Why are noble gases called "safe" gases?

Noble gases are called "safe" gases because they are inert.

General Formulas and Concepts:

Explanation:

Because Noble Gases have filled their outer shell electrons fully, they tend not to form compounds with other elements or react with others to produce another compound.

Take Ne (neon) for instance. It's electron configuration is 1s²2s²2p⁶. The outer most shell is the 2p sublevel, and according quantum numbers, the 2p sublevel only has 3 spaces for 2 electrons each that it can hold. Therefore, Ne has a full outer shell and no electron spots to create a bond with other elements.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.