12.1.2—And explain

A Brønsted–Lowry base accepts a proton. The lone pair on nitrogen lets ammonia accept H⁺: NH₃ + H⁺ → NH₄⁺. In water, NH₃ + H₂O ⇌ NH₄⁺ + OH⁻; the equilibrium lies mainly to the left, so ammonia is a weak base.

NH₄⁺ is tetrahedral with bond angles of about 109.5°. Its fourth N–H bond forms when nitrogen donates its lone pair to H⁺, so it begins as a coordinate bond; after formation all four N–H bonds are equivalent.

Warm an ammonium salt with aqueous alkali: released ammonia turns damp red litmus paper blue. Damp paper is needed because the gas must dissolve before its basic behaviour is shown.

Ammonia does not need an OH group to be basic; its lone pair accepts H⁺. A coordinate bond describes the origin of the shared pair, not a permanently different N–H bond in NH₄⁺.