Editing Quantum number (section)

=== Principal quantum number ===
The principal quantum number describes the [[electron shell]] of an electron. The value of {{mvar|n}} ranges from 1 to the shell containing the outermost electron of that atom, that is<ref>{{cite book|title=Concepts of Modern Physics |edition=4th |first=A. |last=Beiser |publisher=McGraw-Hill (International) |date=1987 |isbn=0-07-100144-1}}{{page needed|date=November 2019}}</ref>
<math display=block>n = 1, 2, \ldots</math>

For example, in [[caesium]] (Cs), the outermost [[valence (chemistry)|valence]] electron is in the shell with energy level 6, so an electron in caesium can have an {{mvar|n}} value from 1 to 6. The average distance between the electron and the nucleus increases with {{mvar|n}}.