Vertebral column

 

Chromium (atomic number 24)

Chromium (atomic number 24) has a special electron configuration because it is more stable when half-filled $d$-orbitals are present.

Step by step:

1. Normally, following the order of filling, we would expect:

$$1s^2 \; 2s^2 \; 2p^6 \; 3s^2 \; 3p^6 \; 4s^2 \; 3d^4$$

2. However, Chromium actually shifts one electron from the 4s orbital to the 3d orbital to achieve a half-filled $3d^5$, which is very stable.

Final electron configuration:

$$\text{Cr: } 1s^2 \; 2s^2 \; 2p^6 \; 3s^2 \; 3p^6 \; 3d^5 \; 4s^1$$

In shorthand (noble gas form):

$$\text{Cr: } [Ar] \; 3d^5 \; 4s^1$$
 This unusual arrangement happens because a half-filled $d$-subshell (3d⁵) gives extra stability due to electron exchange energy and symmetrical distribution.