Question

1.helium 2.argon 3.carbon which is unstable and why?

Answer 1

Carbon is the unstable on because it's last shell is not complete.

Answer 2

Hi there !!

-->Carbon is the element which is unstable here.

-->This is because its shells are not filled.

-->Its atomic no: is 6.

--> No : of electrons is also 6.

-->Its electronic configuration is 2 ,4

-->Thus it needs to lose 4 electrons to combine with another element

-->Its valency is 4

--->Therefore it is unstable
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On the other hand , helium and argon are noble gases , whose shells are filled and both of them have valency 0.