Revise From models to materials for IB Chemistry (S2.4)

Ionic, covalent and metallic bonding are limiting cases. A perfectly ionic bond would mean one atom transfers electrons completely to another; a perfectly covalent bond would mean two atoms share electrons perfectly equally; metallic bonding is a lattice of cations in a sea of delocalised electrons. These extremes are rarely realised exactly. In sodium chloride the bonding is close to the ionic corner, and in the chlorine molecule Cl₂ it is genuinely at the covalent corner, but a huge number of substances — hydrogen chloride, water, silicon carbide, semiconductors, alloys — sit between corners and are best described as a blend of character.

The clearest window onto the continuum is the polar covalent bond. When two different non-metal atoms share a pair of electrons, the more electronegative atom pulls the shared pair closer to itself. The sharing is unequal, so that atom gains a partial negative charge ($\delta-$) and the other a partial positive charge ($\delta+$). This is neither the equal sharing of a pure covalent bond nor the complete transfer of an ionic bond — it is the middle ground. As the electronegativity difference grows, the bond shifts steadily from nonpolar covalent, through polar covalent, toward ionic. The difference in electronegativity, $\Delta\chi$, is therefore a direct measure of ionic character.

The van Arkel–Ketelaar bonding triangle turns the continuum into a map you can read. The three corners are pure ionic (top), pure covalent (bottom right) and pure metallic (bottom left). A substance is plotted using two coordinates calculated from the electronegativity ($\chi$) values of its elements — the difference on the vertical axis and the average on the horizontal axis.

$\Delta\chi = |\chi_A - \chi_B| \qquad\text{and}\qquad \bar{\chi} = \frac{\chi_A + \chi_B}{2}$

The vertical axis, $\Delta\chi$, measures how unevenly the electrons are attracted, i.e. ionic character — a large difference pushes a substance up toward the ionic corner. The horizontal axis, $\bar{\chi}$, measures how electronegative the elements are on average. When $\Delta\chi$ is small the substance is not ionic, and the average then decides between the two lower corners: a low average (two electron-poor metals) sits at the metallic corner, while a high average (two electron-hungry non-metals) sits at the covalent corner. You need both numbers — that is why a single $\Delta\chi$ cut-off cannot, on its own, separate metallic from covalent bonding.

Following the continuum toward the metallic corner leads to alloys. An alloy is a mixture of a metal with one or more other elements, and crucially it keeps its metallic character: metallic bonding — cations in a sea of delocalised electrons — persists, so the alloy still conducts electricity and heat and remains workable. What changes is the mechanical behaviour. Introducing atoms of a different size disrupts the regular layers of the host lattice, so the layers can no longer slide over one another as easily. The result is a harder, stronger material than the pure metal, which is why brass (copper and zinc) is harder than copper and steel is harder than iron. Because the composition can be varied continuously rather than fixed at a formula, an alloy is a mixture — a solid solution — not a compound.

The same logic — bonding and structure set the properties — guides how engineers choose materials beyond the metals. Polymers are long-chain molecules whose behaviour depends on how the chains are held together: chains linked only by weak intermolecular forces can slide and be remoulded on heating (thermoplastic behaviour), while chains joined by strong covalent cross-links form a rigid network that does not soften (thermosetting behaviour). Composites go a step further by combining materials with different bonding and structure — for example stiff glass or carbon fibres embedded in a flexible polymer resin — so that the combination has a property profile neither part has alone, such as high stiffness at low density. Choosing a material is really choosing the bonding you need: metallic for conduction and toughness, giant covalent for hardness, polymers for lightness and mouldability, and composites to combine those strengths.

This is the showcase for a placement question. In Paper 2 the marks are analytic: each is tied to a specific line of working (a method mark, M, or an answer mark, A), and error-carried-forward (ECF) means a slip in an early number does not automatically cost you the marks that follow — provided your method is written down. Study how each of the three marks below is earned by a specific line, and note where the accept ranges and ECF apply.

Seeing bonding as a continuum reframes everything that follows. Intermolecular forces, polarity and solubility all trace back to the $\delta+$/$\delta-$ separation of polar covalent bonds; the properties of giant structures and metals map onto the corners of the triangle; and the materials chapter — alloys, polymers, composites — is the continuum applied to engineering. Master the two-number method (find the difference, find the average, read the triangle) and you can classify the bonding in almost any binary substance and justify it the way the exam rewards.