I thought when writing electron configurations the 4s sublevel is filled before 3d because 4s has a lower energy. However, when I look at the assessment schedules for the exams they don’t follow this rule. For example, in the 2018 exam first question the schedule shows the electron configuration of Br- is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6. Shouldn’t it be 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6. Will I get marked wrong if it write it like this?
Welcome to StudyIt.
You have spotted an interesting anomaly in electron configurations.
You are quite right that when empty the 4s orbital has a slightly lower energy than the 3d orbitals which is why K and Ca appear in the PT before the 10 transition metals starting with Sc. However, once the 4s orbital is occupied by electrons, its energy becomes slightly higher than the 3d orbitals. (You could consider this to be the result of the repulsion between the two electrons in the same orbital). This means that both 4s electrons are more easily removed than the 3d electrons (which is why all transition metals will readily form +2 ions). So to remind us of this, the electron arrangements are often written in the order 3s2 3p6 3d10 4s2, rather than the order of filling which is 3s2 3p6 4s2 3d10.
However, the god news is that both versions will be marked correct in the L3 NCEA external.