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 molecular category. This is because iodine molecules, which occupy lattice points, are held together by weak intermolecular forces such as van der Waals forces, resulting in its relatively low melting point.

Dorothy Hodgkin used x-ray crystallography, also known as x-ray analysis or x-ray diffraction, as the technique to determine the molecular structure of vitamin B₁₂.

Graphite consists of layers of carbon atoms that are strongly bonded covalently in each layer, while the layers themselves are held together by weaker van der Waals forces. This allows the layers to slide over each other easily, making graphite effective as a lubricant.

Silver consists of positively charged ions within a structure where valence (outer) electrons are delocalized. These free-moving electrons act as charge carriers, allowing silver to conduct electricity exceptionally well.