Why is the molecular ion a radical cation in IB Chemistry mass spectrometry?

In IB Chemistry, the molecular ion is described as a radical cation because the ionisation step removes exactly one electron from the molecule. Losing an electron creates a positive charge (a cation) because there is now one more proton than electrons overall. It is also a radical because removing one electron often leaves an unpaired electron behind. That unpaired electron is the dot you see in M+·. This detail matters because radicals are generally reactive and unstable, which helps explain why the molecular ion peak can be weak. In long explanations, you can connect this instability to why fragmentation peaks often dominate the spectrum.

Why do IB Chemistry mass spectra often show whole-number m/z values?

Most ions detected in typical IB-level mass spectrometry questions carry a +1 charge, meaning z = 1. When z = 1, the mass-to-charge ratio simplifies to m/z = m, so you read m/z like a mass value. Also, the common isotopes used at this level are treated using their mass numbers (integers), which makes the spectrum look neatly whole-numbered. Instruments can measure more precisely in real life, but exam questions keep it clean to test interpretation rather than instrumentation. The key is not memorising “whole numbers” but remembering the reason: charge is usually +1. That is why the molecular ion peak gives Mr so directly in IB Chemistry. If you see non-integer ideas elsewhere, it is usually because the question is moving into weighted averages for isotopic abundance rather than organic MS.

Does every compound in IB Chemistry show a clear molecular ion peak?

No, and that is a common misconception. Some molecular ions fragment so quickly after formation that the molecular ion peak is extremely small compared to fragment peaks. The spectrum can still contain the molecular ion peak, but it may be faint and easy to miss unless you deliberately search at the high m/z end. In those cases, isotopic patterns (like M and M+2 for Cl or Br) can be more reliable than intensity alone. Examiners like this because it rewards method over guesswork. If your first pass cannot find a confident M+, your next step should be to interpret fragments and isotopes together, not to abandon the idea of Mr.

IB Chemistry: Molecular Ion Peak in Mass Spectrometry

IB Chemistry

RevisionDojo Team

•11/21/2025•8 min read

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IB Chemistry: Molecular Ion Peak in Mass Spectrometry

IB Chemistry

RevisionDojo Team

•11/21/2025•8 min read

Mass spectra can feel like the exam version of a crowded city skyline. Lots of peaks. Lots of noise. And one quiet detail that changes everything.

In IB Chemistry, the molecular ion peak is that detail. It is the moment a messy unknown compound becomes a number you can trust: its relative molecular mass. Once you learn to find the molecular ion peak quickly, mass spectrometry stops being “guess the tallest peak” and starts being a calm, repeatable method.

Quick checklist for spotting the molecular ion peak (IB Chemistry)

Use this mini-routine whenever you see a spectrum in IB Chemistry:

Look near the highest m/z values for a plausible molecular ion peak (ignore tiny noise spikes).
Remember most ions are +1, so m/z = Mr for the molecular ion.
Check for M+1 and M+2 patterns (isotopes can sit next to the molecular ion peak).
Identify the base peak separately (it is the tallest, not necessarily the molecular ion peak).
If M+ looks weak or missing, use isotopes and fragments more heavily.

For the syllabus-aligned foundations, pair this with RevisionDojo’s notes on S3.2.8 Mass spectrometry (HL) and S1.2.3 Mass Spectrometry (HL).

What the molecular ion peak actually is

In mass spectrometry (as taught in IB Chemistry Topic 11/21 style questions), a sample is ionised, and the machine measures ions by their mass-to-charge ratio, m/z.

The molecular ion (also called the parent ion) forms when the molecule loses one electron but does not break apart:

[ M \rightarrow M^{+\cdot} + e^- ]

That radical cation still represents the whole molecule, so its m/z value corresponds to the compound’s relative molecular mass (the electron’s mass loss is negligible at this level).

The molecular ion peak is the peak on the spectrum that corresponds to this intact ion, M+·.

If you want a clean definition you can reuse in explanations, RevisionDojo’s IB Chemistry glossary phrases these key terms in an exam-friendly way.

How the molecular ion peak forms (and why it is often fragile)

A useful way to picture electron-impact ionisation is: the instrument is trying to tag your molecule with a charge so it can be guided and detected. The problem is the tagging process is violent.

The steps you should hold in your head for IB Chemistry:

The sample is vaporised.
High-energy electrons knock an electron out of the molecule.
The molecule becomes a radical cation, M+·.
The detector records its m/z, producing the molecular ion peak.

Because M+· is often unstable, it can fragment quickly. That is why, in many spectra, the molecular ion peak exists but is not the tallest. In IB Chemistry, students lose marks by assuming “tallest peak = molecular ion peak” instead of separating molecular ion peak vs base peak.

Molecular ion peak vs base peak vs isotope peaks

Molecular ion peak (M+)

Represents the intact molecule.
Usually the highest meaningful m/z peak for the compound (apart from isotope peaks).
Gives Mr directly when charge is +1.

Base peak

The tallest peak.
Represents the most abundant ion (often a stable fragment).
Helpful structurally, but it is not automatically Mr.

Isotopic peaks (M+1, M+2)

Sit near the molecular ion peak.
Come from naturally occurring isotopes (notably 13C; and halogens like Cl and Br).
Act like “fingerprints” for certain atoms.

RevisionDojo’s S3.2.12 Structural Analysis (HL) notes are a strong next step if you want to connect M+, fragments, and identification in one workflow.

Exam patterns IB Chemistry students should recognise fast

Propanone (acetone)

Propanone has Mr = 58, so a molecular ion peak around m/z 58 is your anchor. After that, fragments help confirm functional groups and common cleavages.

Chlorine-containing compounds

Chlorine shows two common isotopes (35Cl and 37Cl), producing an M and M+2 pattern with a 3:1 ratio in peak heights. In IB Chemistry, this is one of the quickest wins on unknown identification.

If you want the isotope story to feel less magical, revisit S1.2.2 Isotopes notes and then come back to spectra.

Bromine-containing compounds

Bromine’s two common isotopes (79Br and 81Br) give an M and M+2 pattern with a 1:1 ratio. When you see equal twins two units apart near the molecular ion peak, you should immediately think “Br”.

When the molecular ion peak is weak (or looks missing)

This is where IB Chemistry questions try to trip you with confidence.

Some compounds fragment so readily that the intact M+· is low intensity. You will commonly see this with molecules that form especially stable fragments, or with structures that break apart easily under electron impact (for example, many alcohols and highly branched molecules).

Your response strategy:

Still scan the highest m/z region for a small but real molecular ion peak.
Use M+2 patterns to confirm halogens even if the molecular ion peak is faint.
Lean on the base peak and common fragment losses (your Data Booklet helps here).

For targeted practice, RevisionDojo’s Structural Analysis Questionbank (HL) is ideal because it forces you to identify the molecular ion peak under exam-style pressure.

Closing: the calm way to earn marks with the molecular ion peak

The molecular ion peak is the quiet anchor point of mass spectrometry. In IB Chemistry, it turns a spectrum from “lots of peaks” into one clear starting number: Mr. From there, isotope patterns and fragmentation stop feeling random and start feeling like clues that agree with each other.

If you want this to stick under exam timing, use RevisionDojo as your daily practice loop: the Study Notes to learn the method, Flashcards for peak pattern recall, the Questionbank to build speed, and AI Chat to talk through spectra you get wrong. When you are close to exams, layer in Predicted Papers, Mock Exams, and Grading tools to simulate pressure, and use the Tutors and Coursework Library when you need targeted support.

For more skills that connect to spectroscopy questions, you can also explore Infrared Spectroscopy Explained and NMR Spectroscopy Explained Simply alongside this IB Chemistry guide.

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IB Chemistry: Molecular Ion Peak in Mass… | RevisionDojo

Mass spectra can feel like the exam version of a crowded city skyline. Lots of peaks. Lots of noise. And one quiet detail that changes everything.