Question

helium carbon argon which element is unstable why

Answer 1

Carbon (C) is the unstable element among the given elements. It is so because Helium (He) and Argon(Ar) are both noble gases and have a complete ducet and octet respectively. Hence they are stable. But Carbon has an incomplete octet, there for it is unstable and reactive.

Answer 2

Helium and argon are stable due to it is Noble gas...

we know that noble gases are stable due to it has complete octet and deplete ..

=) if outer most electron in any atom is 2 & 8 then the atom is stable...

so, He , Ar have 2, & 8 electrons in their outer most shell..

*carbon have 4 electron in their outer most electron so for completing octet carbon required 4 more electron .

=) we can say that carbon is unstable due to incomplete octet.
-------------------------------------------------------------

Hope it helps