Everything Science Grade 10

12.1 CHAPTER 12. THE PARTICLES THAT SUBSTANCES ARE MADE OF

Representing molecules ESADR

The structure of a molecule can be shown in many different ways. Sometimes it is easiest

to show what a molecule looks like by using different types ofdiagrams, but at other times,

we may decide to simply represent a molecule using itschemical formulaor its written

name.

Using formulae to show the structure of a molecule

Achemical formulais an abbreviated (shortened) way of describing a compound. In chap-

ter 2, we saw how chemical compounds can be represented using element symbols from

the periodic table. A chemical formula can also tell us thenumberof atoms of each el-

ement that are in a compound and theirratioin that compound. If the compound is a

covalent molecular compound then we can use the molecular formula.

DEFINITION: Molecular formula

The molecular formula is a concise way of expressing information about the

atoms that make up a particular covalent molecular compound. The molec-

ular formula gives the exact number of each type of atom in the molecule.

For example in figure 12.4 the molecular formula of 2-methyl propane is C 4 H 10. This tells

us that there are 4 carbon atoms and 10 hydrogen atoms in this molecule, i.e the ratio of

carbon to hydrogen is4 : 10. But we can simplify this ratio to:2 : 5. This gives us the

empirical formulaof the molecule.

Note

The empirical and the
molecular formulae
can be the same. For
example in carbon
dioxide the molecular
formula is CO 2. This
is also the empirical
formula since it is the
simplest ratio.

DEFINITION: Empirical formula

The empirical formula is a way of expressing therelativenumber of each

type of atom in a chemical compound. The empirical formula does not

show the exact number of atoms, but rather the simplestratioof the atoms

in the compound.

206 Chemistry: Matter and materials