Crystals are solid with regularly arranged particles held together by electrostatic forces of attraction.The bonds may be strong ionic, covalent or metallic bonds or weaker intermolecular forces such as van der Waal's, dipole-dipole or hydrogen bonds.The strength of the bonds affects the physical properties.The stronger the bonds the greater the enthalpy of fusion and the higher the melting point. Ionic compounds exist as crystals with strong electrostatic forces of attraction between the oppositely charged ions. The bonds are non-directional and the attraction extends throughout the whole structure.All ionic compounds are made of a giant ionic lattice of ions. They exist as solids at room temperature. The stronger the bonds the more energy is needed to break the bond, the higher the melting temperature. Ionic compounds have high melting temperatures e.g. sodium chloride NaCl melts at 801°C. Metallic crystals are made of a lattice of positively charged ions embedded in a sea of delocalised electrons. The strong electrostatic forces of attraction between the positive ions and the electrons extend throughout the structure.A lot of energy is needed to break the strong metallic bonds. Metals have high melting temperatures. Molecular crystals are held together by strong covalent bonds within the molecules and weak intermolecular forces of attraction between the molecules.The crystals are soft and break easily and have low melting points because the intermolecular bonds are relatively weak break more easily.Iodine I2 is a solid at room temperature. The van der Waals forces are strong enough to hold the molecules into a solid crystal lattice because iodine has lots of electrons.