Hot Trends

Periodic trends are trends across the periodic table. One of the ones you need to know is trends in melting point. i.e. how hot does it need to be to melt?

When you start to think about trends in melting points, you've got to think about what bond you need to break to melt it.

If you go through the options, it all becomes fairly self evident...

Let's look at Period 3 (Sodium to Argon)

and let's think about each element in turn and the bonds in the element. I repeat bonding in the ELEMENT. 

Why do I emphasise ELEMENT?

Ionic bonds occur between metals and non-metals. Never in elements. You need that difference in electronegativity for an ionic bond to occur. 

OK, let's look at the contenders for the highest melting point in period 3.

Sodium is a metal, so to melt it you have to break a metallic bond

Magnesium is a metal, so to melt it you have to break a metallic bond

Aluminium is a metal, so to melt it you have to break a metallic bond

Metallic bonds are strong and the trend is, the more electrons in the outer shell (i.e. donated to the sea of electrons) the stronger the metallic bond and the higher the melting point.

So of the three above Aluminium will be the highest (3 electrons donated to the sea), then Mg (2 electrons donated to the sea), then Sodium (1 electron donated to the sea).

Silicon is very similar to diamond in its structure it is a giant covalent compound. Covalent bonds are very strong. Even stronger than metallic bonds so the MP will be higher again than aluminium

Phosphorus, Sulfur, Chlorine are all simple covalent compounds. That is little covalent molecules (P4, Cl2, S8) each compound has covalent bonds in the molecule but these don't break when you melt it so they don't matter. What matters is they have Van der Waals between the molecules and these are very weak. So the melting points are all very low.

Argon is a similar story to P, S and Cl but they don't even have the covalent they are just individual atoms joined by Van Der Waals forces so again very low melting points.

All in all, Silicon has the highest melting point because you have to break the strongest bond (covalent) to melt it.