Achlorine atom in its ground state has a total of seven electrons in orbitals related to the atoms third energy level. how many of the seven electrons occupy the p orbital

5 electrons

The electron configuration of an atom shows or describes the distribution of electrons in their atomic orbitals. In particular, it shows the distribution in terms of the principal energy levels or electron shells (reflected by the numbers 1, 2, 3, 4, 5, 6, 7) and their corresponding subshells (reflected by the letters s,p,d,f,g) composed of atomic orbitals. 

The maximum number of electrons in the s-subshell is 2, in the p-subshell, 6, in the d-subshell, 10, in the f-subshell, 14, and in the g-subshell, 18. In the electron configuration of an atom, the number of electrons in the subshell for a given atom is reflected by a number (in superscript) next to the corresponding letter of the subshell. 
 
The electron configuration of a chlorine atom in its ground state is 


Thus, the number of electrons in the 3rd energy level in the p-subshell is 5 ( as seen in the electron configuration of the atom).