Video Transcript
In this video, we will learn how to
refer to many of the chemical compounds we’ll encounter in chemistry. We’ll learn how to write and
interpret the notations used in chemical formulas. And we’ll learn how to name
different types of compounds.
All matter is composed of different
combinations of atoms that combine in specific ratios to form compounds, which means
that there are hundreds and hundreds of chemical compounds that we encounter on a
daily basis. If we want to refer to all these
compounds and talk about them as scientists, we’re going to need some formalized and
concrete way to do that so that we’re all on the same page.
And since all compounds are made of
a specific ratio of atoms, it’d be helpful if this language that we used referred to
the atomic makeup of the compound. For example, let’s take a look at a
compound that we’re all familiar with, water. A molecule of water is always
composed of two hydrogen atoms and one oxygen atom. How should we refer to this
molecule besides calling it water? Since we would like some way to
refer to the atomic makeup of water, maybe we could use the full names for each
element and call it the molecule composed of two hydrogen atoms and one oxygen
atom.
But that’s a little wordy. We probably wouldn’t want to write
that out every time water participates in a chemical reaction. Perhaps we could shorten things by
using the atomic symbols that we see on the periodic table, H for hydrogen and O for
oxygen, which seems much easier to deal with. If we use these atomic symbols for
the elements in a compound, we have a way to refer to the type of atoms in a
compound. So now we just need a way to refer
to how many of each type of atom are in a compound.
As we mentioned earlier, each unit
of water, one water molecule, contains two hydrogen atoms and one oxygen atom. There’s various different things
that we could think of to put those numbers in, maybe using a ratio or two H one O
or maybe using superscripts or subscripts. Well, as it turns out, by
convention, we use subscripts to indicate the number of atoms. But when we only have one of an
atom in a compound, like we have with oxygen, we drop the subscript, so that leaves
us with H2O. What we’ve arrived at is called the
chemical formula of water.
Chemical formulas tell us the type
and number of each type of atom that’s in one unit of a compound. To indicate the type of atoms that
are in a compound, we use the atomic symbols. And to indicate the number of atoms
in a compound, we use subscripts. Now, what do we do if we have more
than one molecule of water? What if we have two? How do we include this in our
chemical formula? We could try collectively putting
the total number of atoms in our formula, which would give us H4O2. But this could be confusing, since
it could indicate a compound that has four hydrogen atoms and two oxygen atoms. So instead, let’s indicate the
number of units of a compound that we have in front of the chemical formula. So, if we have two water molecules,
we would write 2H2O.
With this knowledge, we know the
basics of chemical formulas. But let’s discuss some specifics so
we can confidently create and name chemical formulas for some of the different kinds
of substances we might encounter in chemistry. With our water example, we’ve been
discussing compounds whose atoms bond covalently to form a molecule. These compounds are composed of
nonmetal elements, which are primarily found on the right-hand side of the periodic
table. Not all molecules have common names
like water, so we need some way to name them based on their atomic makeup.
In this video, we’ll be focusing
our attention on how to name molecules that are composed of two elements. The less electronegative element in
our molecule will be named first. This will be followed by the more
electronegative element, which will have the end of its name replaced with the
suffix -ide. Due to trends in electronegativity
across the periodic table, this will generally mean that the element with the lower
group number will be named first. And if the elements are in the same
group, the element with the higher period number comes first. Though these two rules of thumb
with the period number and the group number will get us pretty far for naming
molecules, it will fail us if we have a compound that’s composed of chlorine,
bromine, or iodine and oxygen, in which case those elements will be named before the
oxygen is.
Next, we’ll use Greek numerical
prefixes to indicate the number of atoms of each element. However, we don’t include the
prefix on the first atom if there’s only one of them. So now that we know these rules,
let’s try naming a molecule to get some practice. Let’s take a look at this molecule,
which is made of one carbon atom and two oxygen atoms. Carbon is less electronegative than
oxygen is, and it’s found in group 14 of the periodic table, while oxygen is found
in group 16. So, in both the name of this
molecule and in our chemical formula, carbon will come first, followed by
oxygen. And since there’s one carbon atom
and two oxygen atoms, the chemical formula for this molecule is CO2. And carbon will also come first in
the name of this molecule, just like in our chemical formula.
Oxygen will come next, but we’ll
replace the end of its name with -ide. Finally, we’ll use Greek numerical
prefixes to tell us how many of each type of atom are in the molecule. There’s one carbon, which
corresponds to mono-, and two oxygens, which corresponds to di-. But since there’s only one carbon
in the molecule and it’s the first element that we name, we can drop the prefix
mono-, which leaves us with the name carbon dioxide.
Now, let’s try one where we go the
other way, where we come up with the chemical formula of a compound from its
name. Let’s try dinitrogen monoxide. As a quick aside, you’ll notice
that we dropped the double vowel here; we have monoxide instead of monooxide. This comes up fairly frequently
when we name chemical compounds. From our name, we know that
nitrogen is the first element in this compound. And since it has the prefix di-,
there must be two of them. Then, oxide corresponds to the
element oxygen. So that must be the second element
in our molecule. And the prefix mono- indicates
one. But of course, we always drop the
subscript one in our chemical formula. So that leaves us with the chemical
formula N2O for the molecule dinitrogen monoxide.
So that covers the basics of naming
and identifying covalently bonded compounds. Now, let’s talk about the other
major class of compounds we’ll run across in chemistry, ionic compounds. Ionic compounds are composed of
positively charged cations and negatively charged anions that are held together by
attractive forces.
When we were looking at molecular
compounds, there could be many compounds that were composed of the same two
elements. For instance, here are just a few
of these compounds that can be composed of nitrogen and oxygen. But in ionic compounds, the cations
and the anions combine in specific ratios to form a compound that’s neutral
overall. Since ionic compounds always
combine in a specific ratio of ions, we won’t need to use those Greek numerical
prefixes. We’ll simply have to work out the
number of ions that are in one unit of a compound from the charges of the ions,
which means that we’re going to need to know the charges of the ions in our compound
to create their formula.
So, here are some common ions and
their charges. Remember that elements tend to form
ions that will have the same electron configuration as a noble gas. We will run across two main types
of ionic compounds in chemistry. The first is the simplest, composed
of only mononuclear ions. “Mononuclear” just means that the
ions are derived from one atom. An example of this kind of compound
is table salt, which is composed of positively charged sodium cations and negatively
charged chlorine anions.
We’ll also run across ionic
compounds that are composed of one or more types of polyatomic ions, which are a
unit of covalently bonded atoms that have a charge. There are many polyatomic ions that
pop up in chemistry. Here are just a few of them and
their names. When we name ionic compounds, we’ll
always name the cation first, followed by the name of the anion. And we’ll use the suffix -ide if
the anion is mononuclear. So now that we know these rules,
let’s try naming some ionic compounds and determining their chemical formula.
Let’s go back to our example of
table salt, composed of positively charged sodium cations and negatively charged
chlorine anions. Since sodium is the cation here,
we’ll name it first, followed by the name of the anion, chlorine. But since chlorine is not a
polyatomic anion — it’s just derived from one atom — we’ll replace the end of its
name with -ide, giving us the name sodium chloride. Now, we need to come up with its
chemical formula. We’ll need to make sure that we end
up with an overall neutral compound. So, the positive charge from the
sodium needs to balance out the negative charge from the chlorine.
Since the sodium cation has a
charge of one plus and the chlorine anion has a charge of one minus, we’ll need one
sodium ion for every one chlorine ion to make sure that we have a neutral
charge. Since we just need one sodium for
every one chlorine, we’ll end up with the chemical formula NaCl for sodium
chloride.
Now, let’s try another one, an
ionic compound composed of a positively charged calcium cation and the negatively
charged polyatomic anion nitrate. Since calcium is the cation, it
will be named first. And nitrate is the anion, so it
will be named second. Since it’s a polyatomic anion, we
don’t need to replace the end of its name with -ide. So, its name would be calcium
nitrate. Now, we need to work out how many
of each ion that we’ll have on our chemical formula so that it has a neutral
charge.
Calcium has a charge of two plus,
and the nitrate has a charge of one minus. We may recognize right away that
we’ll need two of the nitrate anions to balance out the positive charge of the
calcium cation, which would give us a ratio of one calcium cation for every two
nitrate anions. But if we don’t see this right
away, or if we’re dealing with a more complicated ionic compound, we can use this
trick to come up with the chemical formula where we swap the charges and use those
as the subscript of the other ion. Either way, we’ll need one calcium
ion for every two nitrate ions, giving us the chemical formula CaNO32. But we don’t want this to look like
we have one calcium atom, one nitrogen atom, and 32 oxygen atoms in our
compound. So, we’ll put parentheses around
the polyatomic anion to make it clear that we’re talking about two nitrate groups in
our compound.
There’s one last thing that’s worth
mentioning before we move on from ionic compounds. You might notice that some elements
can form multiple ions with different charges, like iron and copper. To differentiate between these
ions, we can use Roman numerals in parentheses following the name of the
element. So, Fe2+ would be called iron(II)
and Fe3+ would be called iron(III). This, of course, means that there
would be two compounds containing iron and chlorine, for instance. But because the ions in ionic
compounds combine in specific ratios based on their charge, we should be able to
work out which cation we’re dealing with based on the chemical formula.
So now that we’ve learned all of
these different rules for naming compounds, it’s worth mentioning that some
compounds have names that just don’t follow the rules. For example, there are many
compounds that have common names. We’ve already run across one of
these. H2O is always called water and not
dihydrogen monoxide. Similarly, NH3 is always called
ammonia and not nitrogen trihydride. This happens most often for
compounds that scientists discovered a long time ago before we standardized the
convention for naming things in chemistry.
Another example is the compound
TiO2, which is commonly found in sunscreens. Ti is the atomic symbol for
titanium, which is one of the metals that can form multiple ions with different
charges. If we work out the charge on
titanium from the chemical formula, we would find out that titanium here has a
charge of four plus. So this compound should be called
titanium(IV) oxide. And while some people do call this
compound titanium(IV) oxide, if you look on the back of your sunscreen bottle,
you’ll find that this compound has the name titanium dioxide instead. But despite this, many of the
compounds that we’ll come across in chemistry will follow the rules that we’ve
learned in this video.
So now that we know all these
rules, let’s look at a couple example problems.
How many hydrogen atoms are there
in a molecule of methane, CH4?
This is the chemical formula for
the molecule methane. The chemical formula tells us the
type and number of atoms that are in one unit of a compound. We indicate the type of atom using
the atomic symbols for each element found on the periodic table. And we indicate the number of each
type of atom using subscripts. So, from the chemical formula CH4,
we can tell that methane must contain carbon and hydrogen.
If we look at the subscript, carbon
has no subscript. This is because, by convention, we
leave off the one in the chemical formula if there’s only one of an atom in a
compound. So, that means that there is one
carbon atom in a methane molecule. And we can see that there’s a
subscript four next to hydrogen, which means that there’s four hydrogen atoms in
methane. This question asked us how many
hydrogen atoms there are in a molecule of methane. And as we just discussed, there are
four.
What would the name of a compound
containing one atom of chlorine and two atoms of oxygen be?
In this question, we need to name a
compound that contains chlorine and oxygen. Chlorine and oxygen are both
nonmetals found on the right-hand side of the periodic table, which means that the
compound that they form will be a molecular compound. When we name molecular compounds,
we name the less electronegative element first. Then, we’ll name the more
electronegative element with the suffix -ide. Finally, we’ll use these Greek
numerical prefixes to indicate the number of atoms that we have in a compound.
Chlorine is less electronegative
than oxygen, so we’ll name it first, followed by oxygen. But we’ll have to replace the end
of its name with the suffix -ide. Finally, we’ll use Greek prefixes
to indicate the number of atoms. We have one atom of chlorine, which
corresponds to mono-, and two atoms of oxygen. But by convention, we drop the
prefix if there’s only one of the first element. So, we would drop the mono-,
leaving us with chlorine dioxide.
Now, let’s sum up with the key
points for this video. Chemical formulas indicate the type
and number of atoms in a compound using atomic symbols and subscripts. If we have more than one unit of a
compound, the number of units goes in front of the chemical formula. When we’re naming molecules, the
less electronegative element will come first. When we’re naming ionic compounds,
the cation is named first. Roman numerals are used to
distinguish elements that form multiple ions.
