Question

which rule was broken? aufbau principle hunds rule pauli exclusion principle drag

Explanation:

The Aufbau principle states that electrons fill lower - energy orbitals before higher - energy ones. In the given electron - filling diagram, electrons are filling the 3d orbital before completely filling the 4s orbital. The 4s orbital has lower energy than the 3d orbital and should be filled first.

Answer:

AUFBAU PRINCIPLE