Atomic Structure and the Periodic Table
This domain tests how evidence for atomic structure explains the layout and trends of the periodic table: subatomic particles, isotopes, mass spectra, electron configuration and ionisation energy.
Core rules
- Proton number (Z) fixes the element; nucleon number (A) = protons + neutrons. Isotopes have the same Z but different A.
- Relative atomic mass is the weighted mean of relative isotopic masses on the 12C scale, not a simple average of mass numbers.
- Electrons fill in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d. The 4s orbital fills before 3d, but transition-metal ions lose 4s electrons first.
- Across Period 3, atomic radius decreases and first ionisation energy generally increases, with dips at Al (3p1) and S (3p4) because of subshell energy and electron-pair repulsion. Down a group, ionisation energy falls as shells increase, shielding rises and the outer electron is farther from the nucleus.
- A mass spectrometer records m/z peaks; peak height gives relative abundance.
Worked example
Natural chlorine is 75.8% 35Cl (relative isotopic mass 34.97) and 24.2% 37Cl (36.97). Ar = (0.758 × 34.97) + (0.242 × 36.97) = 26.51 + 8.95 = 35.46. Convert percentages to fractions, multiply each by its isotopic mass, then add. Do not average the two masses equally or round 35Cl to 35 unless told to.
Traps
Using mass number as relative isotopic mass; writing 3d before 4s; explaining ionisation energy without specifying gaseous atoms; claiming radius increases across a period; forgetting that isotopes have identical chemical properties but different physical properties such as density of the gas.