we know that the number of electrons in an atom is equal to number of protons. So the number of electrons here is 7.
Using Moller chart, the electronic configuration is writen by the electrons first enterring into 1s then into 2s after 2p. The s orbital accomodates maximum of 2 electrons.
∴ for atomic no. 7 nitrogen atom, electronic configuration is 1s2 2s2 2p3.
