The last orbit of argon would have how many electrons?

Between $4s$ and $3d$ orbitals,which one has higher energy and why?

How many electrons can a given orbital accommodate?

If $n$ and $l$ represent the principal and azimuthal quantum numbers respectively, the formula used to determine the number of radial nodes possible for a given orbital is:

What is the number of unpaired electrons in a ferrous ion $(Fe^{2+})$?

The electronic configurations of four elements are given in brackets: $L\,(1s^2, 2s^2 2p^1)$; $M\,(1s^2, 2s^2 2p^5)$; $Q\,(1s^2, 2s^2 2p^6, 3s^1)$; $R\,(1s^2, 2s^2 2p^2)$. Which element would most readily form a diatomic molecule?