CHEMISTRY COMMUNITY

When doing electron configurations, do you do the 3d- first or the 4s- first? Which has the lower energy?
Thanks!

4s comes first because it has a lower energy state.

Hi! 4s is filled first and has lower energy

Hello! When there are zero electrons occupying the 3d orbitals, 4s will have a lower energy than 3d. This means that 4s will go after 2p when writing electron configurations. However, when electrons begin to fill the 3d orbitals, then 4s will have the higher energy than 3d. Therefore, 4s will be written after 3d when writing electron configurations for elements with electrons in the 3d orbital.

After the element Ca, the 4s orbital has a higher energy than the 3d orbital, since 3d will have electrons filling it. Thus, after Ca, 4s will be written after 3d.

From what I understand, once the 4s-state is filled and e^- enter the 3d-state, the 3d- state has less energy and is listed first. However there are two exceptions to this, in the case of the half-full d⁵ state and full d¹⁰ state. Following d³s² would be d⁵s¹, rather than d⁴s². This is because d⁵s¹ has less energy than d⁴s². Similarly, d¹⁰s¹ would follow d⁸s².

4s is filled first!

Why is 4s filled first?

4s comes before 3d, remember what the Aufbau Principle states!

Andy Hernandez wrote:Why is 4s filled first?

4s is filled first only if the 3d orbitals are not being occupied.