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.
