OK, why is there this difference, I mean why is it that the melting and boiling point in Group I decrease as you go down the group and increase as you go up Group VII? Has this got something to do with their valence electrons? Can you please explain that a little further?
Thank you so much 
By the way, we only need to know the trends for Group I and IV for b.p.s and m.p.s.
Right. First you need to understand bonding in metals. Each metal atom loses its outer electrons, which are then free to move between the lattice of positively charged metal ions in the solid. The metal ions are held in a rigid formation by the force of attraction between the positive ions and the 'sea' of negative electrons surrounding them. As you go down the group, however, the atoms become larger so that the positive nucleus gets further away from the negative sea of electrons. The force of attraction between the metal ions and the sea of electrons thus gets weaker down the group and the melting points decrease as less heat energy is needed to overcome this weakening force of attraction.
For Group IV, as you go down the group, the molecules get bigger, so the atomic radius increases as the number of electrons increase. Because halogens exist as diatomic molecules, they have intermolecular forces (of the Van der Waals dispersion forces). So when the molecules are bigger, there is an increase in the chances for setting up temporary dipoles - which causes these attractive forces. So the stronger the attractive forces, the more energy required to break these bonds, so m.p. and b.p. increases.
I have to stress however that at IGCSE you do
not need to know why any of these trends happen. You simply need to know
what the trends are. So don't worry if all this jargon doesn't make sense to you right now, more will become clear if and when you study AS Chemistry.
Anyway, hope I answered your question clearly enough.
