Everything Science Grade 11

1.9 CHAPTER 1. ATOMICCOMBINATIONS

Step 3 : Write the chemical formula

The chemical formula for magnesium oxide is therefore:

MgO

Example 9: Formulae of covalent compounds

QUESTION

Write the chemical formula for copper (II) chloride.

SOLUTION

Step 1 : Write down the elements that make up the compound.

A molecule of copper (II) chloride contains theelements copper and chlorine.

Step 2 : Determine the valencyof each element

The valency of chlorineis 1. Since we are givencopper (II) in the problem the

valency of copper mustbe 2. In a molecule of copper (II) chloride, twoatoms of

chlorine will combine with one atom of copper.

Step 3 : Write the chemical formula

The chemical formula for copper (II) chloride is therefore:

CuCl 2

The formulae of ionic compounds ESBS

The overall charge of anionic compound will always be zero and so the negative and positive charge
must be the same size.We can use this information to work out whatthe chemical formula of an
ionic compound is if weknow the charge on theindividual ions. In the case of NaCl for example, the
charge on the sodium is +1 and the charge on the chlorine is− 1. The charges balance (+1− 1 = 0)
and therefore the ioniccompound is neutral. In MgO, magnesium has a charge of +2 and oxygen
has a charge of− 2. Again, the charges balance and the compoundis neutral. Positive ionsare called
cations and negative ions are called anions.

Some ions are made upof groups of atoms, andthese are called compound ions. It is a good idea to
learn the compound ions that are shown in Table 1.2

In the case of ionic compounds, the valency ofan ion is the same as its charge (Note: valency is
always expressed as a positive number e.g. valency ofthe chloride ion is 1 andnot -1). Since an ionic
compound is always neutral, the positive charges inthe compound must balance out the negative. The
following are some examples: