1.5 SOLID STRUCTURES AND THEIR PROPERTIES & SMART MATERIALS

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Hi everyone! There is only one blog post and one youtube video for this post, so, as you can imagine it is a short topic. After you finish revising this post, you should know how to describe and draw ionic crystal structures of NaCl (sodium chloride) and CsCl (caesium chloride). They are called giant ionic lattices. We are going to start with NaCl first.

Giant ionic lattices

Lattice - they are called lattices because the ions are not bonded to each other. They are only held in this arrangement by some type of bonding.

NaCl

There are two facts that are necessary to know about NaCl.

1. It has a face-centred structure.

2. The co-ordination number is 6:6.

Co-ordination number - number of ions of opposite charge surrounding a given ion.

In the case of sodium chloride, this means that every Na ion is surrounded by 6 chlorine ions and vice versa.

You have a diagram on the picture above that shows a part of the giant ionic lattice of NaCl. There are negative ions (chlorine) and positive ions (sodium). As you can also see, there are 6 sodiums surrounding one of the chlorines. Remember to show at least one chlorine with 6 sodiums. Take note that the ions are not bonded, they are just held in a lattice.

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CsCl

There are two facts that are necessary to know about CsCl.

1. It has a body-cetred structure.

2. The co-ordination number is 8:8.

In the case of caesium chloride, the co-ordination number, 8:8, means that every caesium ion is surrounded by 8 chlorine ions.

The diagram for this giant ionic lattice is slightly different than for NaCl, it has one caesium in the middle and is surrounded by chorines, as you can see in the picture above. Caesium is the positive ion and chlorine is the negative ion.

Be sure to know how to draw these diagrams because you might be asked about this in the exam. Don't worry if they are not perfect, just make sure that you include the positive and negative ions and the rough shape of the lattice.

You are probably aware by now, that the co-ordination number for NaCl was 6:6 and it is 8:8 for Caesium chloride. A reason for this is that caesium is larger than sodium, therefore can hold more chlorine ions than sodium can.

Properties of these giant ionic structures:

1.High melting and boiling point - due to strong ionic bonds.

2. Don't conduct electricity - because the ions are held firmly in place. However, they do conduct electricity when melted.

3.Solubility - most of them are soluble in water. The oxygen negative ions from water connect to sodium ions (or caesium) and positive hydrogen ions from water connect with negative chlorine ions in the lattice.

Sructures of covalent compounds

SIMPLE MOLECULAR STRUCTURE

1.Iodine

In iodine, two iodine atoms bond together by strong covalent bonds. The iodine molecules (I2) bond together by weak Van der Waals forces.

So, although the iodine molecule has a strong covalent bond when these I2 molecules bond together they are only connected by weak forces, then they are easy to separate.

Furthermore, iodine doesn't conduct electricity due to no free moving electrons or ions. It also has a low melting point due to weak forces between iodine molecules.

GIANT MOLECULAR STRUCTURE

1. Diamond

In diamond, each carbon atom is covalently bonded to other 4 carbon atoms.

This means the melting point is high because it takes a lot of energy to separate covalent bonds.

Diamond also doesn't conduct electricity because there are no delocalised electrons or free moving ions.

The other property of the diamond is that it's insoluble in water. This is due to the attraction between the carbon and water molecule not overcoming the covalent bonding in diamond. It is also hard.

2.Graphite

In graphite, each carbon atom is covalently bonded to other 3 out of 4 carbon atoms. Graphite consists of layers of these carbon atoms. There are only weak Van der Waals forces between these layers.

Graphite has a high melting point (like a diamond) due to strong covalent bonds and is also insoluble in water for exactly the same reason as diamond.

Furthermore, graphite is soft because the layers slide over each other.

Graphite also conducts electricity in opposition to diamond because it has free moving delocalised electrons. It has to do with the fact that each carbon atom is bonded to only 3 out of 4 carbons and the electrons from the fourth carbon contribute to the delocalised electron system.

3.Carbon nanotubes

Carbon nanotubes have a similar structure to graphite. They also conduct electricity for the same reason as graphite.

Nanotubes also align themselves in ropes which are held together by Van der Waals forces.

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Smart materials

Smart materials- are materials that change their properties due to environmental factors.

I've got three examples of smart materials for you, however, be sure to know that there are many more.

1. Shape memory alloys - they return to their original shape after they're deformed.

2.U.V. materials - change colour when exposed to UV light.

3. Moisture sensitive materials - change colour when exposed to water.

As I said, they are only examples. There are many more. But this is all for now. Be invited to my youtube channel for past paper questions. Familiarise yourself with this topic and I am sure you will do great. This topic is very short therefore I have no doubts you can learn that.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!