Electronic configuration of group 1 elements is:
1s22s22p23s1 = Na
1s2 2s2 2p2 3s2 3p6 3d10 4s2 4p6 5s1=Rb
1s2 2s2 2p2 3s2 3p6 3d10 4s2 4p6 4d10 5s25p66s1=Cs
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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