Atomic Structure and the Periodic Table
Atomic Structure and the Periodic Table
This domain tests whether you can link electron configuration to chemical behaviour: ionisation-energy values, trends across Periods 2 and 3, and evidence from mass spectra.
Configuring electrons
Fill orbitals in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p. The Pauli principle (two electrons per orbital, paired spins), Hund's rule (singly occupy degenerate orbitals before pairing) and the Aufbau principle generate every configuration. Cr is [Ar]3d5 4s1 and Cu is [Ar]3d10 4s1 because half-filled and full d subshells are extra stable. On ionisation, 4s electrons leave before 3d: Fe is [Ar]3d6 4s2, but Fe2+ is [Ar]3d6.
Ionisation energy
First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms. Four factors control it: nuclear charge, shielding by inner electrons, distance from the nucleus, and electron-pair repulsion. Successive ionisation energies jump sharply when a new shell is breached, which locates the element's group.
Worked example
Magnesium has isotopes 24Mg (79.0%), 25Mg (10.0%) and 26Mg (11.0%). Relative atomic mass = (24 x 79.0 + 25 x 10.0 + 26 x 11.0) / 100 = (1896 + 250 + 286) / 100 = 24.32. For precise data use isotopic masses rather than mass numbers. Now explain why the first ionisation energy of Al (577 kJ mol-1) is lower than that of Mg (738 kJ mol-1): aluminium's outer electron is in a 3p orbital, higher in energy and shielded by the 3s pair, and this outweighs aluminium's larger nuclear charge.
Marks lost here
Omitting the gaseous state from the definition; writing 3d before 4s for atoms but not for ions; saying shielding cancels nuclear charge; missing the small dips from Be to B (2s to 2p) and from N to O (spin-pair repulsion in 2p); and quoting a trend without naming the factor responsible. For periodicity, remember metallic character decreases across a period, so the highest oxides change from basic through amphoteric to acidic.
Key ideas
- Ionisation energy depends on charge, shielding, distance and spin-pair repulsion, in that order of explanation.
- 4s fills before 3d in atoms but empties first in ions.
- Period 3 melts: Na, Mg, Al metallic; Si giant covalent; P4, S8, Cl2, Ar molecular.
- Mass spectra give isotope abundances; the molecular ion peak gives relative molecular mass.
- Small dips in ionisation-energy graphs need a specific electronic reason, not a general trend statement.