Atomic Structure
This domain tests the particles that make up atoms, isotopes, mass spectrometry and the quantum model of the atom. STPM expects precise definitions and calculations from mass spectra rather than simple recall.
Fundamental particles and isotopes
Atoms contain protons, neutrons and electrons. Atomic number Z equals the number of protons; mass number A equals protons plus neutrons. Isotopes have the same Z but different A, so they have identical chemical properties but different physical properties. Relative isotopic mass compares one atom of an isotope with 1/12 the mass of a carbon-12 atom. Relative atomic mass Ar is the weighted mean of the relative isotopic masses.
Mass spectrometry
A mass spectrometer vaporises, ionises, accelerates, deflects and detects ions. The x-axis is m/z; for singly charged ions, m/z equals relative isotopic mass. Peak height gives relative abundance. Ar = sum(relative abundance × m/z)/sum(relative abundances).
Atomic orbitals and quantum numbers
An orbital is a region of space where the probability of finding an electron is high. The principal quantum number n gives the shell; l gives the subshell (0 to n−1); m gives orientation (−l to +l); spin is ±1/2. s orbitals are spherical, p orbitals dumbbell-shaped. Pauli exclusion allows at most two electrons per orbital with opposite spins; Hund’s rule fills degenerate orbitals singly before pairing.
Worked example
Magnesium has peaks at m/z 24 (79.0%), 25 (10.0%) and 26 (11.0%). Ar = (24×79.0 + 25×10.0 + 26×11.0)/100 = (1896 + 250 + 286)/100 = 24.32.
Traps that cost marks
Do not use the mass number of the most abundant isotope as Ar; include every peak. For ions, electron count differs from proton count. A mass spectrometer measures m/z, not mass. Isotope effects are small in chemical reactions but significant in physical properties such as density and diffusion rate.