Compounds

There are about 100 known elements

There are hundreds of thousands different compounds

Why do elements combine to form compounds?

Many elements are chemically reactive. Some elements are so reactive that they must be stored in a specia way to prevent them from reacting e.g. elements from Group I. Sodium and Potassium are so rective with water or moisture that they must be stored under liquid paraffin. Phosphorous must be stored under water to prevent it from reacting with the oxygen in the air and bursting into flames. Other elements such as gold do not react easily and some elements such a the noble gaes do not react at all and will not combine with anything to form a compound.

Many of our everyday substances are compounds:

The properties of compounds are usually totally different to the properties of the elements from which they are made of. One example is NaCl illustrated below.
Images courtesy of webelements.com

Some elements particularly many of the gases occur naturally as molecules e.g. (Hydrogen, Oxygen, Nitrogen and Chlorine)

Molecules may be made up of atoms of the same element such as Hydrogen gas (H2 - two atoms of Hydrogen), or atoms of different elements such as Sodium Chloride (NaCl).

Both elements and compunds are "pure" substances. Hydrogen gas consists only of Hydrogen molecules and salt consists only of salt molecules.

However many substances are just mixtures of different elements or compounds. Air is just a mixture of different gases. Our breathing system can easily separate out the oxygen from the air we breathe in because the gases are not chemically combined.

A tablespoon of sand and salt is just a mixture. They do not chemically combine - they just mingle together.

Identifying the differences between Mixtures and Compounds: