Electronic structure: shells and orbitals
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Summary
Electronic structure: shells and orbitals
In a nutshell
Understanding electronic structure is crucial for understanding the chemistry of reactions. Here, we will review how to deduce electronic configurations of atoms and explain what is meant by the terms shells, subshells and orbitals.
Electron shells
Electrons orbit the nucleus in electron shells, called quantum shells which are given principal quantum numbers. Each quantum shell contains different types of subshells. Each subshell contains a different number of orbitals. The four types of subshells and the number of electrons they can hold are summarised below.
subshell | number of orbitals | maximum electrons |
| | | |
| | | |
| | | |
| | | |
And the subshells and electrons in the first four quantum shells are summarised below.
shell | subshells | number of electrons |
| | | |
| | | |
| | | |
| | | |
Orbitals
Each subshell contains different numbers of orbitals, which can hold up to two electrons. Orbitals have characteristic shapes.
- orbitals are spherical.
- orbitals are shaped like dumbbells, of which there are three.
Below is an -orbital.
Below is a -orbital, of which there are three kinds; , and .
Note: Electrons in each orbital spin in opposite directions, this is called 'spin-pairing'.
Deducing electronic configuration
A simple set of rules must be followed to work out electronic configuration.
Rule | explanation |
| 1. Arrows in boxes | Electrons in orbitals are represented by arrows in boxes. Each box represents one orbital and an arrow represents one electron.  |
| 2. Lowest energy subshells first | Electrons fill up the lowest energy subshells first. From low to high, the order is: . Note, unusually, is filled before because it is lower in energy, even though it's principal quantum number is bigger; this is an exception you must remember. Below is the electronic configuration of calcium. |
| 3. Single before double | Electrons fill orbitals singularly before they start to pair up.  1. Nitrogen's electron arrangement. 2. Oxygen's electron arrangement. |
To represent electronic configuration, you can either draw out the energy levels (as in the image for Rule 2. above) or use subshell notation.
Example
For potassium, the subshell notation is: . You can check this is correct by adding the total of the superscripts. Here, it equals , which tells you it is correct as that is the number of electrons in a atom.
Note: Noble gas symbols (i.e or ) are sometimes used for simplicity. For potassium, it is also correct to write the configuration as [], since the [] corresponds to .
Using the periodic table
The periodic table can be used as it is split into s-block, p-block and d-block elements.
- s-block elements have an outer shell configuration of or .
- p-block elements have an outer shell configuration of to .
- d-block elements have an outer shell configuration of to .
Remember: must be filled to , and only afterwards can be filled.
Exception to the rule
and don't fit the usual rule. Their 4 shell only ever fills to as they donate the second electron to the subshell as it's more stable.
FAQs - Frequently Asked Questions
What is meant by 'spin-pairing'?
Spin-pairing explains that electrons in each orbital spin in opposite directions.
How do you work out electronic configuration?
To deduce electronic configuration: 1. Draw arrows in boxes, 2. Fill lower energy subshells first, 3. Fill electrons singularly before pairing up.
What shapes do orbitals have?
s orbitals are spherical. p orbitals are shaped like dumbbells, of which there are three.