When does 4s come before 3d

It's just become a bad "habit" to write 4s before 3d. It really should be with the forethought of which orbital is next highest in energy , and that one goes next in the configuration.

Hence, you really should write, for the first-row transition metals,. This indicates that the 4s orbital is higher in energy. All data for orbital potential energies I reference in this answer are found here. See how the 3d orbitals are lower in energy than the 4s for the first-row transition metals here:. And you can further see how the Aufbau principle fails for the heavier transition metals, in that the n-1 d and ns orbital potential energies criss-cross rather unpredictably:.

This is because the 4s sublevel has higher energy than the 3d sublevel. As it turns out, for the first row of transition metals, the 4s sublevel has higher energy than the 3d sublevel, which is contrary to what we would expect from an Aufbau diagram. So the electron configuration for chromium is written in order of increasing energy, with the 4s sublevel having the highest energy and written at the end.

Can someone make this clear for me by explaining, why 4s comes before 3d, when writing the electronic configuration of elements? Raising l raises orbital energy. Higher l values result in orbitals with more nuclear nodes a node being a place where the probability of finding the electron is zero. We say high- l orbitals are "less penetrating" because their electrons have a lower probability of being found at or near the nucleus.

That gives high- l orbitals like d orbitals more energy than low- l orbitals like s orbitals within the same shell.

This effect causes 4s orbitals to have lower energy than 3d orbitals for elements lighter than copper. Although for hydrogen, the unoccupied 4s and 3d orbitals have nearly identical energies. Within a subshell, more unpaired spins means a lower overall energy.

Quantum mechanics predicts that the motions of electrons with paired spins are "correlated". Post by tmehrazar » Sat Apr 28, pm.

Laurence Lavelle Skip to content. Quick links. Email Link. Why does the 4s orbital come before the 3d?