Lesson 6: Chemistry: Inorganic — Periodic Table, Bonding, Coordination, p-Block Basics

Inorganic Chemistry is largely NCERT-text chemistry: the facts are in the book, and the marks go to students who can state them precisely and apply a small set of rules — oxidation numbers, the shapes predicted by VSEPR, and the naming of coordination compounds.

This lesson covers periodic trends, chemical bonding, coordination compounds and the basics of the p-block. Learn trends as explanations, not as lists: atomic radius, ionisation enthalpy and electronegativity each follow from nuclear charge, shielding and distance, and every exception in the book has a reason.

For coordination compounds, keep three tools ready: oxidation number of the central metal, coordination number (the number of donor atoms attached), and the magnetic moment formula μ = √[n(n+2)] Bohr magnetons, where n is the number of unpaired electrons.

Periodic trends

Across a period, atomic radius generally decreases (increasing nuclear charge pulls electrons in) while ionisation enthalpy and electronegativity generally increase. Down a group, radius increases (new shells outweigh increased nuclear charge) while ionisation enthalpy and electronegativity generally decrease.

Two classic exceptions to learn with their reasons: nitrogen has a higher first ionisation enthalpy than oxygen because its half-filled 2p subshell is extra stable; beryllium has a higher first ionisation enthalpy than boron because its filled 2s subshell is extra stable. Questions love these exceptions precisely because they test understanding rather than recall of the trend.

Oxidation number rules that settle most questions: oxygen is usually −2 (except in peroxides, −1, and in OF₂, +2); hydrogen is usually +1 (except in metal hydrides, −1); the sum of oxidation numbers in a neutral compound is zero, and in an ion it equals the ion's charge.

Bonding and shapes

VSEPR theory: electron pairs around a central atom repel each other, and lone pairs repel more strongly than bond pairs. Methane (four bond pairs) is tetrahedral; ammonia (three bond pairs, one lone pair) is pyramidal; water (two bond pairs, two lone pairs) is bent. The bond angles shrink as lone pairs are added: about 109.5° in methane, about 107° in ammonia, about 104.5° in water.

Hybridisation follows the same counting: four electron domains around the central atom means sp³, three means sp², two means sp. Ionic character increases with electronegativity difference, and hydrogen bonding — the strong attraction between hydrogen attached to fluorine, oxygen or nitrogen and a lone pair on F, O or N — explains water's high boiling point and ice floating on water.

Coordination compounds and p-block basics

In [Co(NH₃)₆]³⁺, each ammonia is neutral, so cobalt's oxidation number is +3, and the coordination number is 6 (six donor nitrogen atoms). In [Fe(CN)₆]⁴⁻, each cyanide is −1, so iron is +2 (six × −1 + x = −4 gives x = +2). Ligands are named before the metal, anionic ligands end in -o (chloro, cyano), and the metal's oxidation number is written in Roman numerals.

P-block basics: nitrogen's inert pair and small size make its chemistry different from phosphorus (nitrogen forms strong pπ–pπ multiple bonds; phosphorus prefers single bonds and can expand its octet). The noble gases are chemically inert because of their filled valence shells, and the halogens are the most reactive non-metals because gaining one electron completes their octet. Ozone in the upper atmosphere absorbs ultraviolet radiation; at ground level it is a pollutant — the same molecule, different context.

Common pitfalls