Which of the following elements is not a Noble gas?

Correct answer: (c) Actinium

Explanation

Noble gases form Group 18 of the periodic table with completely filled outer orbitals, making them chemically inert.

The noble gases are Helium, Neon, Argon, Krypton, Xenon and Radon.

Actinium is a radioactive metal of the actinide series, not a noble gas.