Chemistry Content: Matter, Atomic Structure, and Periodicity
This domain covers matter classification, atomic structure, electron configuration, and periodic trends. Exam items ask you to interpret atomic models, use quantum numbers, predict properties from position on the periodic table, and explain trends in radius, ionization energy, and electronegativity.
Core rules and concepts
- Matter can be classified as pure substance (element or compound) or mixture (homogeneous or heterogeneous). Physical changes alter form, not identity; chemical changes alter composition.
- Atomic number Z equals protons and identifies the element. Mass number A equals protons plus neutrons. Isotopes have the same Z but different A.
- Electron configuration follows the Aufbau principle, Pauli exclusion principle, and Hund’s rule. Orbitals fill in order of increasing energy.
- Across a period, effective nuclear charge increases, so atomic radius generally decreases and ionization energy and electronegativity increase. Down a group, added shells dominate, so radius increases and ionization energy decreases.
Worked example
Write the ground-state electron configuration of iron (Z = 26). Fill 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or [Ar] 4s² 3d⁶. The 4s orbital fills before 3d, but when transition-metal cations form, electrons are removed from 4s before 3d. For Fe²⁺, the configuration is [Ar] 3d⁶.
Traps that cost marks
Chromium and copper are exceptions: Cr is [Ar] 4s¹ 3d⁵ and Cu is [Ar] 4s¹ 3d¹⁰. Do not assume all cations lose electrons from the highest principal shell last; transition-metal ions lose 4s electrons first. A cation is smaller than its neutral atom, while an anion is larger. Noble gases have high ionization energy but are not automatically the largest atoms.