AS-Level Group 7 (Halogens)
The melting and boiling points of the halogens increase going down group 7.
Halogens exist in elemental form as simple, diatomic molecules held together by weak intermolecular forces (temporary induced dipole-dipole).
The larger the molecules, the larger the intermolecular forces that can arise between them, which require more energy to overcome – giving the substance a high melting point.
The size of the halogen molecules increases down the group, meaning their melting and boiling points also increase.
Boiling Points of the Halogens
The melting and boiling points of the halogens increase as you go down group 7.
To melt a substance, the forces holding the particles of the substance together have to be broken. The stronger these forces are, the higher the amount of energy required to break them. Thermal energy is used to break apart these forces, so a high temperature is needed, giving the substance a high melting point. The opposite is true for substances with weak forces between particles – little energy is required to break the forces, so a low temperature is needed, giving the substance a low melting point.
The halogens all exist di-atomically (two atoms covalently bonded together) as simple molecules. They have only very weak temporary induced dipole-dipole intermolecular forces between molecules.
Temporary induced dipole-dipole interactions arise when electrons are momentarily, unequally distributed in a molecule. The more electrons present in the molecule, the greater and stronger the dipole-dipole interactions. Stronger interactions require more energy to overcome, so the molecules have higher melting points.
The size of the atoms and the number of electrons in the halogens increase as you go down the group, resulting in higher melting and boiling points.