Atomic Structure, Periodicity, and Nuclear Chemistry
What Domain II Tests
This domain covers atomic structure, electron configuration, periodic trends, and nuclear chemistry. Expect questions that connect subatomic structure to position on the periodic table, predict trends in radius and ionization energy, write nuclear equations, and apply half-life reasoning.
Core Rules and Concepts
- Atomic number (Z) equals protons and identifies the element; mass number (A) equals protons plus neutrons. Isotopes have the same Z but different A; ions have different electron counts.
- Electron configuration follows Aufbau (lowest energy first), Pauli (two electrons per orbital, opposite spins), and Hund's rule (maximize unpaired electrons in degenerate orbitals). Exceptions include Cr ([Ar]4s¹3d⁵) and Cu ([Ar]4s¹3d¹⁰).
- Periodic trends: atomic radius decreases across a period and increases down a group; first ionization energy generally increases across a period and decreases down a group; electronegativity follows the same pattern as ionization energy. Effective nuclear charge and shielding explain these trends.
- Nuclear decay: alpha emission loses ⁴₂He; beta-minus emission converts a neutron to a proton and increases Z by 1; positron emission decreases Z by 1; gamma emission changes neither A nor Z. Nuclear equations conserve mass number and atomic number.
- Half-life is the time for half a sample to decay; after n half-lives, the remaining fraction is (1/2)ⁿ.
Worked Example
An 80.0 g sample of a radioisotope has a half-life of 12 days. How much remains after 36 days? Reasoning: 36 days / 12 days = 3 half-lives. Remaining mass = 80.0 g × (1/2)³ = 80.0 g × 1/8 = 10.0 g. The decayed mass is 70.0 g. If the question asked for activity instead of mass, the same fraction applies because activity is proportional to the number of undecayed nuclei.
Traps That Cost Marks
- Confusing mass number with average atomic mass; average atomic mass is a weighted average of isotopes.
- Writing beta decay as if a proton becomes a neutron; beta-minus decay does the reverse.
- Assuming all isotopes of an element have identical chemical behavior in every respect; masses differ, and kinetic isotope effects can matter.
- Forgetting that ionization energy increases across a period but drops at group boundaries because a new shell begins.
- Treating half-life as a linear loss of 50% per interval rather than an exponential fraction.