What a group means
The periodic table has 18 numbered groups. A group is a vertical column. Elements in the same group often show related chemical behaviour because their valence-electron arrangements are related, but the similarity is a trend rather than an absolute rule.
Reading groups 1–18
1–18
Patterns — and exceptions
Moving down a group adds electron shells. Atomic size generally increases, while other properties such as ionization energy and reactivity change in ways that depend on the family. Hydrogen is a good reminder not to infer every property from column position alone.
Use the group as a comparison tool
Start with the group number, then compare valence electrons, common oxidation states and measured properties. Treat the group as a powerful organizing pattern, not as a substitute for element-specific data.