Elements, compounds and mixtures

Elements in the Earth’s crust     Obtaining metals from their ores

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)


Metallic Elements Symbol
Aluminium Al
Calcium Ca
Copper Cu
Iron Fe
Gold Au
Lead Pb
Magnesium Mg
Mercury Hg
Silver Ag
Sodium Na
Tin Sn
Zinc Zn


Non-metallic elements Symbol
Bromine Br
Carbon C
Chlorine Cl
Helium He
Hydrogen H
Iodine I
Nitrogen N
Oxygen O
Sulphur S

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
Water                         H2O
Zinc oxide                   ZnO

Some differences between a metallic and non-metallic element:





Conducts electricity

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,
form an alkaline solution.

The oxides of a non-metalusually form an acidic solution.



Elements, Mixtures and Compounds: Summary

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.

Useful Links

Periodic Table of the Elements