Subatomic particles isotopes and mass spectrometry

An atom is built from a dense central nucleus of protons and neutrons, with electrons occupying shells outside the nucleus.

Protons and neutrons each have a relative mass of 1, while an electron has a relative mass of about 1/1836 (effectively negligible).

Protons carry a charge of +1, electrons carry a charge of −1, and neutrons are uncharged.

Key Definition The atomic number (Z) is the number of protons in the nucleus of an atom.

Key Definition The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus.

Subtracting Z from A gives the number of neutrons; the number of electrons equals Z in a neutral atom and is then adjusted for any ion charge.

Key Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, and therefore different mass numbers.

Isotopes have identical chemical properties because their electron arrangements are the same, but they have different physical properties, such as mass and density.

A mass spectrometer separates positive ions according to their mass-to-charge ratio (m/z) through four stages:

1. Ionisation — the sample is converted into positive ions.
2. Acceleration — the positive ions are accelerated by an electric field to a constant kinetic energy.
3. Deflection — the ions are deflected by a magnetic field, with lighter ions and more highly charged ions deflected more.
4. Detection — the ions are detected and their relative abundances recorded.

Each peak in a mass spectrum gives the m/z of an ion, and the y-axis gives its relative abundance.

The relative atomic mass is calculated as a weighted mean of the isotopic masses using the formula:

$A_{\mathrm{r}}$ = Σ(isotopic mass × relative abundance) ÷ Σ(relative abundance) (when abundances are given as percentages, divide by 100)

For molecules, the peak at the highest m/z (the molecular ion peak) gives the relative molecular mass, $M_{\mathrm{r}}$.

Mass spectrometry links subatomic structure to measurable data and allows the identification of elements and molecules.

Recognising isotope patterns — especially the 9 : 6 : 1 triplet for Cl₂ (at m/z 70, 72, 74) and the 1 : 2 : 1 triplet for Br₂ (at m/z 158, 160, 162) — is a recurring exam skill.

Doubly charged ions appear at half their expected m/z and must be distinguished from lighter species.

Carrying out probability-based calculations for diatomic molecules is the most common predictive task in this topic.

At a Glance