In a simple diatomic molecule such as hydrogen fluoride (chemical formula:
They are therefore unable to overlap and interact with the s-type valence orbital on the hydrogen atom.
An example of a non-similar one is the non-bonding orbital of the allyl anion, whose electron density is concentrated on the first and third carbon atoms.
However, in the context of localized molecular orbitals, the concept of a filled, non-bonding orbital tends to correspond to electrons described in Lewis structure terms as "lone pairs."
As an exception, the LUMO of phenyl cation is an spx (x ≈ 2) atomic orbital, due to the geometric constraint of the benzene ring.)
For example, n→π* transitions can be seen in ultraviolet-visible spectroscopy of compounds with carbonyl groups, although absorbance is fairly weak.