There are about 100 elements which all materials are made of. More are still being 'discovered'.
It is laid out so that elements with similar properties form columns.
The modern periodic table shows the elements in order of ascending atomic number.
These vertical columns are called groups and Roman numerals are often (but not always) used for them.
The group to which the element belongs corresponds to the number of electrons it has in its outer shell. E.g. Group 1 elements have 1 outer shell electron, Group 7 elements have 7 outer shell electrons and so on. Group 8 elements have 8 electrons in their outer shell - this means their outer shell is full.
The rows are called periods. Each new period represents another full shell of electrons.
The period to which the element belongs corresponds to the number of shells of electrons it has.
Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons
Isotopes have the same atomic number but different mass numbers.
If they had different atomic nume