Bonding stops being a set of boxes and becomes a continuum

There are three types of bonding — ionic (metal + non-metal, electrons transferred), covalent (non-metal + non-metal, electrons shared) and metallic (metal + metal, sea of delocalised electrons). Every compound is assigned to exactly one box, and the box is decided by looking at which side of the periodic table the elements come from.

Bonding is a continuum. The ionic, covalent and metallic models are three idealised corners of a triangle (the van Arkel–Ketelaar bonding triangle, Structure 2.4.1–2.4.2). Where a compound sits is determined quantitatively by two numbers computed from electronegativity values in the data booklet: the difference in electronegativity, Δχ (vertical axis — degree of ionic character) and the average electronegativity, χ̄ (horizontal axis — metallic vs covalent). Real substances sit inside the triangle. AlCl₃, BeCl₂, SiO₂, and most transition metal halides are the classic "doesn't fit a box" cases.

Classification stops being a lookup from the periodic table and becomes a calculation with a spectrum of answers. "Polar covalent" stops being a curiosity and becomes the normal case; pure covalent (Δχ = 0) and fully ionic are the limits, not the norm. Bond polarity (Structure 2.2.5) and molecular polarity (2.2.6) then do real explanatory work for solubility, boiling points and reaction mechanisms.

The GCSE binary works for the ~20 compounds GCSE actually asks about (NaCl, MgO, H₂O, CO₂, CH₄), and dot-and-cross diagrams require a clean transferred-or-shared story. Introducing electronegativity requires the periodic trend machinery that GCSE doesn't build.

The student answers "is it ionic or covalent?" for AlCl₃ or for a compound with Δχ = 1.5 and gets it flatly wrong; they cannot explain why HCl dissolves in water but CH₄ doesn't; they cannot rationalise reaction mechanisms in Reactivity 3.4 (nucleophile attacks the δ+ carbon — which requires believing that a covalent bond can be partly ionic). This one breaks Structure 2 entirely and leaks into all of organic chemistry.