What's inside an atom — protons, neutrons and electrons — how we count them, and why isotopes give non-whole-number atomic masses.
Theme · StructureStructure 1.2
This is the Standard Level lesson, covering S1.2.1–S1.2.2. Taking Chemistry at Higher Level? The HL version adds mass spectrometry (S1.2.3) — reading a mass spectrum to find isotopic composition.
👆 Switch atoms in the builder · type abundances into the calculator
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. Pick an atom and watch the numbers — the atomic number (Z) is the number of protons; the mass number (A) is protons + neutrons.
Interactive atom builder
11H
Protons (Z)
1
Neutrons
0
Electrons
1
Mass no. (A)
1
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 — so the same atomic number but different mass numbers. Chlorine, for example, is a mix of chlorine-35 and chlorine-37.
Same 17 protons (both are chlorine), different neutrons → different mass numbers. Isotopes have identical chemistry but slightly different masses.
The relative atomic mass (Aᵣ) printed on the periodic table is the weighted average of the isotope masses, using their natural abundances:
Aᵣ = Σ (isotope mass × fractional abundance)
Aᵣ calculator · try chlorine (75% / 25%)
35.50(35×75 + 37×25) ÷ 100 = 35.50
Chlorine's Aᵣ is 35.45 on the data booklet — close to our 35.50 because the real abundances are ~75.8% / 24.2%.
Going to HL? The HL lesson adds mass spectrometry (S1.2.3): reading a mass spectrum — peak positions give isotope masses, peak heights give abundances — and using it to calculate Aᵣ. → Open the HL version
Common mistakes examiners see
What makes two atoms isotopes of the same element?✗ They have different numbers of protons.✓ Same number of protons (same element), different numbers of neutrons — so the same atomic number but different mass numbers.
How do you find the number of neutrons?✗ Neutrons = mass number + atomic number.✓ Neutrons = mass number − atomic number (A − Z).
Why is relative atomic mass rarely a whole number?✗ Because atoms have half-protons.✓ It is a weighted average of the isotope masses by abundance, so it lands between the whole-number isotope masses (e.g. Cl ≈ 35.45).
Do isotopes react differently?✗ Yes, chlorine-37 is a different element chemically.✓ No — isotopes have the same electron arrangement, so identical chemistry; only physical properties tied to mass differ slightly.