A chemical element is a substance that cannot be changed into any other substance by a normal chemical reaction.
An element is made up of atoms containing electrons, protons and neutrons. Each atom in an element will contain the same number of protons and electrons, for example every carbon atom has six protons and six electrons.
The periodic table of the elements contains lists all of the elements in order of their number of protons and is laid out so that the elements in each group (vertical column) will have similar properties.
Just because two substances contain the same atoms does not always mean that they will have the same properties. Graphite and diamonds are both made completely from carbon atoms, but the way in which they join with each other means that the have different properties.
Atoms of the same element may also contain different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.