Ions: Anions and Cations

Definition of Ions, Cations, and Anions

Let's start by looking at the definition of cations and anions.

As mentioned above, ions are charged molecules. The word “ion” was first introduced by Michael Faraday in 1834 to describe a substance he observed moving through a current.

The term “ion” comes from the Greek word of the same spelling, which means “to go”, while the names “cation” and “anion” mean an item that moves down and up respectively. This is because, during a process known as electrolysis, cations are attracted to the negatively charged cathode, while anions are attracted to the positively charged anode.

Cation and Anion Ion Differences

The difference between cations and anions arises from their different charge.

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Figure 1: Illustration of cations and anions forming from a neutral atom as a result of losing and gaining electrons, respectively. Daniela Lin, StudySmarter Originals

It’s because of these charge differences that cations and anions behave differently in processes like electrolysis.

Within chemistry, we write cations with a + sign and anions with a – sign. The number symbol written next to the charges indicates how many electrons the atom has lost or gained, respectively.

In the case of Na⁺ and Cl^- the ionic reaction results in Na+ losing one electron and Cl- gaining one electron. The illustration above will be expanded upon below with Lewis Dot Diagrams, but for now, it is important to understand the convention associated with how we write ions.

Cation Ion and Anion Radius

Now that we know the definition of ions and the differences between them, it’s time to go over ionic radii.

Recall that the atomic radius is half the distance between two nuclei of neutral atoms. In contrast, ionic radius describes half the distance between two nuclei of non-neutral atoms.

Anions have a greater ionic radius when compared to the same element’s atomic radius. In comparison, cations have a smaller ionic radius when compared to the same element’s atomic radius.

Confused? It’s all right! The illustration below gives a visual representation of radial size differences.

Figure 3: Cations and anions radius compared to their element’s respective atomic radius. Daniela Lin, StudySmarter Originals.

The size differences in radii arise because as neutral atoms gain electrons and become anions, more electrons occupy the outer orbitals, leading to increased electron repulsion. This increase in electron repulsion pushes electrons further apart, resulting in a larger ionic radius.

The opposite occurs with cations, which result from the loss of electrons. Less electron repulsion results in a smaller ionic radius.

In other words, cations have a smaller ionic radius, while anions have a larger ionic radius when compared to their element’s respective atomic radius.

Cation and Anion Ion Exchange Resin

Earlier in the article, we mentioned that certain substances can act as media for ion exchange.

One of these substances is resin. Resin is a highly viscous substance, often made using plants. It’s insoluble and contains microbeads that are porous enough to trap specific ions, according to charge, facilitating the process known as ion exchange.

Cation-exchange resins are composed of negatively charged sulfonate groups. Meanwhile, anion-exchange resins contain positively charged amine surfaces.

Figure 4: Ion exchange illustration. Daniela Lin, StudySmarter Originals

The process of softening water, through ion exchange, is shown above. This particular cation exchange involves exchanging magnesium and calcium for sodium ions. There are many other types of ion exchange and also many other applications of ion exchange chromatography in organic chemistry and biochemistry. We will not discuss these in detail here, however, all these advanced chemistry techniques are based on the simple application of ion exchange depicted above.

Examples of Ions Cations and Anions

Before looking at the formation of ionic compounds, we need to understand which elements on the periodic table are likely to form cations or anions.

• Noble gases are stable because they have full valence electrons; thus they do not tend to form ions.

• Metals tend to create cations, while non-metals tend to create anions.

• Elements on the left side of the periodic table tend to make cations, compared to the right side of the periodic table, which tends to create anions.

Figure 5: Illustration of the periodic table with ionic charges shown. Daniela Lin, StudySmarter Originals.

The picture above shows that:

• Cation formation (+): Groups 1, 2, 13, and 14 tend to form cations by losing electrons.

• Anion formation (-): Groups 15, 16, and 17 tend to form anions by gaining electrons

Now that we have learned about which elements tend to create cations and which ones tend to create anions. The next step is to look at how ionic compounds form. To achieve this, we will use Lewis Diagrams.

Simplified illustrations of a molecule's valence electrons are known as Lewis dot diagrams. We can also use Lewis dot diagrams to show electron transfer in ionic compounds, which is precisely what we are going to do now.

We will use the same ions shown in our writing ions graphic above.

Figure 6: Examples of ion transfer shown in an ionic compound reaction where Sodium Chloride and Magnesium Oxide are produced. Daniela Lin, StudySmarter Originals

Now that we’ve taken a look at some examples of cations and anions through an ionic compound reaction. We should be comfortable identifying ions, cations, and anions. We should also be able to understand which ions will gain or lose electrons. Finally, we should comprehend exchange resins and ionic radii trends.