Why does a 4s electron have higher energy than a 3d electron in a chromium atom?

A 4s electron has higher energy than a 3d electron in a chromium atom because of the way electrons fill energy levels. In chromium, the 4s orbital is filled before the 3d orbital due to the stability gained from having a half-filled or fully-filled d orbital. This results in the 4s electron having higher energy than the 3d electron in a chromium atom.