Question

why do elements He , Ne and Ar have zero valencies

Answer 1

Because they are noble gases ..And their octets and duplets are complete..i.e. they can not accomodate anymore electrons in their shells..

Answer 2

because they haves full electrons in the outer most shell or valency shell

ex:He=2 electrons in valency shell

so first shell is enough to fill the electrons

the capcity of the shell is 2

If the electrons are more the half of the capacity of the shell

then,
the valency will be the capacity of shell- the number of electrons in the shell

so valency of He = 2-2
= 0

similarly we can find the valencies of Ar ,Ne etc...

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