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Elements in the same group have similar ... and behave in a similar way ...

Same group element have same Valence electron and behave similarly in Chemistry.

Explanation:

For example. First group elements Alkali metals: - H, Li, K, Rb, Cs, Fr

Valance electron will take part in forming a bond with other elements and compound will form. All the above-given elements (H-Fr) have valence electron 1 in outer most 'S' shell. All have electronic configuration S1

Behavior: Since valence electrons are the same so the behavior of all the elements in this group is the same. All are metal (from Li-Fr, except Hydrogen), all are very reactive, does not found in native state in the environment, and all react with water.