Atomic Structure & Amounts is a core part of A-Level Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
ProtonPositive subatomic particle in the nucleus; relative mass 1.
NeutronNeutral subatomic particle in the nucleus; relative mass 1.
ElectronNegative particle in shells around the nucleus; negligible mass.
Atomic numberThe number of protons, which identifies the element.
Mass numberThe total number of protons and neutrons.
IsotopeAtoms of the same element with different numbers of neutrons.
s orbitalSpherical orbital holding up to 2 electrons.
p orbitalDumbbell-shaped orbital; 3 per subshell, 6 e- max.
d orbitalComplex shape; 5 per subshell, 10 e- max.
Ionisation energyEnergy to remove 1 mol e- from gaseous atoms.
Aufbau principleElectrons fill lowest energy orbitals first.
Hund's ruleElectrons singly occupy degenerate orbitals first.
ElectronegativityAtom's ability to attract bonding electrons.
Polar bondBond with unequal electron sharing, has dipole.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
Where is almost all the mass of an atom located?✗ Electrons contribute significantly to the mass of an atom. ✓ In the nucleus, made up of protons and neutrons.
What is different between two isotopes of the same element?✗ Isotopes have different numbers of electrons, which is why their mass differs. ✓ They have the same number of protons but different numbers of neutrons.
Why is the 4s orbital filled before the 3d orbital?✗ Because the 4s shell is closer to the nucleus than the 3d shell. ✓ Because the 4s orbital has a lower energy than 3d when both are empty, so electrons fill it first.
What is a hydrogen bond?✗ A hydrogen bond is a covalent bond involving hydrogen. ✓ A strong dipole-dipole interaction between an H atom bonded to N/O/F and a lone pair on another electronegative atom.
Why does graphite conduct electricity but diamond does not?✗ Graphite conducts because it is a metal, while diamond is a non-metal. ✓ Graphite has delocalised electrons between layers; diamond has all four outer electrons in localised covalent bonds.
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