What does a crystal lattice refer to?

A crystal lattice refers to a three-dimensional infinite array of lattice points, which represent the positions of atoms, ions, or molecules within a crystalline solid. This structure is key to understanding how solids are organized at the atomic level. Each lattice point corresponds to the position of a repeat unit of the crystal, which can be an atom, ion, or a group of atoms.

In a crystal lattice, the entire structure is characterized by translational symmetry, meaning that its pattern repeats itself in three dimensions. This repetitive arrangement is essential for defining the physical properties of the crystal, such as its symmetry, stability, and how it interacts with light, heat, and other forms of energy.

The other options do not accurately depict a crystal lattice. A flat structure of bonded atoms describes a two-dimensional arrangement, which does not conform to the three-dimensional nature of lattices. A repeating unit of molecular formula refers more to the concept of a formula unit in crystalline solids but does not encompass the full dimensionality and infinite nature of a crystal lattice. Lastly, a single atom arrangement in a molecule focuses on molecular structure rather than the extended, ordered arrangement characteristic of crystal lattices.