The Periodic Table is an arrangement of the elements in order of atomic number.
The elements in any one vertical column are said to be in the same group or family.
Eight of the groups are numbered from left to right. Elements in the same group have similar chemical properties.
The group number is the same as the number of electrons in the outer shell of all the elements in the group.
All the Group 1 elements: Li, Na, K, Rb, Cs and Fr, have one electron in the outer shell.
The Group 7 elements: F, Cl, Br, I and As have seven electrons in the outer shell.
Group 0 elements: He, Ne, Ar, Kr, Xe and Rn have full outer shells. (Group 0 is sometimes called Group 8.)
Elements in the same horizontal row are said to be in the same period. The periods are numbered from top to bottom.
The period number is the same as the number of electron shells that are occupied.
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