Question

Why does the first member of a group differ from the rest ?

Answer 1

the first member of a group differ from its other members by in electronetavity , Size and ionization energy etc.
the first member will have higher electronetavity compared to its other group members
the first member will have smaller atomic size compared to its other group members
the first member will have higher ionization energy compared to its other group members
still there are lots of properties they will change :)

Answer 2

since why? u have asked
the reason is filling of electron from first to last group member in the same group takes to the new shell and not to the same shell:)
for example Na- 3S1
K - 4s1
here # and 4 makes lots of sense which are called shells