arrangement of the spots, known as the diffraction pattern, is determined by the structure of the
crystal (the dimensions and shape of the unit cell) and the arrangement of particles within it.
Obtaining the diffraction pattern, then, allows one to deduce the crystal structure.
Most solids are crystalline in structure, and can be further categorized by the nature of the forces
that hold the particles together.
A CLOSER LOOK
The structure of crystalline solids can be determined using the technique of X-ray
crystallography. When electromagnetic radiation (of which X-ray is a type) passes through
matter, it interacts with the electrons and is scattered in different directions. If the matter is
made up of a regularly spaced array of structural units (as is the case for crystals), the X-ray is
scattered in such a way that it produces a characteristic pattern of spots. This phenomenon
is known as diffraction.Ionic Crystals
Ionic solids are generally formed between atoms with significantly different electronegativities. The
atoms exist in their ionic form in the solid, with the positive and negative ions occupying alternate
sites within the unit cell. Examples include NaCl and CaF 2 . Ionic solids are brittle and are
characterized by high melting points, high boiling points, and poor electrical conductivity in the
solid phase. These properties are due to the compounds’ strong electrostatic interactions, which
also cause the ions to be relatively immobile. Because they are aggregates of positively and
negatively charged ions, there are no discrete molecules, and their formulas are empirical formulas
that describe the ratio of ions in the lowest possible whole numbers. For example, the empirical
formula BaCl 2 gives the ratio of Ba2+ to Cl– ions within the crystal.
Covalent Crystals
Covalent crystals are composed of atoms linked by strong covalent bonds into a large three-
dimensional structure that can be thought of as one giant molecule. Because of the strong forces
binding the atoms together, covalent crystals have high melting points and are hard. The most