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.
Electronic configuration of group 1 elements is:
1s22s22p63s1 = Na11
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1=Rb37
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s25p66s1=Cs55
Number of electrons in the valence shell determine the group of an element. All these elements have one electron in their valence shell.
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