Answer
The electronic configuration of $P$ is $1s^22s^22p^63s^23p^3$ or $[Ne] 3s^23p^3$.
The 5 electrons in the $3s^23p^3$ orbitals are the valence electrons.
The 10 electrons in the $1s^22s^22p^6$ orbitals are the core electrons.
Work Step by Step
$P$ has 15 electrons. It has 5 more electrons than its previous noble gas, $Ne$. The electrons are distributed in the orbitals to get the electronic configuration as : $1s^22s^22p^63s^23p^3$ or $[Ne] 3s^23p^3$.
The 5 electrons in the $3s^23p^3$ orbitals (outermost orbital) are the valence electrons.
The 10 electrons in the $1s^22s^22p^6$ orbitals are the core electrons.
