4. Types of Solids

L2CHEM

---

In L2 Bonding and Structure, there would always be a question on solids. Identifying what type of solids are necessary.

In this standard, there are 4 types of solids that can come out:

1. Covalent Network
2. Ionic
3. Molecular
4. Metallic

To identify by just looking at the formulas, look at what type of atoms/molecules/ions there are.

If there are only metals, then it’s straight up metallic solid.
If it’s a metal + non-metal, then it’s straight up ionic solid.

It works differently for non-metals only — you need additional information. Very common information given in the exam is the melting point (“MP”). If it has a VERY HIGH MP, then it’s a covalent network solid, and if it has a VERY LOW MP, then it’s a molecular solid.

If you aren’t given the MP, then you rely on their structure — how it bonds to one another, and whether if it can continuously bond, or not.

---

First, draw the lewis structure for the molecule.

From here, my explanation gets quite intense, but it works. If someone has a better explanation, PLEASE contact me 😭.

Then, we should know how many ‘bonds’ the molecule can have, to achieve a full octet. This can be calculated by using this formula:

$$\text{Possible Bonds}=8-\text{Valence Electron(s) of the Molecule}$$

For instance, for oxygen $O$, it can have 2 bonds to achieve a full octet:

$$\text{Possible Bonds}=8-6=2$$ $$⟹\text{Possible Bonds}=2$$

Likewise, chlorine $Cl$ can have 1 bonds, since $8-7=1$.

Now, look at the lewis structure you’ve drawn.

I’m going to use $Si{O}_2$ as an example:

Count how many bonds $Si$, and $O$ can have. If the number of bonds in the lewis diagram doesn’t match the number of possible bonds (that you calculated), then it’s pretty likely a covalent network, since that means that the structure can infinitely expand like this to achieve its full octet WITH FULL POSSIBLE BONDS:

See that expanded lewis diagram? That’s a lattice, which is known to be super hard to break, which implies that it’s a covalent network solid.

On the other hand, take $SiC{l}_4$ as an example. It’s lewis structure probably looks like this:

Now, compare the bond count in the lewis structure to the possible bond counts (calculated from the formula). $Cl$ can have 1 possible bond, and $Si$ can have 4 possible bonds. This matches the lewis diagram’s bond count, which means that the lewis diagram can’t expand like the $Si{O}_2$ example. This means that the molecule stops here. Since it doesn’t expand, it is a molecular solid!

Summary

Metals only? $⟹$ Metallic Solid
Metal + Non-metal? $⟹$ Ionic Solid
Non-metals only? Count possible bonds, and compare it to the bonds in the lewis diagram!
$\to$ if bond count = possible bond count $⟹$ Molecular Solid
$\to$ if bond count $\ne$ possible bond count $⟹$ Covalent Network Solid