AI Tutor
If you follow the standard rules for building atoms, element 24 (chromium) should end in 4s23d44s^2 3d^4, but real-world measurements show it actually ends in 4s13d54s^1 3d^5.
Why do transition metals break the rules, and how do we write their true electron configurations?

Filling the d-Subshell

An electron configuration is the address list of where an atom's electrons live across energy levels and subshells (s,p,d,fs, p, d, f).
The Aufbau principle states that electrons fill the lowest available energy orbitals first before moving to higher ones. In the 1920s, Niels Bohr developed this principle by matching atomic light emissions to distinct energy rungs.
Notice that the 4s4s subshell fills right before the 3d3d subshell because 4s4s has a slightly lower energy level in isolated neutral atoms.
To save time, we use noble gas shorthand, replacing the first 18 electrons (1s22s22p63s23p61s^2 2s^2 2p^6 3s^2 3p^6) with the bracketed core [Ar][\text{Ar}].
Let's see how a typical transition metal follows this standard pattern.

Standard Transition Metal Configurations

Let's write the configuration for Titanium (Z=22Z = 22, where ZZ is the atomic number, the total count of protons and electrons in a neutral atom).
We start with the [Ar][\text{Ar}] core of 18 electrons, place 2 electrons into the 4s4s orbital, and put the remaining 2 into the 3d3d orbitals: [Ar] 4s23d2[\text{Ar}]\, 4s^2 3d^2.