molecular ion
/ muh-LEK-yoo-ler EYE-on /
Imagine taking a photograph of a whole, intact vase before anyone drops it — the picture of the complete object, before it shatters into shards. In a mass spectrum, the molecular ion is that intact-vase photograph: it is the peak from the whole molecule, charged but not yet broken into fragments.
Precisely, the molecular ion is the ion formed by ionizing the entire molecule while leaving its atoms together — typically by removing one electron (giving a radical cation written M+•) or by adding or removing a proton. Its mass-to-charge value reveals the molecule's molecular mass directly, which is why it anchors the interpretation of the whole spectrum.
It matters because the molecular ion answers the first question about an unknown — how heavy is the whole molecule? — and everything else is read relative to it. The honest caveat is that the molecular ion is not always visible: harsh ionization may fragment the molecule so thoroughly that the intact-molecule peak is faint or absent, which can make assigning the molecular mass tricky.
In the mass spectrum of ethanol, the molecular ion appears at m/z 46, matching ethanol's molecular mass; smaller fragment peaks below it come from pieces that broke off the intact ion.
The molecular ion gives the mass of the whole, unbroken molecule.
The molecular ion is not always the base peak (the tallest peak): a fragment can be more abundant than the intact molecular ion. The two coincide only when the whole molecule happens to be the most common ion.