Which is the first element that has electrons lying in a p sub-shell.
According to the electron configuration model conceived by Niels Bohr, the electrons in an atom are placed in shells and and sub-shells. Each electron is uniquely identified by four quantum numbers. The primary quantum number identifies the shell in which the electron lies. There can be 2*n^2 electrons in each shell. The first shell can accommodate only 2 electrons and does not have a p sub-shell. The second shell can accommodate 2*4 = 8 electrons. Two of these lie in the s sub-shell and the others lie in the p sub-shell.
The first element to have electrons in the p sub-shell requires an atomic number of 5. This gives it an electron configuration of 1s2, 2s2, 2p1. The element with this electron configuration is boron.