Electron Configuration

The purpose of introducing quantum numbers has been to show that similarities in the electron arrangement or electron configuration lead to the similarities and differences in the properties of elements. But writing the quantum numbers of electrons of an element in set notation like 2,1,-1,1⁄2 is time consuming and difficult to compare so an abbreviated form was developed. An electron configuration lists only the first two quantum numbers, n and (ell), and then shows how many electrons exist in each orbital. For example, write the electron configuration of scandium, Sc: 1s2 2s2 2p6 3s2 3p6 4s2 3d1 . So for scandium the 1st and 2nd electron must be in 1s orbital, the 3rd and 4th in the 2s, the 5th through 10th in the 2p orbitals, etc.

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This is a memory device to remember the order of orbitals for the first two quantum numbers. Follow the arrow starting in the upper right, when the arrow ends go to the next arrow and start again.

In Scandium, the 4s has lower energy and appears before 3d (the complexity of the d-orbitals leads to its higher energy), so it is written before adding 3d to the electron configuration. But it is common to to keep all the principle quantum numbers together so you may see the electron configuration written as Sc: 1s2 2s2 2p6 3s2 3p6 4s2 3d1. Writing electron configurations like this can cause difficulties in determining the element that matches an electron configuration. But if you just count the number of electrons it will equal the number of protons which equals the atomic number which is unique for each element. For example: “Which element has the electron configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d104s24p6 4d8 5s2 ?” Counting the electrons gives 46, which is the atomic number of palladium.

Here’s a diagram of the first several electron configurations. David’s Whizzy Periodic Table is a visual way of looking at the changing electron configuration of elements.

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Rules for Filling Orbitals

Aufbau Principle states that the lowest energy orbital is filled first. So electrons usually fill the lowest energy level and the simplest orbital shape first. Pauli Exclusion Principle states that no two electrons can have the same four quantum numbers. This is why each orbital only has two electrons, one spin up ( 1⁄2) and one spin down (–1⁄2). Hund’s Rule states that orbitals of the same energy, those which differ only in their orientation, are filled with electrons with the same spin before the second electron is added to any of the orbitals. This is why electrons have up spin, ↑, in the orbital diagrams of B to N and of Al to P in the diagrams above.

More Examples of Orbital Diagrams

Here are some orbital diagrams of elements with more electrons to help you understand the rules, electron configuration, orbital diagrams, and quantum numbers.

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