A group is a vertical column in the Periodic Table.

There are 18 main groups, often identified by Roman numerals or by a key element in the group, e.g., Alkali Metals (Group 1) or Halogens (Group 17).

Elements in a group have similar chemical properties because they share the same number of outer electrons (valence electrons).

For example, all alkali metals react similarly with water.

The group structure helps predict properties and reaction patterns.