The bromine atom possesses 3s electrons. It contains six electrons in 2p orbitals, six electrons in 3p orbitals and five electrons in 4p orbitals. Which of these electrons experience the lowest effective nuclear charge?
4p
4p electrons having the largest value of n are farthest from the nucleus, and hence experience the lowest effective nuclear charge. For orbitals of the same type of sub-shell, the greater the value of n the smaller the effective nuclear charge.