A Level Chemistry OCR Practice Exam

Which of the following describes a pair of delocalised electrons?

1. Electrons that are fixed in position within an atom

2. Electrons that move freely within a metal lattice

3. Electrons found in high-energy orbitals only

4. Electrons that belong to specific bonds only

The correct answer is: Electrons that move freely within a metal lattice

Delocalised electrons are best described as electrons that move freely within a metal lattice. This concept is fundamental in understanding metallic bonding, where electrons are not localized to individual atoms or bonds but instead form a "sea of electrons." This allows them to move throughout the lattice, which accounts for many physical properties of metals, such as electrical conductivity and malleability. In contrast, the first option refers to electrons that are bound to specific positions within an atom. The third option describes electrons that are confined to high-energy orbitals, which does not encompass the idea of delocalisation. The last option focuses on electrons that belong to specific bonds, which contradicts the concept of delocalised electrons that are shared across multiple atoms in a structure. Thus, the option indicating that electrons move freely within a metal lattice accurately captures the essence of what delocalised electrons are.