Melting/Boiling point of group 1 and 2

Could someone explain why the melting/boiling points of group 1 and 2 decrease as you go down the group please?

But the why do the melting/boiling points of group 7 increase? Im confused! lol

Thanks

Reply 1

Tes

This is to do with the intermolecualr forces. Group 1 and 2 both use metallic bonding. This involves the delocalisation the electrons from the metal atoms to form ions. When you melt any of these metals, the metallic bond is weakened enough for the atoms to move around, and is then broken completely when you boil the metal. The fall in melting and boiling points reflects the fall in the strength of the metallic bond. The atoms in a metal are held together by the attraction of the nuclei to the delocalised electrons. As the atoms get bigger, the nuclei get further away from these delocalised electrons, and so the attractions fall. That means that the atoms are more easily pulled apart to make a liquid and finally a gas.

Group 7 increases because the intermolecular forces involved are van de waals forces, and van de waals forces increas in strength as you increase the total number of electrons per molecule. As you go down group 7 the total number of electrons per molecule increase and therefore the van de waals forces increase and so the melting and boiling points increase

Hole that helps :P

(Lol that is long!)

Reply 2

sufiankane

wewll groups 1 and 2, they are both metallically bonded, now as you descend the group the size of the cation increases, meaning 2 things.

1) the atom is larger and therefore the sea electrons have to cover a larger area and is therefore weaker a they are more spead out.

2) The cation is larger, but has the same charge, so its charge desnity decreases meaning it is not as strongly attracted tot eh sea of electrons and is therefore weaker.

For group 7

As you descend there are more electrons in the outer shell, and more electrons = stronger van der waals forces, as there is a bigger chance of the charge density in the clouds being off balanced, since stronger van der waals, melting temp goes up as you descend the group for group 7

any queries feel free to pm me

EDIT: CURSES I WAS BEATEN

Reply 3

Tes

sufiankane

EDIT: CURSES I WAS BEATEN

Don't worry i dont have a life so its acceptable to be beaten by me :P

Reply 4

sufiankane

BlakDog

Don't worry i dont have a life so its acceptable to be beaten by me :P

~.~ well i have les of a life...i errr....i play final fantasy without a memory card, so i have to keep my PS1 on 24.7! beat that

Reply 5

Tes

sufiankane

~.~ well i have les of a life...i errr....i play final fantasy without a memory card, so i have to keep my PS1 on 24.7! beat that

I have been on TSR since 9 this morning so there!

Reply 6

HayleyChem

Yeah thats helps alot thanks, was really quick too!

Reply 7

HayleyChem

Also do you know the trends in boiling temps for the noble gasses and for group 4 5 6 and 7 hydrides please? thanks

Reply 8

Tes

noble gases are the same as group 7 and ill check for the hydrides

edit: lol i know the hydrides

methane = low boiling point as no hydrogen bonds are present
ammonia = higher as one hydrogen bond can form to a lone pair per molecule
water = very high as 2 hydrogen bonds to a lone pair can be formed per molecule
hydrogen fluoride = similar to ammonia as one hydrogen bond per molecule

If that helps :P