Group (periodic table)

A group is a vertical column of the periodic table; the elements in it have the same number of valence electrons and therefore similar chemical properties. Malay: kumpulan · Chinese: 族.

Because a whole group shares its number of valence electrons, the group is the single most useful thing to know about an element’s chemistry. For a main-group element the group number even tells you that count directly: Group 1 elements have one valence electron, Group 17 elements have seven. Reading down a group the atoms gain an extra occupied shell each time, so they grow larger, and their reactivity changes in a steady, predictable direction, the alkali metals grow more reactive down the group, while the halogens grow less reactive. That trend is one of the most heavily tested ideas in the chapter, and it follows directly from how tightly the outer shell is held.

Example. Lithium, sodium and potassium all sit in Group 1 and all have one valence electron, so all three react with water to give an alkaline metal hydroxide and hydrogen gas. That shared behaviour is exactly why we group them together. Fluorine, chlorine, bromine and iodine share Group 17 and all form diatomic molecules and coloured salts called halides.

Confusion to avoid. Do not swap “group” with “period”. A group is a vertical column and fixes the valence electrons and hence the chemistry; a period is a horizontal row and fixes the number of occupied shells. Elements in the same group are chemically alike; elements in the same period are not.

In the Periodic Table of Elements chapter the group is the organising idea behind every family you study, the alkali metals of Group 1, the halogens of Group 17 and the noble gases of Group 18 are all simply groups, learned one column at a time.