molecular ion
/ moh-LEK-yoo-lar EYE-on /
In the mass spectrometer, the very first thing that happens to your molecule is that it gets hit hard enough to lose one electron. The intact molecule, now missing a single electron and carrying a positive charge, is the molecular ion. It is the molecule itself, whole, just ionized — and because it has not yet broken into pieces, its mass is the mass of the whole molecule. This makes it the single most valuable line in the spectrum: read its position and you have the molecular weight.
It is written M+ or, more honestly, M+• with a dot, because knocking off one electron leaves an odd number of electrons — so the molecular ion is a radical cation, both a radical (unpaired electron) and a cation (positive charge) at once. Its m/z value equals the molecular weight. With a high-resolution instrument you measure that value to several decimals and can determine the exact molecular formula, because each formula made of C, H, N, O has a unique exact mass.
There is an honest catch: the molecular ion is often unstable and may break apart so completely that its line is tiny or even absent. Some molecules, especially branched or alcohol-containing ones, fragment so readily that almost no whole molecular ion survives to reach the detector. So a missing M+ does not mean the experiment failed; it means the molecule is fragile, and softer ionization methods are then used to coax a molecular ion into surviving.
Benzene gives a tall molecular-ion line at m/z 78, equal to its molecular weight (C6H6 = 78). Because the benzene ring is stable, that whole-molecule line dominates the spectrum, unlike fragile alcohols whose M+ can nearly vanish.
The molecular ion's m/z is the molecular weight — when it survives to be seen.
Do not assume the tallest peak is the molecular ion. The tallest peak is the base peak, which is usually a stable fragment; the molecular ion is the highest-mass peak (ignoring small isotope lines), which can be quite short.