Have you ever heard of the old wife's tale that spilling salt on the table is bad luck, and you must toss a pinch of salt over your shoulder for good luck? Well, the table salt (NaCl) that’s thrown is known as an ionic solid, and luckily we will explore what that means all without any salt throwing!
Ionic solids are solids made up of cations and anions held together by ionic bonds that form due to the electrostatic attraction between positive and negative ions.
- In this overview, we will look at the basic structure of ionic solids.
- Then, we will zoom in on the crystal lattice structure of ionic solids.
- Once we understand the structure, we will investigate factors affecting the structure of ionic solids.
- Finally, we will discuss the significant characteristics of ionic solid structures.
Basic Structure of Ionic Solids
Before we zoom in to the crystal lattice structure of ionic solids, it's important to know that ionic solids are types of crystalline solids!
Crystalline solids are solids whose particles are arranged in a very organized, repeating 3D arrangement.
Fig. 1: Particle arrangement of crystalline solids, Vaia Original. The basic structure to keep in mind is that ionic solids will be made up of rigid, repeating patterns of ions.
There are four types of crystalline solids: ionic solids, molecular solids, covalent network solids, and metallic solids! The difference between the four is based on the type of bonds and molecular forces holding them together. Our focus is on the structure of ionic solids but check out Types of Solids to learn more about the others.
Lattice Structure of Ionic Solids
So, now that we know the big picture of ionic solids, let's look at the specific structure of ionic solids known as lattices.
Lattices refer to the repeating arrangement of alternating positive cations and negative anions in an ionic solid.
Fig. 2: 2D particulate model of NaCl lattice. Vaia Original.
Often, ionic solids will be depicted in a 2D particulate model (as seen above) because it allows us to accurately represent and determine the size and arrangement of the ions in the solid. The particulate model is essentially a slice of the 3D lattice structure.
Crystal Structure of Ionic Solids
So combining what we know about crystalline solids and ionic lattices, what structure do ionic solids form? The answer is Ionic crystal lattices!
Crystal lattices refer to the repeating 3D arrangement of alternating positive cations and negative anions that make up an ionic solid.
Fig. 3: The 3D crystal lattice structure of NaCl., Vaia Original
Why do crystal lattices form?
You may be wondering, how do we know that ionic solids form this way? Well, this is because the ions are always going to arrange themselves to minimize repulsive forces between similarly charged ions and maximize attractive forces between oppositely charged ions. Naturally, this means that cations and anions will attract each other (hence why they are next to each other in the model), while cations will repel cations, and anions will repel anions.
One important aspect to grasp is that ions can have an equal electrostatic attraction to any number of oppositely charged ions nearby. Each ion is surrounded by oppositely charged ions in all directions creating these lattice structures; that is why one Na+ ion can be experiencing electrostatic attraction with six Cl- ions in the NaCl ionic solid above.
Check out Coulomb's Forces and Interaction Strength to learn more about this concept!
Based on what we have covered so far, is the below model of NaF correct? If so, explain why. If not, explain why not and draw it correctly. As a hint, the sizes of these ions are identical, so size isn't relevant to this particular structure, and we will get into how size affects the structure later on.
Fig. 3: KF ionic solid model. Vaia OriginalFirst, do you think this is the right structure?
The correct answer is: no! Looking at the arrangement, can you figure out what's wrong?
1) The first step is always to figure out the cation and anion.
It's pretty easy in this case because we are given the charges so
2) Next, we can ask ourselves if this arrangement maximizes attractive forces between Na+ and F- and minimizes repulsive forces by having an alternating, repeating pattern.
And bingo, there's the problem! This model doesn't have the optimal arrangement because Na+ and F- are next to each other.
The correct model should look like the below:
Fig. 4: Correctly arranged KF ionic solid. Vaia Original.
The above example should have been pretty straightforward, but it is a good reference for your understanding of ionic solids so far!
Factors Affecting the Structure of Ionic Solids
We will now investigate the factors that directly affect the crystal lattice structure of ionic solids. You may have guessed that the two main factors are:
- Charge of the Ions.
- Size of the Ions.
If you think these factors sound familiar, they are also some of the factors that determine the strength of ionic bonds according to Coulomb's Law! As the charge of ions increases and the size of the ionic radius decreases, the crystal lattice structure becomes more tightly packed, and the electrostatic forces between ions are stronger.
Charge of Ions
As should be clear from what we've covered so far, the charge of ions affects the structure because the oppositely charged ions attract one another, and the similarly charged ions repel one another, leading to the formation of the lattice structure in the first place! Based on Coulomb's Law, the stronger the charges of the ions, the more stable the lattice structure will be!
Size of Ions
How does the size of the ions affect the structure? Let's revisit the structure of NaCl to explore this!
Fig. 5: 2D particulate model of NaCl. Vaia Original.
What do you notice about the sizes of ions in the NaCl lattice?
Well, the Na+ cations are noticeably smaller than the Cl- anions! The larger anions form the outline of the crystal lattice while the smaller cations fill the space in between.
This is important to keep in mind because it means:
- the ionic radius of the ions affects how the lattice structure forms and is depicted.
- according to Coulomb's Law, the smaller the ionic radius, the more stable the lattice and the stronger the electrostatic forces between ions.
Because the charge and size of an ion directly affect the structure of ionic solids, it is crucial that you can correctly identify the cation and anion within an ionic compound, compare sizes between ions, and compare charges.
We can quickly identify cations and anions based on the ionic compound name and formula. The cation always comes before the anion in an ionic formula or name. Based on this knowledge, in LiF which ion is the cation and anion?
Let's look at the periodic table below to determine how to compare ionic radii and charges!
The general trend for ionic radius is that the ionic radius increases as we go down a group. As we go across a period from left to right, the ionic radius decreases.
The general charges for ions are labeled on the periodic table below and correspond to the group number. If you want a refresher on how to determine this, check out Periodic Table for more!
Fig. 5: Charges of Elements on Periodic Table, Vaia Original.
Characteristics of Ionic Solids Due to Structure
Finally, we will briefly look at how this crystal lattice structure leads to specific characteristics among ionic solids.
| Characteristic | Reasoning |
| High Melting Point | a lot of energy is required to break the strong ionic bonds which means high temperature to have enough energy |
| Hard and Brittle | the rigid, repeating lattice structure creates a very hard solid, but when there is a slight shift in arrangement, repulsive forces between similarly charged ions will break the crystal apart |
| Poor electric conductors as solid | the crystal lattice structure is highly organized and keeps the charged ions locked in place |
| Good electric conductors when dissolved | when the crystal lattice is dissolved, the charged ions can move freely and thus conduct an electrical charge |
This is a summary of all the important properties. Please check out Ionic Solids and Ionic Bonding for more detail on these characteristics!
Structure of Ionic Solids - Key takeaways
- Ionic solids are solids made up of cations and anions held together by ionic bonds
- they are a type of crystalline solid
- Ionic solids are structured in crystal lattices which refers to the repeating 3D arrangement of alternating positive cations and negative anions
- 2D particulate models often represent them
- The charge of ions and size of ions directly affect the stability and strength of the lattice structure of ionic solids.
- Ionic solids tend to be hard and brittle, have high melting points, and are poor conductors unless dissolved in a solution.
References
- Brown, Theodore L, H E. LeMay, Bruce E. Bursten, Catherine J. Murphy, Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science. , 2018.
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