Classification of matter

There are 112 officially named elements and about 118 known elements. Most of these are natural, but some are man-made. The elements we know are represented in the Periodic Table of the Elements , where each element is abbreviated to a chemical symbol . Examples of elements are magnesium ( $\mathrm{Mg}$ ), hydrogen ( $\mathrm{H}$ ), oxygen ( $\mathrm{O}$ ) and carbon ( $\mathrm{C}$ ). On the Periodic Table you will notice that some of the abbreviations do not seem to match the elements they represent. The element iron, for example, has the chemical formula $\mathrm{Fe}$ . This is because the elements were originally given Latin names. Iron has the abbreviation $\mathrm{Fe}$ because its Latin name is 'ferrum'. In the same way, sodium's Latin name is 'natrium' ( $\mathrm{Na}$ ) and gold's is 'aurum' ( $\mathrm{Au}$ ).

Compounds

A compound is a chemical substance that forms when two or more elements combine in a fixed ratio. Water ( $\mathrm{H}{}_2\mathrm{O}$ ), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride ( $\mathrm{NaCl}$ ) is a compound made up of one sodium atom for every chlorine atom. An important characteristic of a compound is that it has a chemical formula , which describes the ratio in which the atoms of each element in the compound occur.

Compound

A substance made up of two or more elements that are joined together in a fixed ratio.

[link] might help you to understand the difference between the terms element , mixture and compound . Iron (Fe) and sulphur (S) are two elements. When they are added together, they form a mixture of iron and sulphur. The iron and sulphur are not joined together. However, if the mixture is heated, a new compound is formed, which is called iron sulphide (FeS). In this compound, the iron and sulphur are joined to each other in a ratio of 1:1. In other words, one atom of iron is joined to one atom of sulphur in the compound iron sulphide.

[link] shows the microscopic representation of mixtures and compounds. In a microscopic representation we use circles to represent different elements. To show a compound, we draw several circles joined together. Mixtures are simply shown as two or more individual elements in the same box. The circles are not joined for a mixture.

We can also use symbols to represent elements, mixtures and compounds. The symbols for the elements are all found on the periodic table. Compounds are shown as two or more element names written right next to each other. Subscripts may be used to show that there is more than one atom of a particular element. (e.g. $\mathrm{H}{}_2\mathrm{O}$ or $\mathrm{NaCl}$ ). Mixtures are written as: a mixture of element (or compound) A and element (or compound) B. (e.g. a mixture of $\mathrm{Fe}$ and $\mathrm{S}$ ).

One way to think of mixtures and compounds is to think of buildings. The building is a mixture of different building materials (e.g. glass, bricks, cement, etc.). The building materials are all compounds. You can also think of the elements as Lego blocks. Each Lego block can be added to other Lego blocks to make new structures, in the same way that elements can combine to make compounds.