Lesson Video: Ions Chemistry

In this video, we will learn about what cations and anions are, how to deduce the charge on monatomic ions with help from the periodic table, and finally we will look at some common polyatomic ions.

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Video Transcript

In this video, we will learn about what cations and anions are, how to deduce the charge on monoatomic ions with help from the periodic table, and finally, we will look at some common polyatomic ions.

First, let’s look at what we mean by anions and cations. As we know, atoms are made from protons, neutrons, and electrons. Protons have a positive charge, electrons have a negative charge, and neutrons are neutral. When we look at ions, we’re interested in protons and electrons, since these are the particles which have a charge. So, we can ignore the neutrons for now. Remember that protons are found locked away inside the nucleus of the atom. This means it’s difficult for an atom to gain or lose protons. In order to form an ion, the atom needs to gain or lose electrons.

In our initial atom, the number of protons and the number of electrons are the same. This means that the positive charges on the protons and the negative charges on the electrons cancel each other out, making our atom overall neutral. If our atom gains an electron, we’re going to end up with more electrons than there are protons. This is going to result in an overall negative charge. On the other hand, if our atom loses an electron, we’re going to have more protons than electrons. This will result in an overall positive charge.

When an atom gains or loses electrons, it becomes an ion. A positively charged ion is called a cation, while a negatively charged ion is called an anion. Of course, an atom could gain or lose more than one electron at a time. The more electrons that it gains or loses, the more highly charged the ion becomes. For example, if our atom were to gain three electrons, it would become a three minus anion. On the other hand, if our atom were to lose two electrons, it would become a two plus cation.

But how do we remember whether a cation or an anion is the positive ion and which is the negative ion? You could remember that the extra n in anion stands for negative. This can remind you that anions are negatively charged ions. Of course, if you’re a lover of cats, you may want to remember that cats are positive. Therefore, cations are positive ions. But it’s up to you.

Of course, another question you may ask is, how do we know whether an atom is more likely to form a cation or an anion? And if it does form an ion, what charge might it have? Luckily, for many cases, the periodic table can give us a hand. Here’s a diagram of the main part of the periodic table, with the group numbers listed at the top of the columns. On the far right of our periodic table in group 18, we have the noble gases. You may remember that the noble gases are very stable. This makes them highly unlikely to form ions at all. So, we’ll list these as normally having a charge of zero.

In group 17, we have the halogens. Let’s look at the first of these halogens, fluorine. When we look closely at fluorine on our periodic table, we can see that it has the atomic number of nine. This means that one atom of fluorine contains nine protons. Remember that protons have a positive charge. To balance out the charges on these nine protons, our atom of fluorine also needs nine electrons. This makes our fluorine atom neutral. But what if we kept our nine protons and instead had 10 electrons? Would this make it an ion? The answer is yes. Because we no longer have the same numbers of protons and electrons, we’re going to end up with an overall charge, which is not zero. This makes it an ion.

Because we have more electrons than protons, we’re going to have an overall negative charge. In fact, what we end up with is F−. We would call this a fluoride ion, but is it an anion or a cation? Remember that anion is negative. So, this is a fluoride anion. Fluoride anions, of course, are often found in toothpaste. In some places, extra fluoride is even added to drinking water. This is because fluoride anions are important for looking after our teeth. As it turns out, fluorine can often be found in its F− anion form.

The same can be said for the other elements in the same group. For example, chlorine can be found as the chloride anion and bromine, the bromide anion. Of course, the chloride anion is a very common sight as we find it in table salt. So, halogens are most often found in their one minus anionic form. We get to these by adding one electron to the initial atom. But what about elsewhere on the periodic table?

Let’s look now to the left-hand side in group one of the periodic table. Here we have elements like hydrogen and sodium. What sort of ions do you think these form? Let’s think about the salt that we put on our food. The chemical name for the sort of salt we put on our food is sodium chloride. This is made up of both sodium and chlorine. We’ve already seen that chlorine tends to form a one minus charge, the chloride anion. Since table salt is overall neutral, our sodium must have a charge that balances out the chloride. So, the sodium exists as a one plus cation. Remember, of course, that a cation is a positively charged ion.

It turns out that other elements in the same group also form one plus cations easily. A good example is H+, which you will have come across in lots of your other chemistry, for example, when looking at acids. We can extend this and say that group two atoms tend to form two plus cations, and atoms from group 16 tend to form two minus anions. When it comes to other groups, particularly, the transition metals, which you can find within d-block, things are not quite so simple.

For example, many of the transition metals can form more than one ion easily. A couple of examples are iron and copper. Iron can easily form Fe2+ or Fe3+, while copper can often form Cu+ or Cu2+. When it comes to elements like these, it’s probably best to make yourself familiar with the most common ions that each of them forms. The same is true of groups 13, 14, and 15, which again are not always straightforward. As a general rule of thumb, you can assume that group 13 form three plus cations, that group 15 form three minus anions, and that those in group 14 tend to form covalent bonds instead.

So far, we’ve only talked about ions that contain one atom. So what about ions that have more than one atom? These are the polyatomic ions. Here, we’ll look at a few common examples of polyatomic ions. The poly- in polyatomic means more than one. So, it literally means ions with more than one atom. The opposite to poly- would be mono-, like in monatomic, meaning one atom. Here are some examples of polyatomic anions.

The carbonate anion is CO3 2−. Many carbonate minerals can be found in rocks, while other forms of carbonate can be used as cleaning products. The sulfate anions is SO4 2−. Each time you’ve used sulfuric acid in the laboratory, you’ve been using the sulfate anion. The formula for sulfuric acid is, of course, H2SO4. By remembering that hydrogen tends to form an H+ cation and that there are two of these in H2SO4, we can work out that the SO4 anion has a charge of two minus.

A similar method can be used for remembering the charge on the nitrate anion, NO3−. Nitric acid has the formula HNO3. Again, if the hydrogen has a one plus charge, the NO3 must have a one minus charge to balance it out. Our phosphate anion is PO4 3−. Phosphate anions are very common in many biological systems, for example, DNA. Our hydrogen carbonate anion has the formula HCO3−. Hydrogen carbonate is the main component in baking soda. When exposed to acid or heat, hydrogen carbonate decomposes, producing lots of carbon dioxide gas. It’s this gas production that helps our lovely baked goods to rise.

Of course, there are polyatomic cations as well, with one of the most common being the ammonium ion. Our ammonium cation is NH4+. You can form this ammonium cation by protonating ammonia, NH3. Now that we’re familiar with common polyatomic and monatomic ions, let’s try answering some questions.

How does an atom form an anion?

First, let’s remind ourselves what an anion is. An anion is a negatively charged ion. To form an anion, our atom, which is neutral, needs to gain negative charge. It does this by gaining electrons. Normally, our atom has the same number of protons as it does electrons. This gives our atom an overall neutral charge. If we add an extra electron to our atom, we’re going to tip the balance of the charge.

Now that we have more negative charges than positive charges, the overall charge on our atom is negative. An atom with a negative charge is called an anion. Remember that if an ion has a positive charge, it’s called a cation. So, how does an atom form an anion? The answer is by gaining electrons.

Which of the following is a polyatomic ion? (A) F−, (B) O2, (C) NO3−, (D) SO2, or (E) Mg2+.

The first thing to note is that we’re being asked to find a polyatomic ion. The poly- in polyatomic means more than one. So, we’re looking for an ion that contains more than one atom. So, any answer with only one atom, monatomic, can be ruled out. This rules out F− and Mg2+. Next, we’re looking for an ion. An ion is something that has a nonzero charge. We can spot an ion because it will have either a positive or negative charge notation.

For example, in answer (A), F− shows a negative charge on our fluorine atom. This makes it an ion. So, let’s look at our three remaining answers and see if any of them have a charge. Both O2 and SO2 are neutral, so we can rule these out. Answer (C) NO3− does indeed have a charge. A negative charge means that we can call this an anion. Remember that a positive charge is called a cation. So, here, we have our answer. The polyatomic ion in this question is answer (C) NO3−, the nitrate anion. Out of interest, you can find the nitrate anion in nitric acid, HNO3.

Let’s summarize our key points about ions. Anions are negatively charged ions, while cations are positively charged ions. Anions are formed by gaining electrons, and cations are formed by losing electrons. We found that we can use the periodic table to deduce the charges on ions in certain cases. For example, group one elements tend to form one plus cations, while group 17 form one minus anions. We also became familiar with some common polyatomic ions. That is ions which contain more than one atom, for example, sulfate, nitrate, ammonium, and carbonate.

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