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Now there is another facet to the table because of the way it's laid out. Each element is a member of a group that goes up and down the table, and a period that goes across the table. There are seven horizontal periods running from left to right. Each one has been assigned a number which is placed at the left of the period. There are also 18 vertical columns called groups are families. Each group is designated by a number and a letter, which is placed at the top of the column. Elements within a family all have similar properties. as you go from the top to the bottom of the column. The atoms of each element become larger. Here we see a jump in atomic size in the next alkali metal, and again a decrease along the period.
This pattern repeats itself throughout the table and is called periodicity.