Why do metals expand more than plastics when heated?

Metals expand more than plastics when heated due to their stronger atomic bonds and higher thermal conductivity.

When a substance is heated, its particles gain energy and start to vibrate more vigorously. This increased vibration causes the particles to move further apart, leading to an expansion of the substance. The degree of expansion depends on the nature of the substance, particularly the strength of the bonds between its particles and its thermal conductivity.

Metals have a unique atomic structure with a sea of delocalised electrons, which allows them to conduct heat more efficiently than plastics. This means that when a metal is heated, the heat energy is quickly and evenly distributed throughout the substance, causing a uniform expansion.

On the other hand, plastics are made up of long, tangled polymer chains. These chains are held together by weaker intermolecular forces, which are less efficient at conducting heat. When a plastic is heated, the heat energy is not distributed as evenly or as quickly as in a metal. This results in a slower and less uniform expansion.

Furthermore, the bonds between metal atoms are generally stronger than the intermolecular forces between polymer chains in plastics. When heated, the strong metallic bonds can withstand more vigorous vibrations before breaking, allowing the metal to expand more. In contrast, the weaker bonds in plastics break more easily when heated, limiting the amount of expansion.

In summary, the higher thermal conductivity and stronger atomic bonds in metals allow them to expand more than plastics when heated. This is why, for example, a metal railway track or a bridge needs to have gaps to allow for expansion on hot days, while a plastic object of the same size would not require such considerations.