Atomic Structure

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Edexcel IAL Chemistry · Unit 1

Atomic Structure

Every atom is a tiny solar system: a dense, positive nucleus (protons + neutrons) surrounded by electrons — and a mass spectrometer is basically a machine that weighs individual atoms one at a time by flinging them through a magnetic field and seeing how much they bend.

Summary — What This Chapter Covers

Atoms are built from three subatomic particles: protons, neutrons, electrons — each with a relative mass and relative charge you must know cold.
Atomic number (Z) = protons. Mass number (A) = protons + neutrons. These two numbers tell you everything about an atom's identity and composition.
Ions are atoms that have gained or lost electrons (never protons or neutrons) — this changes electron count only, not the element's identity.
Isotopes = same protons, different neutrons. Same chemistry, different physical properties (mass, density, boiling point).
The mass spectrometer vaporises, ionises, accelerates, deflects, and detects charged particles to find their mass and relative abundance.
Relative atomic mass (Aᵣ) is a weighted average of all isotopes, calculated from a mass spectrum using percentage abundances.
Molecules show a molecular ion (M⁺) peak at the highest m/z value — this gives the relative molecular mass — plus a small [M+1] peak from carbon-13.
Chlorine and bromine each exist as two isotopes, so compounds containing them show extra [M+2] and [M+4] peaks in fixed, predictable ratios (3:1, 9:6:1, 1:1, 1:2:1).

Topic 1 — Sub-Atomic Particles

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.

Analogy: Picture a football stadium. If the nucleus were a marble placed at the centre spot, the electrons would be like specks of dust swirling around near the very top row of seats. Atoms are almost entirely empty space — the "solid" stuff you touch every day is really just electron clouds repelling other electron clouds.

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.

Subatomic Particle	Relative Charge	Relative Mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1836 (≈ negligible)

Why is the electron's mass "negligible"?

An electron is about 1836 times lighter than a proton or neutron. That's why, when you calculate an atom's mass number, you only add up protons and neutrons — electrons barely register.

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