Periodicity (2) Melting and boiling points of the elements of Period 3

Here we go again with the concept of periodicity.

We’ve already said what periodicity is:

Periodicity is the regular recurrence of similar properties of the elements across the periodic table.

I think one of the ways we can get this circularity or regular recurrence of similar properties is to view the periodic table in a 3D circular form as here in this diagram:

As you can see from both views the first three rows are connected; there is no break in the flow of atomic numbers.  

Now as the elements with similar properties appear above each other we can see the regular recurrence of their properties. 

So this graph I made of the melting and boiling points ought to be circular–yes!!

What we see first about these values is that there is a pattern but unlike ionisation energy and electronegativity the trends in melting and boiling point across a period are not straightforward.

Let’s define boiling and melting first and then get to an explanation of what is going on here.

Then you can construct the chart for the Period 2 values and see how they compare with Period 3. 

What is meant by boiling point?

Boiling point is the temperature at which the vapour pressure of the element is equal to the current atmospheric pressure.  The energy being supplied to the element is used to break interatomic or intermolecular bonds in the liquid element to allow its particles to exist much further apart and move at great velocity in the gaseous state.  The stronger these forces of attraction are the higher the boiling point. 

What is meant by melting point?

Melting point is the temperature at which the particles of the element transition from the solid into the liquid state. To transition, energy is supplied to break bonds between particles in the solid state instead of raising the temperature of the element. The particles remain about as close as before but they now move randomly rather than vibrating on the spot. 

What these two definitions tell you is that the explanation for the variation in melting and boiling point is going to be to do with the bonding and structure of these elements. 

Explanation

Let’s now look at and try to explain the periodic pattern.

Sodium, Magnesium, Aluminium

The first thing we can see is that there are three metallic elements: sodium, magnesium and aluminium, with relatively low melting and boiling points.

Metallic bonds hold the atoms together in these three metals. 

Here is a pictorial description of metallic bonding for a group 1 and a group 2 metal.

So notice this as we go from sodium to magnesium to aluminium the charge on the metal ion increases from +1 to +3 that also means that the number of delocalised electrons per atom increases and that has the effect of increasing the strength of the metallic bonds. 

Silicon

So what happens with silicon?

Well, silicon is not a metal it is a metalloid i.e. it has some metallic characteristics (it looks like a metal grey and shiny) but crucially its structure and bonding is not metallic. 

The atoms of silicon are held together by strong directional covalent bonds in a huge atomic network. 

This is much like the structure of diamond that you might already be familiar with.

So each silicon atoms as we can see is covalently bonded to four other atoms in a tetrahedral arrangement. 

Each of these four covalent bonds is strong.

This tetrahedral arrangement extends to edge of a silicon crystal and is sometimes called a giant structure for obvious reasons. 

To merely melt this stuff, energy will need to be given to it to break each covalent bond if the atoms are to be set free to move randomly around each other. 

As you can see that energy value and hence the temperature at which silicon melts will be very high. 

Phosphorus, Sulfur, Chlorine and Argon

Again these are not metal but non–metals. 

They have molecular structures. 

They exist in small groups of atoms: P₄, S₈, Cl₂ and Ar.

There are strong forces of attraction between their atoms within each molecule; what are called intra–molecular bonds. 

But what matters for our explanation are the inter–molecular bonds.

These inter molecular bonds are very weak.

These are called van der Waals forces.

The strength of these van der Waals forces depends n the number of electrons in each molecule and so the melting and boiling points follow this pattern: S₈ > P₄ > Cl₂ > Ar.

The bigger the molecule, more electrons, the stronger the van der Waals force, the higher the melting and boiling point.