What is the inert-pair effect?

In the heavier p-block elements the valence s electrons become increasingly reluctant to participate in bonding, so oxidation states two units below the group maximum become more stable, as in the preference of lead for the +2 over the +4 state.

Why is carbon's chemistry so much richer than that of the heavier group 14 elements?

Carbon forms exceptionally strong carbon–carbon and carbon–hydrogen bonds and readily makes multiple bonds, allowing extensive catenation into chains and rings; the heavier elements form weaker bonds and catenate far less, limiting their structural variety.

p-Block Elements and Nonmetals

The p-block spans groups 13 to 18 and contains most of the nonmetals and metalloids, displaying the richest variety of oxidation states, allotropes, and bonding in the periodic table.

Definition

The p-block elements are those of groups 13 to 18, in which the highest-energy electrons fill a p subshell; they encompass metals, metalloids, and nonmetals and show variable oxidation states, extensive catenation, and diverse molecular structures.

Scope

This topic covers the descriptive chemistry of groups 13 to 18 (excluding the noble gases treated separately): the metalloid–nonmetal gradient, multiple oxidation states and the inert-pair effect, catenation and allotropy in carbon, phosphorus, and sulfur, the hydrides, oxides, oxoacids, and halides of the nonmetals, and the anomalous chemistry of the first-row elements. It emphasizes periodic trends and characteristic compounds rather than cluster bonding, which is treated under boron and electron-deficient clusters.

Core questions

How does character shift from metallic to nonmetallic across and down the p-block?
Why do heavy p-block elements favour lower oxidation states (the inert-pair effect)?
How do catenation and allotropy give carbon, phosphorus, and sulfur their varied forms?
What governs the strength of the nonmetal oxoacids?

Key concepts

Metalloids and the diagonal band
Inert-pair effect
Catenation and allotropy
Hydrides and halides of nonmetals
Oxides and oxoacids
First-row anomaly

Key theories

Periodic trends and the inert-pair effect: Across the p-block electronegativity rises and metallic character falls, while down each group the increasing reluctance to use the valence s electrons stabilizes oxidation states two below the group number.
Catenation and allotropy: Light p-block elements form strong element–element bonds, giving carbon its vast catenated chemistry and producing the allotropes of carbon, phosphorus, and sulfur with distinct structures and reactivities.
Oxoacids and the nonmetal oxides: Nonmetals form a graded series of oxides and oxoacids whose acid strength correlates with the oxidation state and electronegativity of the central atom, systematized by rules such as Pauling's for oxoacid strength.

Clinical relevance

The p-block supplies the carbon, nitrogen, oxygen, and phosphorus of biology, the silicon of semiconductors and glass, the halogens of disinfectants and pharmaceuticals, and the fixed nitrogen and phosphate of fertilizers.

History

The p-block elements were central to the construction and testing of the periodic table, with Mendeleev predicting the properties of then-undiscovered members such as gallium and germanium. Moseley's X-ray work fixed their atomic numbers, and Pauling's electronegativity scale systematized the trends in their bonding and acidity.

Key figures

Dmitri Mendeleev
Henry Moseley
Linus Pauling

Seminal works

greenwood1997
weller2018
housecroft2018

Frequently asked questions

What is the inert-pair effect?: In the heavier p-block elements the valence s electrons become increasingly reluctant to participate in bonding, so oxidation states two units below the group maximum become more stable, as in the preference of lead for the +2 over the +4 state.
Why is carbon's chemistry so much richer than that of the heavier group 14 elements?: Carbon forms exceptionally strong carbon–carbon and carbon–hydrogen bonds and readily makes multiple bonds, allowing extensive catenation into chains and rings; the heavier elements form weaker bonds and catenate far less, limiting their structural variety.