CHEM 121 glossary
Orbital diagram
A configuration drawn with one box per orbital and an arrow per electron (↑ or ↓ for the two spins). It shows what the superscript notation hides: which electrons are paired and how many are unpaired.
Oxygen: one orbital is paired, two hold single electrons.
Example
Oxygen, : the set is drawn (↑↓)(↑)(↑), so O has 2 unpaired electrons.