Question

what is a delocalised electron?

Answer 1

Delocalised electrons are spread across more than one atom. Usually electrons in materials are bound to one atom, and atoms are held together by the interactions of the charges on different atoms. In some cases, electrons can be shared between atoms, and are then called delocalised. In the case of hydrocarbons, delocalisation occurs in Benzene rings, where a hexagon of six carbon atoms has delocalised electrons spread over the whole ring. In metals, electrons are delocalised over the whole crystal structure, and carry currents - the outer electrons of the metal atoms are shared in an electron sea, and are not confined to particular atoms. 

To really visualize this you have to think of an electron as a probability cloud, and just see that cloud spread over more than one atom; then it's not such a leap to see a delocalised cloud spread all through a metal. The planetary orbit picture of electrons isn't much use for delocalisation; forget about orbits, and think of clouds instead!
Answered by: Paul Hodgkinson, Physics Undergrad, Cambridge University, UK.

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