What's actually inside an atom?

Every atom — no matter which element — is made of the same three building blocks: protons, neutrons, and electrons. Protons and neutrons are packed tightly together in a super-dense central nucleus, while electrons zip around in the mostly-empty space surrounding it, occupying regions called orbitals/shells.

The nucleus is positively charged overall (protons are +, neutrons are neutral), while the electron cloud around it is negatively charged. In a normal, uncharged atom these balance out perfectly.

Why "relative" mass and charge?

Protons, neutrons and electrons are absurdly small — so small that measuring them in grams or coulombs would give you awkward numbers like 0.0000000000000000001602 coulombs. Instead, chemists compare particles to each other, giving simple relative values.

Atomic number (Z) and Mass number (A)

These two numbers are the "ID card" of every atom and appear on every entry in the periodic table.

Isotope notation puts A top-left and Z bottom-left of the element symbol, e.g. for carbon-12: ¹²₆C means 12 = mass number, 6 = atomic number.

Working out protons, neutrons, electrons in ions

This is the single most-tested skill in this section, so let's nail the logic once and for all:

1. Protons never change for a given element — they define which element it is. Protons = atomic number, always.
2. Neutrons = mass number − atomic number. Charge has zero effect on neutron count.
3. Electrons change with charge. A neutral atom has electrons = protons. A positive ion (cation) has lost electrons, so electrons = protons − charge. A negative ion (anion) has gained electrons, so electrons = protons + charge.

What makes atoms "isotopes" of each other?

Isotopes are atoms of the same element (same number of protons and electrons) that have a different number of neutrons. Because neutrons only add mass and don't affect charge or how electrons behave, isotopes of the same element are chemically identical twins that just weigh slightly different amounts.

The classic example: hydrogen has three natural isotopes.

Same chemistry, different physics

Chemical properties (identical): Isotopes react the same way in chemical reactions because chemistry is driven by electrons — specifically the electrons in the outer shell — and isotopes have identical electron arrangements.

Physical properties (slightly different): Extra neutrons add extra mass without adding charge. More mass per atom means small but measurable differences in density, melting point, and boiling point.

What does it actually do?

A mass spectrometer is a machine that weighs individual atoms or molecules with incredible precision by turning them into charged particles and watching how a magnetic field bends their path. It gives you two crucial pieces of information: the relative isotopic mass of each isotope present, and the relative abundance (what percentage of the sample each isotope makes up).

Real-world uses: detecting illegal drugs, forensic science, analysing samples from space missions, and carbon-14 dating of archaeological finds.

The five-step journey through the machine

1. Vaporisation: The sample is injected and heated until it turns into a gas.
2. Ionisation: An electron gun fires high-energy electrons at the gas particles, knocking electrons off them and creating positive ions.
3. Acceleration: Charged plates attract these positive ions, accelerating them into a beam moving at a controlled speed.
4. Deflection: The beam passes through a curved flight tube surrounded by electromagnets. The magnetic field bends the path of each ion — how much it bends depends on its mass-to-charge ratio (lighter ions deflect more sharply).
5. Detection: Ions strike a detector plate, each impact generating a tiny electrical current that's amplified. By varying the magnetic field strength, every ion in turn can be steered onto the detector, building up a full picture of masses and abundances.

Calculating Relative Atomic Mass (Aᵣ) from a spectrum

Since isotopes of an element have different masses, the "atomic mass" you see on the periodic table is really a weighted average — it accounts for how common each isotope actually is.

The molecular ion (M⁺) peak

When a whole molecule (not just an atom) is bombarded with electrons in the spectrometer, it loses one electron and becomes a molecular ion, written M⁺. On the resulting graph (mass spectrum), the peak furthest to the right — i.e. at the highest m/z value — is the molecular ion peak, and its m/z value equals the relative molecular mass of the whole compound.

The [M+1] peak

Just to the right of the main M⁺ peak, you'll often spot a much smaller peak one mass unit higher. This is the [M+1] peak, caused by the natural presence of carbon-13 (a heavier, rarer isotope of carbon that makes up about 1.1% of all carbon atoms) somewhere in the molecule.

The more carbon atoms a molecule contains, the more "chances" there are for one of them to be a ¹³C instead of ¹²C — so the [M+1] peak is taller for molecules with more carbon atoms (e.g. hexane's [M+1] peak is bigger than ethane's).

Chlorine and bromine peak patterns

Chlorine and bromine each naturally exist as two isotopes in significant amounts, so any compound containing them shows extra peaks — and the pattern of peak heights is a distinctive fingerprint you can learn to recognise instantly.