IB Chemistry HL — Structure 1.2: The Nuclear Atom

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What makes two atoms isotopes of the same element?

This is the Higher Level lesson. The core S1.2.1–S1.2.2 (subatomic particles, isotopes, relative atomic mass) is shared with SL; the HL-only statement S1.2.3 — mass spectrometry carries a purple HL badge.

Core — SL & HL

HL Additional Higher Level only

👆 Switch atoms · click the mass-spectrum peaks

1. Inside the atom S1.2.1

An atom has a tiny, dense nucleus of positively charged protons and neutral neutrons, surrounded by fast-moving negatively charged electrons. The atomic number (Z) is the number of protons; the mass number (A) is protons + neutrons.

Interactive atom builder

11H

Protons (Z)

Neutrons

Electrons

Mass no. (A)

Neutral atom → electrons = protons. Mass number A = protons + neutrons. (Shells show the 2,8,8 filling pattern — the detail comes in Structure 1.3.)

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	≈ 1/1836 (negligible)	Outside the nucleus

2. Isotopes & relative atomic mass S1.2.2

Isotopes are atoms of the same element (same protons) with different numbers of neutrons. The relative atomic mass (Aᵣ) is the weighted average of the isotope masses by their natural abundances:

Aᵣ = Σ (isotope mass × fractional abundance)

Aᵣ calculator

Isotope 1 mass

Abundance 1 (%)

Isotope 2 mass

Abundance 2 (%)

35.50(35×75 + 37×25) ÷ 100 = 35.50

3. Mass spectrometry HL

A mass spectrometer ionises the sample, accelerates the ions and separates them by mass-to-charge ratio (m/z). The result is a mass spectrum: each peak is an isotope. Peak position = isotope mass (m/z); peak height = relative abundance. Click a peak to read it, then calculate Aᵣ.

Interactive mass spectrum · click a peak

Click a peakEach peak is one isotope: its position on the x-axis is the isotope mass, its height is the relative abundance.

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What makes two atoms isotopes of the same element?✗ They have different numbers of protons. ✓ Same protons (same element), different neutrons — same atomic number, different mass numbers.

On a mass spectrum, what does peak height tell you?✗ The mass of the isotope. ✓ Peak height = relative abundance; peak position (m/z) = isotope mass. HL

How do you get the number of neutrons?✗ mass number + atomic number. ✓ neutrons = mass number − atomic number (A − Z).

Why is Aᵣ rarely a whole number?✗ Atoms have half-protons. ✓ It's a weighted average of isotope masses by abundance, so it sits between the whole-number isotope masses.

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The Nuclear Atom