Identify the element that has a ground state electron configuration of [ar]4s23d104p5.

4s²3d¹⁰4p⁵

4 in 4p⁵ shows that an element is in the 4th period,

4s² 4p⁵ (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.