Trends in Physical and Chemical Properties — Core Principles

Core Principles

Group 16 elements, or Chalcogens (O, S, Se, Te, Po), exhibit systematic trends in their properties. Atomic and ionic radii increase down the group due to the addition of new electron shells. Consequently, ionization enthalpy, electron gain enthalpy (with oxygen as an exception), and electronegativity all decrease down the group, reflecting a weaker hold on valence electrons.

Metallic character increases from non-metallic oxygen and sulfur, through metalloid selenium and tellurium, to metallic polonium. Melting and boiling points generally increase down the group due to increasing atomic mass and stronger van der Waals forces, though oxygen and sulfur show distinct molecular structures.

Chemically, they all have $ns^2np^4$ valence configuration, commonly showing -2 oxidation state. Heavier elements also exhibit +2, +4, and +6 states, with the stability of +4 increasing down the group due to the inert pair effect.

Their hydrides ($H_2X$) show increasing acidic and reducing character but decreasing thermal stability down the group, with $H_2O$ being anomalous due to hydrogen bonding. Oxygen's unique behavior is attributed to its small size, high electronegativity, and absence of d-orbitals.

Important Differences

vs Group 15 Elements (Pnictogens)

Aspect	This Topic	Group 15 Elements (Pnictogens)
Valence Electron Configuration	Group 16: $ns^2np^4$	Group 15: $ns^2np^3$
Ionization Enthalpy (within same period)	Lower than Group 15	Higher than Group 16 (due to stable half-filled $np^3$)
Common Negative Oxidation State	-2 (to achieve $ns^2np^6$)	-3 (to achieve $ns^2np^6$)
Electronegativity (within same period)	Higher than Group 15	Lower than Group 16
Tendency to form Hydrogen bonds	Strong for Oxygen (in $H_2O$)	Weak for Nitrogen (in $NH_3$)

Group 16 elements (Chalcogens) differ significantly from Group 15 elements (Pnictogens) primarily due to their valence electron configuration. Group 16 has $ns^2np^4$, needing two electrons to complete its octet, hence a common -2 oxidation state. Group 15 has $ns^2np^3$, needing three electrons, leading to a common -3 state. Interestingly, Group 16 elements generally have lower ionization enthalpies than their Group 15 counterparts in the same period, because removing an electron from Group 16's $np^4$ configuration leads to a more stable half-filled $np^3$ configuration. Group 16 elements are also more electronegative than Group 15 elements.