Hey there!:
The 1s, 2s and 2p subshells are completely filled (a maximum of two electrons go into the 1s subshell and a maximum of two electrons go into the 2s subshell. The 2p subshell includes 3 orbitals, with 2 electrons maximum per orbital). The 3s subshell has only one of a maximum of two electrons.
Hope that helps!
