Periodic Table and Atomic Structure
Periodic Table and Atomic Structure
This domain tests the evidence for atomic structure, electron configuration and the periodic trends that follow from nuclear charge and shielding. Expect definitions, isotope calculations, orbital diagrams and explanations of trends rather than simple recall.
Core rules
- Atomic number Z is the number of protons; mass number A is protons plus neutrons. Isotopes have the same Z but different A.
- Electrons fill in order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Use Aufbau, Pauli exclusion and Hund's rule; Cr and Cu are exceptions.
- Across a period, effective nuclear charge rises and atomic radius falls; down a group, extra shells increase radius.
- First ionisation energy generally rises across a period and falls down a group. Exceptions occur where a new subshell starts or extra shielding outweighs charge, e.g. Al after Mg.
- Electronegativity rises across and falls down.
Worked example
Chlorine has isotopes 35Cl (75.0%) and 37Cl (25.0%). Relative atomic mass = (35 × 75.0 + 37 × 25.0)/100 = (2625 + 925)/100 = 35.5. The answer is a weighted mean, not a simple average of 35 and 37.
Traps
Do not confuse mass number with relative atomic mass; the periodic table value is an average. When writing configurations for ions, remove electrons from the highest-energy occupied subshell first, not always the outermost shell by n only. Ionisation-energy explanations must mention attraction, shielding and subshell stability.