What are exceptions to electron configurations?

Chromium (Cr) and copper (Cu) are the two main exceptions to expected electron configurations that IB Chemistry students need to know. Instead of following the normal filling pattern, chromium has the configuration $\ce{[Ar]} 4s^1 3d^5$ rather than the expected $\ce{[Ar]} 4s^2 3d^4.$ Similarly, copper has $\ce{[Ar]} 4s^1 3d^{10}$ instead of $\ce{[Ar]} 4s^2 3d^{9}.$ In both cases, an electron from the $4s$ orbital is promoted to the $3d$ orbital, leaving only one electron in the $4s$ subshell rather than the expected two. This occurs because half$-$filled $(d^5)$ and fully$-$filled $(d^{10})$ $d$ subshells have special stability.

The key principle behind these exceptions is that atoms adopt the electron configuration that gives them the lowest overall energy.

While there are other exceptions in the periodic table (particularly among heavier transition metals), chromium and copper are the only ones IB chemistry students are expected to know and explain.