Which element has the ground state electron configuration [ar]4s²3d¹⁰4p⁵?

4s²3d¹⁰4p⁵ 4 in 4p⁵ shows that an element is in the 4th period, 4s²4p⁵   (2+5=7) shows that element is in 17 group, if we have p-electrons on the last level, we need to count s-electrons and p-electrons, and then we will see number of the group, where an element is.  7 s- and p- electrons on the last level, so element in 17th group. 4th period, 17th group - it is Br.

Explanation: