How do you explain the conditions under which the orbital notations for helium is possible ?
the notation is 1s^2....can anyone explain it is like this, unlike the other noble gases ?
Helium can be classed as a noble gas because it is rather unreactive because it has a full energy shell. It's outer valence shell is in energy level 1, which only has an s-orbital, and is full with 2 electrons. Hence 1s^2.
Looking at neon, its outer shell is energy level 2, and it is also full. For this to be full we have to fill the 2s and 2p orbitals - giving 1s^2, 2s^2 2p^6.
What matters is that the relevant outer orbital (1s for He, 2p for Ne, 3p for Ar etc) is full for each noble gas.