Video Transcript
In this video, we will learn how to
write chemical formulas for covalent and ionic compounds and how to determine their
names from their formulas. We will use valence values to write
the chemical formulas of ionic compounds.
What is a chemical formula? Compounds differ from each other
because they contain different elements in different proportions. For example, the compound water has
one oxygen atom and two hydrogen atoms, while the compound hydrogen peroxide
contains two oxygen atoms and two hydrogen atoms. We can use simple chemical formulas
instead of diagrams like this to show which elements are present in a compound and
how many atoms of each element there are. A chemical formula is an expression
consisting of chemical symbols and numerical subscripts that represents the
composition of a molecule or one unit of a compound.
We can write the chemical formula
for water by writing the chemical symbol of each element present, H for hydrogen and
O for oxygen. Next, we write subscript numbers to
indicate how many hydrogen atoms there are and how many oxygen atoms there are. Since there are two hydrogen atoms,
we write a subscript of two next to H. And since there is one oxygen atom,
we write a subscript of one next to O. We can see from the chemical
formula that a water molecule consists of three atoms, specifically two hydrogen
atoms and one oxygen atom. Note that we usually leave off a
subscript of one. So, if no subscript appears next to
the chemical symbol of an element, then the number one is implied. For hydrogen peroxide, the chemical
formula is H2O2, since there are two hydrogen atoms and two oxygen atoms.
Now, let’s look at an example of a
slightly more complicated chemical formula. Here is a picture of a molecule of
the compound ethanol, and here is its chemical formula, C2H6O. We can see that ethanol contains
two carbon atoms, six hydrogen atoms overall, although the hydrogen atoms are spread
out over the molecule, and an oxygen atom. In total, there are nine atoms and
three different elements. Sometimes a chemical formula is
written in such a way that it gives us some extra information about the molecule or
compound. Ethanol’s chemical formula can also
be written as CH3CH2OH. When the chemical formula is
written in this way, it gives us information about how the atoms are bonded
together.
Some compounds contain sets of
multiple atoms. For example, the compound iron
carbonate, also called iron(III) carbonate, has the following chemical formula. This compound contains an atomic
group. An atomic group is a set of atoms
joined together that have a combined valence and behave as one unit during a
chemical reaction. This atomic group contains a carbon
atom and three oxygen atoms. Together they are called carbonate
or the carbonate ion, as together they have an overall charge. Carbonate has an overall charge of
negative two.
We can see that in this compound,
there is more than one carbonate atomic group. In fact, there are three atomic
groups, as the subscript outside the parentheses tells us. When there is more than one atomic
group, parentheses must be used. We can determine that this compound
has two iron particles.
To determine how many of each type
of atom in the atomic groups there are in the compound, we multiply the subscript
next to each atom by the subscript outside the parentheses. So, for carbon we say one
multiplied by three, which gives three carbon atoms in the compound. And for oxygen we say three
multiplied by three, which gives nine oxygen atoms in the compound. So, one unit of iron carbonate
contains two atoms of iron, three atoms of carbon, and nine atoms of oxygen. Overall, a unit of this compound
has three elements and 14 atoms.
So far, we have learnt how to write
and understand chemical formulas. Now let’s learn about naming some
simple compounds. We will learn how to name simple
covalent compounds and simple ionic compounds. Let’s start with covalent
compounds. Covalent compounds are typically
composed of nonmetal atoms bonded together. For example, when nitrogen and
oxygen bond together, a covalent compound forms, since both nitrogen and oxygen are
nonmetals. We can see this covalent compound
has one nitrogen atom and two oxygen atoms.
We can determine the name of a
covalent compound by first writing the name of the first element in the chemical
formula. For this compound, the element that
appears first is nitrogen. Next, we write the name of the
second element that appears in the chemical formula. The second element in this chemical
formula is oxygen. Third, we change the end of the
second element’s name to -ide. This gives us nitrogen oxide. Finally, we add a prefix in front
of the elements in the name to indicate how many atoms of that element there
are.
The table shows the prefixes. Mono- means one, di- means two,
tri- means three, and so on. Since there are two oxygen atoms,
we write the prefix di- in front of the term oxide. This gives us a name for this
compound of nitrogen dioxide. We do not need to use the prefix
mono- when there is only one atom of the first element. So, this compound is just called
nitrogen dioxide, not mononitrogen dioxide.
Here are some more examples of
covalent compound formulas and their names. CO is carbon monoxide. SO3 is sulfur trioxide. Covalent compound names can be
easily used to deduce the chemical formula. In other words, we can work
backwards from a name to a chemical formula. Here is an example. Sulfur dioxide is the name of a
covalent compound. The name tells us that the elements
present are sulfur and oxygen. There is no prefix before the word
sulfur, so we know there is one sulfur atom in a molecule of sulfur dioxide. And the prefix before the oxide
term is di-, so we know that there must be two oxygen atoms in this compound. Therefore, the chemical formula of
sulfur dioxide is SO2. Remember, we can leave off the
subscript of one.
Now, how do we name ionic
compounds? Ionic compounds are typically
composed of positively charged metal ions and negatively charged nonmetal ions. Here is an example. This ionic compound has the
chemical formula AlCl3. For an ionic compound, we first
write the name of the metal in the chemical formula. Aluminum is the metal. Second, we write the name of the
second element, the nonmetal. Chlorine is the nonmetal. Finally, we change the end of the
second element’s name to -ide. This gives us the name aluminum
chloride for this compound. Notice that we do not use prefixes
in ionic compound names to indicate how many of each type of ion there are.
Now we know how to write the name
of an ionic compound from its formula. We can also do the opposite. We can write the chemical formula
of a simple ionic compound from the name if we know the valence or valence numbers
of the elements in the compound. Valence is the number of electrons
that an atom gains, or loses, or shares when forming a bond during a chemical
reaction. This periodic table shows the
valence of some elements. Transition elements in the middle
of the periodic table can have variable valence values. But we will focus on elements in
groups on the left and right sides of the periodic table.
Let’s use this information to
determine the chemical formula of calcium oxide. Calcium oxide is composed of
calcium and oxygen. We can see that calcium in group
two has a valence of two and oxygen in group 16 has a valence of two as well. To write the formula, we first
write the symbols of the metal and nonmetal in the ionic compound and the valence of
each. Next, we cross over the valence
values and write them as the subscripts of the opposite chemical symbol. Lastly, we must simplify the values
if they have a common factor. In this example, both subscripts
are divisible by two. So, we simplify and write
Ca1O1. But subscripts of one need not be
shown in chemical formulas, so we can remove these. Therefore, calcium oxide has the
chemical formula CaO.
We can also use valence to write
the chemical formulas of ionic compounds containing atomic groups. This is done in much the same way
as when writing the chemical formula of simple ionic compounds. But we would need to make use of a
table such as this to determine the valence value of atomic groups. We can see that the carbonate
atomic group, for example, has a valence of two, while the nitrate atomic group has
a valence of one.
Now, let’s practice what we have
learnt.
What is the chemical formula of the
ionic compound magnesium chloride?
A chemical formula is an expression
of chemical symbols and numerical subscripts that represents the composition of a
molecule or one unit of a compound. For example, water has the chemical
formula H2O. We can see from the chemical
symbols H and O that the elements present in a molecule of this compound are
hydrogen and oxygen. There is also a numerical subscript
shown in this chemical formula. Hydrogen has a subscript of
two. And since there is no subscript
written next to oxygen, a subscript of one is implied. So, the chemical formula for water
tells us that one molecule of water consists of two hydrogen atoms and one oxygen
atom.
Now, the compound whose chemical
formula we are asked to find is magnesium chloride. We are told that magnesium chloride
is an ionic compound. To write the chemical formula of an
ionic compound from its name, we can follow three simple steps. First, write the symbols of the
metal and the nonmetal elements and their valence values. The metal is magnesium, whose
chemical symbol is Mg. The nonmetal is chlorine, which we
get from the term chloride. Chlorine has the symbol Cl. Now, we need to write their valence
values. Valence is the number of electrons
that an atom gains, loses, or shares when forming a bond during a chemical
reaction.
This diagram shows only the left
and right sides of the periodic table. It has the valence values of the
elements in groups one and two. It also depicts the valence values
of some elements in groups 15 to 17. We can see that magnesium in group
two has a valence value of two. Chlorine in group 17 has a valence
value of one.
Step two is to cross over the
valence values and make them the subscripts of the opposite chemical symbol. When we do this, we get the formula
Mg1Cl2. The third step is to simplify the
subscripts. In this case, the subscripts cannot
be simplified further. Remember, we can leave off any
subscripts of one. This leaves us with a chemical
formula for magnesium chloride of MgCl2.
Now, let us summarize what we know
about writing chemical formulas. We learnt that a chemical formula
is an expression of chemical symbols and numerical subscripts that represents the
composition of a molecule or one unit of a compound. Subscripts in a chemical formula
tell us how many atoms or ions of each element are present. We do not write subscripts of one
in chemical formulas. When naming covalent compounds, we
use prefixes to indicate the number of each type of atom. When naming ionic compounds, we do
not use prefixes. And finally, we cross over valence
values to determine the chemical formula for an ionic compound.
