Electronic Configuration: The 2n² Rule, Aufbau, Pauli, Hund and the Cr and Cu Exceptions

Electronic Configuration: The Address of Every Electron

An atom's chemistry is decided almost entirely by how its electrons are arranged. Electronic configuration is that arrangement written as an address: which shell, which subshell, and how many electrons sit there. Once you can write it, you can predict an element's valency, its group and period in the periodic table, and whether it behaves like a metal or a non-metal.

Exams test this at two depths. School-level papers use the Bohr-Bury shell picture, written as 2, 8, 8, 1 for potassium. Higher papers use subshell notation such as 1s² 2s² 2p⁶, and they ask the three filling rules and the two famous exceptions. This short builds from the first picture to the second, so that each rule arrives with a worked element and the trap that papers attach to it.

1. What is the maximum number of electrons the M shell can hold?

2. What is the shell-wise electronic configuration of potassium (Z = 19)?

3. Which principle states that no two electrons in an atom can have the same set of four quantum numbers?

4. What is the correct ground-state electronic configuration of chromium (Z = 24)?

5. What is the maximum number of electrons a d subshell can hold?

6. According to the Aufbau order, which subshell fills immediately after 4s?

7. How many unpaired electrons does a nitrogen atom (Z = 7) have in its ground state?

8. An element has the configuration 1s² 2s² 2p⁶ 3s² 3p⁵. Where is it placed in the modern periodic table?