Group 1: Alkali Metals?

Question

According to the van der waals dispersion forces, the larger the atomic size, the more electrons are there and the chances of dipole happening increases and therefore the intermolecular force increases. But why does the melting point decreases down Group 1 even though the atomic size increases?

Answer 1

Now, you have used the concept of van der Waals to explain it. I think your usage is wrong. van der Waals is used for molecules or monoatomic compounds, for e.g. Br2, Ar, He and many others more. They employ covalent bonding.

Group 1 elements are not molecules formed by covalent bonding. The bonding is metallic bonding. Metallic bonding is determined by the strength of the positive centres (the ions) and the amount of the delocalised electrons (which is the amount of electrons released in the formation of the bond). Metallic bonding forms between the positive ions and the sea of delocalised electrons.

Now as we go down group 1, the amount of delocalised electrons is the same (1 electron per atom). However, the attraction strength of the positive centres (which is the nucleus) to towards the delocalised electrons decreases down the group due to the increased size of atom and the shielding effect.

To explain shielding effect, imagine you in a lecture hall. You can see a girl sitting two rows behind you but you can see clearly a girl sitting twenty rows behind you as there are many people blocking your view. The same thing applies to the positive centre. The increased number of orbitals and electrons 'shield' the nucleus from attracting the delocalised electrons. Thus weaker metallic bond is formed at the bottom of group 1. Thus, the melting point decreases down the group.

To explain the trend, we must first analyze the bonds form before applying the right concept. Hope this helps:P

Answer 2

The alkali metals have the largest atomic radii in their periods in the periodic table. That is because there is a single electron in the outer most shell of each alkali metal, and it is so far way from the nucleus of the atom, that it is easily lost. As a result they easily form positive stable ions. The ease that the valence elctron is lost increases as the atomic radii increases, that is down the group. Now a solid melts when the kinetyic energy of molecules becomes great enough to overcome the intermolecular forces(Vanderwals) holding them fixed, so your solid will begin to melt.So, the amount of energy needed to change a solid to a liquid alkali decreases down a group the larger the radii get, the weaker the attractive forces become.

Answer 3

Note that van der waals dispersion forces refer to stable and non-polar molecules (ie noble gases, CO2, F2....etc). Alkali Metals usually have a +1 charge. The real reason to your question, however, is that the atoms have greater size as we go down the periodic table because it has more energy shells that contain electrons. Since distance and electron shielding (think trying to look through a bunch of dirty windows) weakens the nucleus' ability to attract valence electrons and electrons from other atoms, the metal atoms tend to separate much easier with less force (in your case, heat). Therefore, the melting point decreases down Group 1.