Topic 2.1 Types of Chemical Bonds: classify bonds as ionic, covalent (polar or nonpolar), or metallic using electronegativity and the elements involved, and relate bond type to properties.

What this topic is asking

The College Board (Topic 2.1) wants you to classify the bonding in a substance as ionic, covalent (further split into polar and nonpolar), or metallic, using the elements involved and their electronegativity difference, and to connect bond type to the substance's macroscopic properties.

The three bond types

A quick guide from the elements: metal + non-metal usually gives ionic; non-metal + non-metal gives covalent; metal + metal (or a pure metal) gives metallic. But bonding is really a continuum, and electronegativity tells you where on that continuum a bond lies.

Electronegativity and bond polarity

So $\text{Cl}_2$ has identical atoms ($\Delta\text{EN} = 0$) and is nonpolar; $\text{HCl}$ has a moderate difference and is polar covalent; $\text{NaCl}$ has a large difference and is ionic. There is no sharp cut-off; the College Board cares that you reason from the difference rather than memorize exact boundaries, though roughly above $1.7$ to $2.0$ a bond is treated as ionic.

Bond type explains properties

The bonding model accounts for what a substance is like in bulk:

• Ionic compounds form rigid lattices, so they have high melting points, are brittle (shifting the lattice brings like charges together and it shatters), and conduct electricity only when molten or dissolved, when the ions become mobile.
• Covalent molecular substances have strong bonds within molecules but weak forces between them, so they have low melting points and do not conduct (no free charges).
• Metals have delocalised electrons, so they conduct electricity and heat, are malleable (the electron sea lets cations slide past one another without breaking the bond), and are typically lustrous.

This is the heart of Unit 2: structure determines properties. The same Coulombic reasoning from Unit 1 governs all three bond types, because each is ultimately the attraction between positive and negative charges; what differs is whether the electrons are transferred, shared, or pooled. Keeping that single picture in mind lets you predict, rather than recall, why molten salt conducts but solid salt does not, or why metals bend while ionic crystals snap.

Try this

Q1. Classify the bonding in (a) $\text{KBr}$, (b) $\text{O}_2$, (c) copper metal. [3 points]

• Cue. (a) ionic; (b) nonpolar covalent; (c) metallic.

Q2. Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. [1 point]

• Cue. In the solid the ions are fixed in the lattice; when molten the ions are free to move and carry charge.