What are the group 18 elements called?
That's why, they are called Noble Gases.
The cause of there inertness is: Electronic Configuration
Every Noble Gas has completed its octet, means it has 8 electrons in its valence shell. That's why it doesn't want to make ions or mutually share its electrons to make covalent bonds.
Here are the Electronic configurations of every element of group 18:
1) Helium: [
2) Neon : [
3) Argon : [
4) Krypton : [
5) Xenon : [
6) Radon : [