The periodic table is a way of organising all of the elements. Elements are what all substances are made up of, and which contain only one type of atom. When elements combine together they make compounds such as hydrogen and oxygen combining to make water. The periodic table also tells us about the elements symbol, its mass number and its atomic number.
In the modern periodic table elements are ordered by their atomic number, but before we knew much about the atom they were ordered by the mass number (how heavy they are).
Mendeleev looked at the elements more closely and he grouped those that had similar properties together. All metals are to the left hand side, he very reactive metals are in group 1 – The Alkali metals, and the very reactive non-metals are in group 7 – The Halogens. Gases that do not react are all in Group 0 – The Noble gases.
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