Chemistry › Atomic structure
Atomic structure
Unit C01 · 4 lessons.
An atom is a nucleus of protons and neutrons with electrons around it, and atomic structure puts numbers on that picture. A time-of-flight mass spectrometer weighs ions to give a relative atomic mass, electrons fill shells, subshells and orbitals in a set order, and successive ionisation energies give the evidence for that order.
The trap is in the two dips across Period 3. First ionisation energy falls from magnesium to aluminium because aluminium's outer electron is in a 3p subshell, higher in energy than 3s. It falls from phosphorus to sulfur because sulfur's electron comes from a 3p orbital that already holds a pair, and the two repel. The sulfur dip is not more shielding: shielding hardly changes from phosphorus, and the mark is for the repulsion in the pair.
AQA Papers 1 and 3, OCR A Papers 1, 2 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1 and 2 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.1.1.1, 3.1.1.2, 3.1.1.3 · OCR A 2.1.1, 2.2.1, 3.1.1 · Edexcel 1.1 to 1.25, 15.1 · CAIE 1.1, 1.2, 2.1, 22.2, 1.3, 1.4
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Protons, neutrons, electrons and isotopes
The proton, neutron and electron with their relative masses and charges, atomic number and mass number, nuclide notation, isotopes and why they have the same chemical properties, counting the particles in atoms and ions, the deflection of beams of protons, neutrons and electrons in an electric field, and how the model of the atom changed from the solid sphere and the plum pudding to the nuclear atom, Bohr's energy levels and orbitals.
- Time-of-flight mass spectrometry and relative atomic mass
The four stages of a time-of-flight mass spectrometer, with electron impact and electrospray ionisation, acceleration to the same kinetic energy, ion drift in the flight tube and detection; flight times from KE = ½mv² and t = d/v; reading a mass spectrum; relative isotopic mass and relative atomic mass calculated from abundances; the molecular ion; the spectra of magnesium, chlorine and bromine; and the M+1 peak from carbon-13 used to count carbon atoms, with the M+2 peaks of organic chlorine and bromine compounds.
- Electron configuration: shells, subshells and orbitals
Shells and energy levels, s, p, d and f subshells and how many electrons each holds, orbitals and the shapes of s and p orbitals, the order of filling up to krypton with 4s before 3d, the Pauli principle and Hund's rule, electrons in boxes, configurations of atoms and ions including chromium and copper, 4s electrons lost first, ground and excited states, the principal quantum number, counting unpaired electrons in transition metal ions, and the s, p, d and f blocks of the periodic table.
- Ionisation energies and the evidence for subshells
First ionisation energy and its equation, second and successive ionisation energies, nuclear charge, distance and shielding, the decrease down a group, the increase across Period 3 and the dips at aluminium and sulfur, the same dips in Period 2, the large jump that identifies a group, log graphs of successive ionisation energies as evidence for shells, atomic radius, and the energy needed to form an ion such as Al³⁺ from the sum of its ionisation energies.
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