Properties of Solids

Properties of Solids

First, let's recall the definition of matter.

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.

But, what exactly are properties? And how are properties classified? Properties are classified as either physical or chemical properties:

Physical properties can be further broken down into two categories:

1. Intensive properties - properties that are independent of the size or amount of substance.
2. 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 - StudySmarter 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 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 - StudySmarter 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 - StudySmarter Original.

Solids can be:

• Ionic solids.
• Covalent network solids.
• Molecular solids.
• Metallic solids.
• Amorphous solids.

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 - StudySmarter 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 - StudySmarter 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 - StudySmarter Originals.

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 - StudySmarter 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.

Now, I hope that you feel more at ease knowing the different properties of solids and how to tackle problems involving them!