Everything Science Grade 11

CHAPTER 1. ATOMICCOMBINATIONS 1.9

can help to work out how many atoms of eachelement are in the compound, and therefore what its
formula is. The following are some exampleswhere this informationis used to write the chemical
formula of a compound.

Example 7: Formulae of covalent compounds

QUESTION

Write the chemical formula for water

SOLUTION

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

A molecule of water contains the elements hydrogen and oxygen.

Step 2 : Determine the valencyof each element

The valency of hydrogen is 1 and the valency of oxygen is 2. This means that

oxygen can form two bonds with other elementsand each of the hydrogen atoms

can form one.

Step 3 : Write the chemical formula

Using the valencies ofhydrogen and oxygen,we know that in a single water

molecule, two hydrogenatoms will combine with one oxygen atom. Thechemi-

cal formula for water istherefore:

H 2 O

Example 8: Formulae of covalent compounds

QUESTION

Write the chemical formula for magnesium oxide

SOLUTION

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

A molecule of magnesium oxide contains the elements magnesium and oxygen.

Step 2 : Determine the valencyof each element

The valency of magnesium is 2, while the valency of oxygen is also2. In a

molecule of magnesiumoxide, one atom of magnesium will combine with one

atom of oxygen.