in a lattice
    3. On melting or dissolving they are released

    Answer: No. It is ionic, and it will conduct the moment the ions can move

Common mistakes

Balancing the number of atoms instead of the charges
What has to cancel is the total charge, not the particle count. Hence MgCl2 and not MgCl, even though that is one atom against two.
Expecting a solid ionic compound to conduct
The ions are charged, but in a lattice they are fixed in place. No movement means no current, so it conducts only when molten or dissolved.
Calling every bond between two atoms ionic
An ionic bond needs a giver and a taker, which means a metal and a non-metal. Two non-metals share electrons, and that is a covalent bond.

What to remember

More in The Periodic Table