Why Cr and Cu are famous exceptions
Real electron configurations result from subtle energy balances. Some do not exactly follow a naive filling order.
Start with the idea
Real electron configurations result from subtle energy balances. Some do not exactly follow a naive filling order.
A concrete example
Chromium and copper are classic examples where observed configurations require more than mechanical use of the n+l rule.
Mendeleev’s tip
Do not start with a formula. Identify what changes, what stays constant and which quantity you are trying to explain or calculate.
Go one step deeper
Writing [Ar] 3d⁵ 4s¹ for Cr and [Ar] 3d¹⁰ 4s¹ for Cu is a reminder that a mnemonic cannot replace experimental data.
A trend or model is not an exceptionless law. Always check the quantity’s definition and the conditions being compared.
Check your understanding
Can you explain the phenomenon in your own words before looking at an equation?