Think about what you call your friends. Some of your friends might have full names (long names) and a nickname
(short name). These are the words we use to tell others who or what we are referring to. Their full name is like
the substances name and their nickname is like the substances formulae. Without these names your friends would
have no idea which of them you are referring to. Chemical substances have names, just like people have names. This
helps scientists to communicate efficiently.
It is easy to describe elements and mixtures. We simply use the names that we find on the periodic table for
elements and we use words to describe mixtures. But how are compounds named? In the example of iron sulfide that
was used earlier, the compound name is a combination of the names of the elements but slightly changed.
The following are some guidelines for naming compounds:
The compound name will always include the names of the elements that are part of it.
A compound of iron (\(\text{Fe}\)) and sulfur (\(\text{S}\)) is
ironsulfide (\(\text{FeS}\))
A compound of potassium (\(\text{K}\)) and bromine (\(\text{Br}\)) is
potassiumbromide (\(\text{KBr}\))
A compound of sodium (\(\text{Na}\)) and chlorine (\(\text{Cl}\)) is
sodiumchloride (\(\text{NaCl}\))
In a compound, the element that is on the left of the Periodic Table, is used first when naming the
compound. In the example of \(\text{NaCl}\), sodium is a group 1 element on the left hand side of the table,
while chlorine is in group 17 on the right of the table. Sodium therefore comes first in the compound name.
The same is true for \(\text{FeS}\) and \(\text{KBr}\).
The symbols of the elements can be used to represent compounds e.g. \(\text{FeS}\),
\(\text{NaCl}\), \(\text{KBr}\) and \(\text{H}_{2}\text{O}\). These are called chemical
formulae. In the first three examples, the ratio of the elements in each compound is 1:1. So, for
\(\text{FeS}\), there is one atom of iron for every atom of sulfur in the compound. In the last example
(\(\text{H}_{2}\text{O}\)) there are two atoms of hydrogen for every atom of oxygen in the compound.
A compound may contain ions (an ion is an atom that has lost or gained electrons). These
ions can either be simple (consist of only one element) or compound (consist of several elements). Some of the
more common ions and their formulae are given in Table 2.3 and
in Table 2.4. You should know all these ions.
Prefixes can be used to describe the ratio of the elements that are in the compound. This is
used for non-metals. For metals, we add a roman numeral (I, II, III, IV) in brackets after the metal name to
indicate the charge on the ion. You should know the following prefixes: “mono” (one),
“di” (two)
and “tri” (three).
\(\text{CO}\) (carbon monoxide) - There is one atom of oxygen for every one atom of
carbon
\(\text{NO}_{2}\) (nitrogen dioxide) - There are two atoms of oxygen for every one atom
of nitrogen
\(\text{SO}_{3}\) (sulfur trioxide) - There are three atoms of oxygen for every one atom
of sulfur
When numbers are written as “subscripts” in compounds (i.e. they are written below and to the right
of the element symbol), this tells us how many atoms of that element there are in relation to other elements in
the compound. For example in nitrogen dioxide (\(\text{NO}_{2}\)) there are two oxygen atoms for every one atom
of nitrogen. Later, when we start looking at chemical equations, you will notice that sometimes there are
numbers before the compound name. For example, \(2\text{H}_{2}\text{O}\) means that there are two
molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom.
The above guidelines also help us to work out the formula of a compound from the name of the compound. The
following worked examples will look at names and formulae in detail.
We can use these rules to help us name both ionic compounds and covalent compounds. However, covalent compounds
are often given other names by scientists to simplify the name (or because the molecule was named long before its
formula was discovered). For example, if we have 2 hydrogen atoms and one oxygen atom the above naming rules would
tell us that the substance is dihydrogen monoxide. But this compound is better known as water!
Some common covalent compounds are given in Table 2.5
Name
Formula
Name
Formula
water
\(\text{H}_{2}\text{O}\)
hydrochloric acid
\(\text{HCl}\)
sulfuric acid
\(\text{H}_{2}\text{SO}_{4}\)
methane
\(\text{CH}_{4}\)
ethane
\(\text{C}_{2}\text{H}_{6}\)
ammonia
\(\text{NH}_{3}\)
nitric acid
\(\text{HNO}_{3}\)
Table 2.5: Names of common covalent compounds
Worked example 3: Writing chemical formulae 1
What is formula of sodium fluoride?
List the ions involved
We have the sodium ion (\(\text{Na}^{+}\)) and the fluoride ion (\(\text{F}^{-}\)). (You can look these up on
the tables of cations and anions.)
Write down the charges on the ions
The sodium ion has a charge of \(\text{+1}\) and the fluoride ion has a charge of \(-\text{1}\).
Find the right combination
For every plus, we must have a minus. So the \(\text{+1}\) from sodium cancels out the \(-\text{1}\) from
fluoride. They combine in a \(\text{1}\):\(\text{1}\) ratio.
Write the formula
\(\text{NaF}\)
Worked example 4: Writing chemical formulae 2
What is the formula for magnesium chloride?
List the ions involved
\(\text{Mg}^{2+}\) and \(\text{Cl}^{-}\)
Find the right combination
Magnesium has a charge of \(\text{+2}\) and would need two chlorides to balance the charge. They will combine
in a \(\text{1}\):\(\text{2}\) ratio. There is an easy way to find this ratio:
Draw a cross as above, and then you can see that \(\text{Mg}^{2+}\) →\(\text{1}\) and \(\text{Cl}^{-}\)
→\(\text{2}\).
Write down the formula
\(\text{MgCl}_{2}\)
Worked example 5: Writing chemical formulae 3
Write the chemical formula for magnesium oxide.
List the ions involved.
\(\text{Mg}^{2+}\) and \(\text{O}^{2-}\)
Find the right combination
\(\text{Mg}^{2+}\):\(\text{2}\)
\(\text{O}^{2-}\):\(\text{2}\)
If you use the cross method, you will get a ratio of \(\text{2}\):\(\text{2}\). This ratio must always be in
simplest form, i.e. \(\text{1}\):\(\text{1}\).
\(\text{Cu}^{2+}\) (the questions asks for copper(II) not copper(I))
\(\text{NO}_{3}^{-}\)
Find the right combination
Write the formula
\(\text{Cu}(\text{NO}_{3})_{2}\)
Notice how in the last example we wrote \(\text{NO}_{3}\) inside brackets. We do this to indicate that
\(\text{NO}_{3}\) is a compound ion and that there are two of these ions bonded to one copper ion.
The ions dating game
Your teacher will assign each of you a different ion (written on a piece of card). Stick this to yourself. You
will also get cards with the numbers \(\text{1}\)–\(\text{5}\) on them. Now walk around the class and try
to work out who you can pair up with and in what ratio. Once you have found a partner, indicate your ratio using
the numbered cards. Check your results with your classmates or your teacher.
Naming compounds
Textbook Exercise 2.3
The formula for calcium carbonate is \(\text{CaCO}_{3}\)
Is calcium carbonate an element or a compound? Give a reason for your answer.
What is the ratio of \(\text{Ca}\):\(\text{C}\):\(\text{O}\) atoms in the formula?
Solution not yet available.
Give the name of each of the following substances.
\(\text{KBr}\)
\(\text{HCl}\)
\(\text{KMnO}_{4}\)
\(\text{NO}_{2}\)
\(\text{NH}_{4}\text{OH}\)
\(\text{Na}_{2}\text{SO}_{4}\)
\(\text{Fe}(\text{NO}_{3})_{3}\)
\(\text{PbSO}_{3}\)
\(\text{Cu}(\text{HCO}_{3})_{2}\)
Solution not yet available.
Give the chemical formula for each of the following compounds.