Ionic, covalent (including dative) and metallic bonding, electronegativity and bond polarity, the shapes of simple molecules and ions, and the four types of crystal structure.

What this topic is asking

Edexcel Topic 2 wants you to classify bonding as ionic, covalent (including dative), or metallic, use electronegativity to judge bond polarity, predict molecular and ionic shapes using electron-pair repulsion theory, and relate the four crystal structures to physical properties.

The answer

The three bond types

Electronegativity and polarity

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. A difference in electronegativity makes a bond polar, giving a permanent dipole. A molecule can contain polar bonds yet be non-polar overall if the dipoles cancel by symmetry, as in $CO_2$.

Shapes of molecules

Electron pairs in the outer shell repel and arrange themselves as far apart as possible. Lone pairs repel more strongly than bonding pairs, reducing bond angles by about $2.5^\circ$ each.

The four crystal structures

• Giant ionic (e.g. $NaCl$): high melting point, conducts when molten or dissolved.
• Giant covalent (e.g. diamond, graphite, $SiO_2$): very high melting point; graphite conducts due to delocalised electrons.
• Simple molecular (e.g. $I_2$, ice): low melting point, weak intermolecular forces, non-conductor.
• Metallic: good conductor, malleable, melting point depends on charge and ion size.

The link to properties is the marking point: melting points reflect the strength of the bonds or forces that must be overcome, and electrical conduction needs mobile charge carriers (ions in molten ionic compounds, delocalised electrons in metals and graphite).

Examples in context

Example 1. Why diamond tips drill bits. Diamond is a giant covalent structure in which every carbon is bonded to four others by strong covalent bonds in a rigid three-dimensional lattice. Breaking it requires breaking many strong bonds, which is why diamond is the hardest natural material and is used on drill bits and saw blades. Graphite, by contrast, has layers held together only by weak forces, so it flakes and is used as a lubricant and in pencils, a vivid demonstration that structure, not just composition, controls properties.

Example 2. Alloys and why they are harder than pure metals. Pure metals are malleable because layers of identical ions slide over one another. Alloys such as steel contain atoms of different sizes that disrupt the regular layers, so the layers cannot slide as easily and the alloy is harder. This explains why bronze, brass and steel are used in tools and structures rather than the soft pure metals, applying the metallic-bonding model from Topic 2.

Try this

Q1. State and explain the shape and bond angle of an ammonia molecule. [3 marks]

• Cue. Pyramidal, $107^\circ$; three bonding pairs and one lone pair, and the lone pair repels more, reducing the angle from $109.5^\circ$.

Q2. Explain why magnesium oxide has a higher melting point than sodium chloride. [2 marks]

• Cue. $Mg^{2+}$ and $O^{2-}$ carry higher charges than $Na^+$ and $Cl^-$, so stronger electrostatic attraction in the lattice.