How bonding and structure are related to properties

Chemistry · Bonding, structure, and the properties of matter. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The properties of a substance depend on its structure and on what must be overcome to melt it. A substance is a solid below its melting point, a liquid between its melting point and boiling point, and a gas above its boiling point.

Giant structures (ionic, giant covalent, metallic) have high melting points because many strong bonds must be broken. Substances made of small molecules have low melting and boiling points because only the weak intermolecular forces between the molecules are overcome.

To conduct electricity a substance needs charged particles that can move: delocalised electrons in metals, or ions that are free to move in molten or dissolved ionic compounds. The simple particle model is limited because it shows particles as solid spheres with no forces between them.

Rules to remember

Ionic compounds: high melting points; conduct when molten or dissolved in water, but not when solid, because the ions are then fixed in place.
Larger ion charges give stronger attraction and a higher melting point.
Small molecules: low melting and boiling points; do not conduct because the molecules have no overall charge.
Bigger molecules have stronger intermolecular forces, so polymers are solids at room temperature.
Giant covalent structures: very high melting points; all atoms joined by strong covalent bonds.
Metals: conduct electricity and heat through delocalised electrons; layers of atoms slide, so metals can be bent and shaped.
Alloys are harder than pure metals: atoms of different sizes distort the layers so they slide less easily.

Common mistake

Students say that covalent bonds break when a simple molecular substance melts or boils. Only the weak forces between the molecules are overcome; the covalent bonds inside each molecule stay intact.

Worked example

A substance melts at −101 °C and boils at −35 °C. What is its state at −50 °C?

−50 °C is higher than the melting point (−101 °C), so the substance has melted.
−50 °C is lower than the boiling point (−35 °C), so it has not boiled.
A temperature between the melting point and the boiling point means liquid.

Answer: It is a liquid at −50 °C.

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