chem 難題 ........

Fluorine, chlorine, bromine and iodine are Group VII elements (halogens). All of them have simple molecular structure. They exist as diatomic molecules, such that F2, Cl2, Br2 and I2 molecules. Two identical halogen atoms are joined together with strong covalent bonds to form a halogen molecule, and the molecules are held together by weak van der Walls' forces. The stronger the van der Waals' forces between molecules, the higher the melting point and boiling point are. This is because more energy is needed to overcome the van der Waals' forces in melting and in boiling.

The van der Waals' forces between molecules depend on the molecular size. The larger the molecules, the stronger the van der Waals' forces between molecules are. Down Group VII, the atomic sizes are increasing, and thus the molecular sizes are also increasing. Therefore, melting point and boiling point: F2 < Cl2 < Br2 < I2 because molecular size: F2 < Cl2 < Br2 < I2.

At room conditions, the van der Waals' forces between I2 molecules are strong enough to hold the I2 molecules together to be in solid state, but the van der Waals' forces between F2 or Cl2 molecules are too weak and thus F2 and Cl2 are gases. The strength van der Waals' forces between Br2 molecules is between those of Cl2 and I­2. The melting point of Br2 is below the room temperature, but the boiling point of Br2 is above the room temperature. Therefore, bromine is a liquid at room conditions.
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