What is the relationship between melting point and atmoic radius?
It depends on which elements we are comparing.
If we are talking aout the whole of the Periodic Table, there is no direct link between atomic radius and melting point. This is because melting point depends on the structure of the substance rather than just on the atoms themselves.
However, if we look at a group of elements such as Group I (the alkali metals) where the atoms all form the sam kind of structure, we find that the is a relationship. To understand why this relationship exists, you need to understand about how metal atoms stick together to form a large piece of metal.
In metals, positive metal ions are held together by electron clouds. This is known as metallic bonding. The metal atoms are so tightly packed that their outside shells overlap. This means that there is a "delocalised sea of electrons" which surrounds the metal ions (ions are atoms that have gained or lost one or more electrons).
Look at the revisionworld section on metallic bonding for a diagram.
"Deocalised" means that these electrons are free to move through the structure, this is why metals conduct electricity.
This can explain the change in melting points as you go down group I.
The melting points decrease as you go down the group.
The atoms get larger as you go down the group (because they have more shells of electrons).
The group I metals all have one electron in the outside shell.
Think of this as the glue that holds the metal structure together.
If we have small atoms and a certain amount of glue, we will have fairly big attractions between the atoms and so the melting point will be quite high.
If we have large atoms and the same amount of glue (still one electron available from the outside shell) there will be less attraction and so the melting point will be lower.
This can also help to explain why the group I metals are soft.
Hardness depends on the attraction between the atoms.
Lots of attraction means the metal will be hard.
None of the group I metals can be described as “hard” but as you go down the group, they get even softer.
A lot of trends like this can help to predict the proeprties of elements you may not have yet discovered. As an example, francium is rare and radioactive so there is no chance of trying out the experiments on a normal size piece of the metal. We can, however, look at the trend through the other alkali metals and suggest that it will be the most reactive of the alkali metals, it will be the softest and it will have the lowest melting point.
I hope this helps. If you need more help, please reply to this and tell what you need to know.
By the way, this might be helpful to direct you to the metallic bonding bit of revisionworld.
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