Classification of Elements and Periodicity
Classification of Elements and Periodicity tests the periodic law, the modern periodic table, and trends in atomic radius, ionization enthalpy, electron gain enthalpy, electronegativity, and metallic character. Questions often ask you to explain an exception rather than state a trend.
Core rules
- Across a period, effective nuclear charge rises and radius falls; ionization enthalpy and electronegativity generally increase.
- Down a group, added shells outweigh the nuclear charge increase; radius rises and ionization enthalpy falls.
- Cations are smaller than their atoms; anions are larger. Isoelectronic species: higher nuclear charge gives smaller radius.
- Diagonal relationships arise from similar charge/radius ratios, as in Li–Mg and Be–Al.
- Transition-series radii change only slightly because 3d shielding offsets the rising nuclear charge.
Worked example
Compare the first ionization enthalpy of Mg and Al. Mg is [Ne]3s2; Al is [Ne]3s2 3p1. Al's 3p electron is higher in energy and is shielded by the 3s pair, so it is removed more easily even though Al has the larger nuclear charge. Mg is about 738 kJ/mol and Al about 578 kJ/mol. The expected period trend is therefore reversed.
Traps
Do not apply the period trend blindly to Group 15/16: N has a higher first ionization enthalpy than O because N has a stable half-filled 2p3 set. Electron gain enthalpy of F is less negative than that of Cl because the small F atom has strong electron–electron repulsion. Do not confuse atomic radius with metallic radius or van der Waals radius.