Atomic Structure is a core part of IB Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Proton+1 charge, mass 1, in nucleus
Neutron0 charge, mass 1, in nucleus
Electron-1 charge, ~1/1836 mass, in shells
Atomic number ZNumber of protons
Mass number AProtons + neutrons
IsotopeSame Z, different A (e.g. ¹²C, ¹⁴C)
Mass spectrometryIonises atoms, sorts by m/z
m/zMass-to-charge ratio
AbundanceRelative % of each isotope
Emission spectrumDiscrete lines from e⁻ falls
Energy levelQuantised orbit energy
ConvergenceLines converge → ionisation
Aufbau principleFill lowest energy orbitals first
Pauli exclusionMax 2 electrons/orbital, opposite spins
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
If a complex ion has a coordination number of 4, what shape does it have?✗ A coordination number of 4 always means tetrahedral, because that gives the maximum bond-angle separation. ✓ It can be either tetrahedral OR square planar — the shape depends on the metal and ligands (e.g. [Pt(NH3)4]2+ is square planar; [CuCl4]2- is tetrahedral).
In 1H NMR, what does the n+1 rule predict?✗ Splitting tells you how many protons are in the same chemical environment as the one giving the peak. ✓ A proton with n equivalent neighbouring protons will appear as a peak split into n+1 sub-peaks.
What does the M+ peak in a mass spectrum represent?✗ The M+ peak shows the mass of the largest fragment after the molecule breaks apart. ✓ The molecular ion — formed when a single electron is removed from the whole molecule, giving the relative molecular mass.
Why are most transition metal complex ions coloured?✗ Transition metal ions are coloured because they have a half-full d-subshell which naturally absorbs light. ✓ Ligands split the d-orbitals into two energy levels; electrons absorb visible light to jump between them (d-d transitions), and the complementary colour is transmitted.
Why can transition metals show variable oxidation states?✗ Transition metals have variable oxidation states because they have many shells of electrons. ✓ The 4s and 3d orbitals are similar in energy, so different numbers of electrons can be removed without a large energy cost.
Practise Atomic Structure — free games
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