The Periodic Table is a chart of all the 100-plus different chemical elements.
The Periodic Table was devised by Russian Dmitri Mendeleyev. He realized that each element is part of a complete set, and so he predicted the existence of three then unknown elements – gallium, scandium and germanium.
The Periodic Table arranges the elements according to their Atomic Number, which is the number of protons in their atoms. The table lists the elements in order of Atomic Number, starting with hydrogen at 1.
Atoms usually have the same number of electrons as protons. So the Atomic Number also indicates the normal number of electrons an atom has.
Atomic mass is the average weight of an atom of an element and corresponds to the average number of protons and neutrons in the nucleus. The number of neutrons varies in some atoms so the atomic mass is never a round number.
Columns in the Periodic Table are called Groups. Rows are called Periods.
The number of electron layers (shells) in the atoms of an element increases by one down each Group. The elements in each Period have the same number of electron shells.
The electrons in the atoms outer shell increases by one across each Period.
Each Group is made up of elements with a certain number of electrons in their outer shell. This is what largely determines the element’s character. All the elements in each Group have similar properties. Many of the Groups have a name as well as a number, as shown opposite.
Each Period starts on the left with a highly reactive alkali metal of Group 1, such as sodium. Each atom of elements in Group 1 has an electron in its outer shell. Each Period ends on the right with a ‘noble’ gas of Group 0, such as argon. These elements have the full number of electrons in their outer shell and do not react.