What is a crystal? Crystals may be classified as ionic, molecular, metallic, and covalent macromolecular - Leaving Cert Chemistry - Question B - 2019

Crystalline iodine (I₂) belongs to the category of molecular crystals. This is because iodine molecules occupy lattice points and are held together by weak intermolecular forces, such as van der Waals forces, leading to a low melting point.

Dorothy Hodgkin used X-ray crystallography, also known as x-ray scattering or x-ray analysis, to determine the molecular structure of vitamin B₁₂. This technique leverages the diffraction of X-rays through the crystal lattice to reveal the arrangement of atoms within the molecule.

Graphite consists of layers of carbon atoms that are strongly bonded together within each layer by covalent bonds. However, the layers themselves are held together by weak van der Waals forces. This allows the layers to slide over each other easily, making graphite an effective lubricant.

In metallic crystals like silver, positively charged silver ions are arranged in a lattice structure, with valence (outer) electrons being delocalized. These delocalized electrons are free to move throughout the crystal, allowing silver to conduct electricity effectively due to the mobility of these charge carriers.