With most atoms, the number of protons is equal to the number of electrons. This means that normally an atom has zero charge. An atom can become negatively charged when it gains electrons (anions) and vice-versa (positively charged) when it loses electrons (cations). The term "ion" is used to refer to a charged atom, whatever the sign of the charge might be. Understanding ions is vital when it comes to electron movement and bonding in chemistry.
- This article is about the two different types of ions (cations and anions).
- We'll begin by understanding what ions are and then distinguishing their differences.
- Next we will learn about the difference in radius and what an exchange resin is.
- Finally, we will cover examples of common cations and anions.
Definition of Ions, Cations, and Anions
Let's start by looking at the definition of cations and anions.
Ion: a molecule with a net charge (+ or -).
Cation: an ion with a positive (+) net charge.
Anion: an ion with a negative (-) net charge.
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.
For more detailed information regarding electrolysis, please reference our “Electrolysis” article.
Cation and Anion Ion Differences
Now that we understand what ions are, we can now focus on the difference between them.
The difference between cations and anions arises from their different charge.
Cations: are positively (+) charged ions. Their positive charges come from the fact that they have more protons than electrons. They are formed when an often neutral atom loses one or more electrons.
Anions: are negatively (-) charged ions. Their negative charges come from the fact that they have more electrons than protons. They are formed when a neutral atom gains one or more electrons.
A quick way to remember that anions are negatively charged is to think of the N in aNion as negative and the t in caTion as a + sign.
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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, Vaia Originals
It’s because of these charge differences that cations and anions behave differently in processes like electrolysis.
Electrolysis is the process, whereby, an electric current passes through a material, creating a chemical reaction.
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.
Keep in mind that electrons are negatively charged, (-) which means that when we LOSE them our atom becomes positively charged,+, and when an atom gains electrons it becomes negatively charged, -.
Figure 2: Metals lose electrons while non-metals gain electrons. Daniela Lin, Vaia Originals.
For more detailed naming conventions for ionic compounds, please refer to our “Ionic and Molecular Compounds” main article.
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.
Ionic radius: half the diameter of an ion
For more detailed information regarding periodic trends, please reference our “Periodic Trends” or “Periodic Trends: General Trends” articles.
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, Vaia 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.
Ion exchange removes undesirable ions, typically from liquids, and replaces them with more desirable ions.
This process is frequently used to purify and soften water for drinking purposes.
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, Vaia 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, Vaia 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
Carbon can gain or lose electrons based on the situation but the formation of carbocations or carbanions is typically difficult to stabilize.
This means carbon usually shares its 4 valence electrons through covalent bonds of either single, double, or triple bonds with other molecules.
For more detailed information regarding valence electrons or Lewis Diagrams, please reference our “Valence Electrons” or “Lewis Diagrams” articles.
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, Vaia 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.
Ions: Anions and Cations - Key takeaways
An ion is a molecule with a net charge that’s not zero. Ions are an important chemistry concept because it describes electron movement and has commercial applications like water purification.
A cation is a type of ion with a positive (+) net charge
An anion is a type of ion with a negative (-) net charge
The ionic radius is half the diameter of an ion when compared to the atomic radius, which is half the diameter of a neutral atom.
Lastly, 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.
References
- Libretexts. (2020, September 14). Periodic trends in Ionic radii. Chemistry LibreTexts.
- 7.3 lewis symbols and structures - chemistry 2E. OpenStax. (n.d.).
- Libretexts. (2022, May 2). 3.2: Ions. Chemistry LibreTexts.
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