Almost all elements found in nature are solids. Solids can hold their shape, and it is because of the solid's unique properties that we can drink coffee in our favorite mug, without the mug changing its shape, or sleep on the bed without the bed turning into a liquid or disappearing into a gas!
- This article is about the properties of solids.
- We will start by talking about the properties of solids.
- Then, we will look at the different types of solids and what makes them unique: ionic, covalent, molecular, metallic and non-crystalline solids.
- Lastly, we will solve some questions related to what you learned.
Properties of Solids
First, let's recall the definition of matter.
Matter is referred to as anything that has mass and occupies space.
Matter can be classified into three different states: solids, liquids, and gases. However, we will only be talking about solids in this article. Remember: the basic properties of solids are that solids have a fixed shape and definite volume, they are incompressible, and their molecules are tightly arranged together in a fixed position.
To learn more about the differences between these three states of matter, check out "Solids, Liquids, and Gases"!
But, what exactly are properties? And how are properties classified? Properties are classified as either physical or chemical properties:
Physical properties can change the physical appearance of a substance, but allow the chemical composition to stay the same.
Chemical properties can change the chemical composition of a substance. These are usually seen during chemical reactions.
Physical properties can be further broken down into two categories:
- Intensive properties - properties that are independent of the size or amount of substance.
- Extensive properties - properties that are dependent on the size or amount of substance present.
How can we know whether a property is physical or chemical? Look at the image below to find out!
Fig. 1: Examples of Physical and Chemical Properties, Isadora Zaghini - Vaia Originals
Now that we have covered what makes properties physical or chemical, let's talk about the different types of solids and their properties.
Solids are classified into crystalline and non-crystalline (amorphous) solids. Crystalline solids are further divided into four different categories: Ionic, molecular, covalent network, and metallic solids.
- Solids are placed into their corresponding category based on differences in the type of particle (ion, atom, or molecules) and the type of attractive force present.
Crystalline solids have a definite, well-arranged 3D shape
Amorphous solids lack an organized arrangement/patterns and melt over a range of temperatures.
Crystalline solids are formed from small, repeating units called unit cells. Unit cells are repeated and arranged, giving rise to a 3-D crystal structure known as a crystal lattice. The blue dots seen in the lattice corners represents a lattice point and consists of a specific atom, molecule, or ion.
Fig. 2: Crystal Lattice Basic Structure, Isadora Zaghini - Vaia Originals.
Although crystalline solids can have many arrangements, we will only focus on the following unit cell types:
- Primitive unit cell: a unit cell where the lattice points are found in the corners of the unit cell.
- Body-centered unit cell (BCC): a unit cell that has one particle at the center of the lattice and one particle in each of the corners.
- Face-centered unit cell (FCC): a unit cell that contains one particle on each of the faces of the lattice, and one particle in each of the corners.
- Side-centered unit cell (SCC): a unit cell that has one particle on two opposite faces, and one particle in each of the corners.
Fig. 3: Types of Cubic Unit Cells, Isadora Santos - Vaia Original.
Solids can be:
- Ionic solids.
- Covalent network solids.
- Molecular solids.
- Metallic solids.
- Amorphous solids.
STOP - If you don't know what each of these solid types are or need a refresher, read "Solids" first and then come back to learn about their properties!
Let's dive into some properties of the different crystalline solids: ionic solids, covalent network solids, molecular solids, and metallic solids.
Properties of Ionic Solids
Ionic solids consist of ions joined together by ionic bonds, a type of chemical bond that occurs between a positive and a negatively charged ion. Ionic bonding is a strong electrostatic interaction, and this strong attraction is responsible for many of the properties of ionic solids.
The properties of ionic solids include hard texture, high melting, and high boiling points of ionic solids. Ionic solids also have low vapor pressure and are brittle. These solids are not good conductors of electricity unless they are heated to become molten ionic compounds (liquid state) or if they are dissolved in water. When it comes to heat conduction, ionic solids are not the best solids for the job, as they are poor conductors of heat. Ionic solids also have varying solubility in water.
Ionic bonding is a strong electrostatic interaction, and this strong attraction is responsible for many of the properties of ionic solids such as the hard texture, high melting, and high boiling points of ionic solids. Ionic solids also have low vapor pressure and are brittle. These solids are not good conductors of electricity unless they are heated to become molten ionic compounds (liquid state) or if they are dissolved in water.
Ionic solids have lattice energy, the energy that gets released when ions join together to form ionic solids. Lattice energy also tells how strong the attractive force between the ions is.
When a potassium ion (K+) and a chlorine ion (Cl-) join together through ionic bonding, they form potassium chloride (KCl), an ionic solid.
Fig. 4: KCl Face-centered cubic lattice structure, Isadora Zaghini - Vaia Originals.
Properties of Covalent Network Solids
Covalent network solids are composed of atoms, and these atoms are joined together by covalent bonds. Covalent bonds happen when non-metals equally share their valence electrons to fill their outer shell.
This type of bond is also strong, giving covalent network solids their very hard texture and very high melting points. Covalent network solids in this group are unable to conduct electricity and are also poor conductors of heat (except for diamond, which is a good heat conductor). Solids in this category are also insoluble in water.
Diamond is a type of covalent network solid, and also an allotrope of carbon. Allotropes are different crystalline arrangements of the same element.
Fig. 5: Diamond FCC lattice structure, Isadora Zaghini - Vaia Originals.
Properties of Molecular Solids
Molecular solids consist of molecules (either polar or non-polar) that have intermolecular forces as their attractive force. Polar molecules usually have dipole-dipole forces and London dispersion forces present, while Non-polar molecules only have weak London dispersion forces.
Polar molecular solids have high melting points, whereas nonpolar molecular solids have low melting points. Molecular solids are soft and are unable to conduct electricity. In terms of solubility, nonpolar molecular solids are insoluble in water and polar molecular solids have some degree of solubility in water.
Dry ice (solid CO2) is considered a non-polar molecular solid, whereas ice (H2O) is a polar molecular solid.
Fig. 6: Solid CO2 FCC crystal lattice, Isadora Zaghini - Vaia Originals.
Polymers, which are large molecules, are also molecular solids. These polymers are composed of small, repeating units called monomers. Polyethylene terephthalate (PET) is a type of polymer that is used to make plastic bottles. You can learn more about polymers by reading "Crystalline Polymers"
Properties of Metallic solids
The last category of crystalline solids is metallic solids. Metallic solids are composed of positive ions that are attracted to each other by delocalized electrons in the electron cloud.
These solids are great conductors of electricity due to the electron's ability to move. They are also good conductors of heat. Metallic solids do not have specific melting points. Instead, parts of the solid melt at different temperatures. The hardness of metallic solids also varies. These solids also have unique characteristics such as being malleable, ductile, and shiny.
Elemental Silver (Ag) is an example of a metallic solid. The silver ion, which is positively charged, is surrounded by delocalized electrons.
Fig. 7: Metallic solid structure using Ag as an example, Isadora Zaghini - Vaia Originals.
Non-crystalline (amorphous) solids
Amorphous solids do not have an arranged shape like the other solid types. They look disorganized. Amorphous solids are brittle and can conduct electricity. A great example of an amorphous solid is glass. Glass is made up of silica and a varying substance, such as B2O3 or K2O. The components are first heated until passed their boiling points and then allowed to cool rapidly.
Fig. 8: Comparison of a crystalline and an amorphous solid structure, Wikimedia Commons.
In your AP Chemistry exam, you will be expected to relate some physical properties to the type of solid and the type of electrostatic force present.
What type of solid has a high melting point and can conduct electricity when molten?
We know that ionic and covalent network solids can have high melting points due to the strength of their bonds. However, covalent network solids do not conduct electricity. So, the answer would be ionic solids.
Which type of solid is the best at conducting electricity?
Metallic solids are good conductors of electricity in their solid phase due to the free movement of electrons.
Now, I hope that you feel more at ease knowing the different properties of solids and how to tackle problems involving them!
Properties of Solids - Key takeaways
- Physical properties are properties that change the physical appearance of a substance. Chemical properties are properties that change the chemical composition of a substance.
- Ionic solids only conduct electricity when molten, have high melting and boiling points due to their ionic bonds, and are brittle.
- Covalent network solids have covalent bonds, are nonconductors of electricity, and also have high melting points.
- Molecular solids are made up of molecules that are attracted to each other by intermolecular forces. They have relatively low melting points.
- Metallic solids are composed of positive ions surrounded by free electrons. They are good conductors of electricity, and their melting points vary (although they are usually low).
References
- Brown, T. L. (2009). Chemistry: the central science. Pearson Education.
- Tro, N. J. (2012). Principles of chemistry: a molecular approach. Pearson Higher Ed. The Princeton Review. (2019).
- Cracking the AP Chemistry Exam 2020. Princeton Review.
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