There are about 100 elements in total, although only about 20 are in everyday use.
An element is the simplest of all chemical substances.
It is made up of only one kind of atom and cannot be decomposed.
A MOLECULE is formed when two or more atoms join together.
Two (or more) elements can combine together to form a compound.
A compound must be made of at least two elements chemically joined together.
|magnesium + oxygen → magnesium oxide
(element) (element) (compound)
Table showing the common elements with their symbol
(You only need know the names of the coloured symbols)
Some common compounds
Carbon dioxide C02
Copper oxide CuO
Copper Sulphate CuSO4
Copper carbonate CuCO3
Magnesium oxide MgO
Salt (sodium chloride) NaCl
Sulphur dioxide SO2
Zinc oxide ZnO
Does not conduct electricity (except carbon, which does conduct)
High melting point (usually solid)
Low melting point (usually gas or liquid)
The oxides of a metal, when soluble,
The oxides of a non-metalusually form an acidic solution.
A substance is an element if it contains only a single kind of atom and can NEVER be decomposed.
eg copper, oxygen, carbon, iron, sulphur, zinc, gold
A substance is a compound if it contains more then one element chemically joined together.
eg water, magnesium oxide, copper sulphate, calcium carbonate, cobalt chloride, petrol
A mixture exists when there are at least two substances present (either elements, compounds or a combination of the two).
These two substances are not chemically joined and can often be easily separated.
eg air, ink, orange juice, wood, salty water
The main differences between a compound and mixture are summarized in the table below:
|A mixture contains two or more substances NOT chemically joined.||Contains at least two elements chemically joined together|
|The different substances may be visible and the appearance may not be uniform||Will look uniform in appearance (ie the same all the way through).|
|The substances can often be easily separated eg by filtration, evaporation, distillation, chromatography, using a magnet or a separating funnel.||The elements in a compound cannot be separated without a chemical change. ie a new substance will always be formed.|
|The substances can be mixed in any proportion.||The elements in a compound are always present in a fixed proportion.|