In the outermost shells the electrons are distributed as per the atomic numbers due to which similar properties occur. As we go down the group or along the period there are many chemical properties which depend on the atomic number.

For example,

Electronic configuration of group 1 elements is:

1s²2s¹ =Li₃

1s²2s²2p⁶3s¹ = Na₁₁

1s²2s²2p⁶3s²3p⁶4s¹=K₁₉

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 5s¹=Rb₃₇

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s²5p⁶6s¹=Cs₅₅

[Rn] 7s¹=Fr₈₇

Number of electrons in the valence shell determine the group of an element. All these elements have one electron in their valence shell.