Periodic Trends

Objective: Qualitatively compare trends and their limitations.

Across a period of main-group elements, increasing effective nuclear attraction often reduces atomic radius, though electron configurations and definitions affect specific comparisons. Down a group, additional occupied shells generally increase radius. First ionization energy is energy required to remove an electron from a gaseous neutral atom; it generally rises across a period and falls down a group with known exceptions. Electronegativity describes an atom’s tendency to attract shared bonding electrons under a defined scale and often increases toward the upper right among comparable elements. These are qualitative trends with exceptions, not monotonic laws that override measured values. Ion size can differ substantially from the corresponding neutral atom.

Objective: Apply periodic trends to calculations, evidence, and specified situations.

Typical main-group atomic radius increases down a group as additional principal shells are occupied. Across a period, increasing effective nuclear attraction generally reduces radius. First ionization energy is the energy required to remove an electron from a gaseous atom; it generally rises across a period and falls down a group, with configuration-related exceptions. Cations often have smaller radii than their neutral atoms; anions often have larger radii. Comparisons should state whether they involve neutral atoms or ions.