Question

An inert gas (At. no < 20 )

Answer 1

Dear friend,

Examples of an inert gases having atomic number less than 20 are :-

1. Helium (A. no. is 2)
2. Neon (A. no. is 10)
3. Argon (A. no. is 18)

Hope it helps !!!

Answer 2

atomic number < 20
noble gas are
Argon atomic number=18
neon atomic number= 10
helium atomic number=2
octet shells are completed