Helium is different than neon and argon because it does not fulfill the octet rule. Helium fills its outer (and only) orbital, the 1s. There are only two electrons in this orbital. Another difference, now that I mentioned s orbitals, is that helium does not have any filled p orbitals like neon and argon. Neon has a complete 2p orbital, whereas argon has a complete 3p orbital.
They are similar because they are extremely stable. Each of these atoms completes their outer shells, reducing their reactivity with other atoms to practically nothing. This makes them all noble gases.
They are similar because they are extremely stable. Each of these atoms completes their outer shells, reducing their reactivity with other atoms to practically nothing. This makes them all noble gases.