Molecular size is a major factor affecting the melting temperature of a compound. The larger the molecular size and the more rigid the structure, the higher is the temperature at which the substance melts. For example, in the case of straight-chain alkanes, longer chains with an even number of carbon atoms have a well-packed structure that is more difficult to break. As a result, they have a higher melting point than smaller-chain alkanes that have odd numbers of carbon atoms.
Compounds with molecules that exhibit a high degree of symmetry have more efficient packing; this results in greater lattice energy, which requires higher energy input to break the lattice arrangement. Therefore, compounds with a more ordered arrangement melt at higher temperatures. Neopentane, with a more symmetrical structure, melts at a higher temperature than the similar compound isopentane, which has a less ordered structure.
Intermolecular forces determine the ease or difficulty in breaking the bonds and causing melting. Generally, the stronger the intermolecular forces are, the greater is the energy required to break them; consequently, such compounds show higher melting temperatures. Covalent bonds are easier to break than ionic bonds; covalently bonded compounds usually melt at lower temperatures than ionic compounds. Covalent compounds with nonpolar molecules are held together by weak van der Waals forces of attraction. Such compounds have lower melting temperatures than polar molecules held by electrostatic forces of attraction.
The melting temperature is a characteristic property of a compound. Although expressed as a range of temperature, the melting point of pure substances lies within a narrow range that is close to the expected temperature. With an impure sample, contaminants interfere with the melting process; such samples often melt over a wider range that is lower than the expected melting temperature.