Thread: Melting point

Why does the melting point decrease as you move down in the alkalimetal group and why does it increase as you move up in the halogen group?

Melting in context of metals refers to the breaking of the metallic bond within the metal. The strength of the metallic bond depends on the extent/level of attraction between the atoms of the metal and their sea of delocalised electrons (i.e. attraction between the positive ion of the atom of the metal and the sea of electrons) hence, the strength of a metallic bond is inversely related to the size of the ion of the metal's atom. The bigger the ion, the weaker the electrostatic force of attraction between the positive ion and the relevant electrons (why so? coz the positive force comes from the ion's nucleus and with a bigger sized ion, the attractive strength is hindered by both the distance *think a magnet attracting a metal ball 3cm and 30cm away* and the shielding of the nucleus by the electrons in their orbitals within the ion). Hence, as the strength of the bond between the ions and the sea of delocalised electrons gets weaker, it thus requires less energy to break them apart and thus, as you go down the group, the melting point decreases.