In this video, we’re talking about
atomic isotopes. We’ll learn what this term
means. And we’ll also see how the periodic
table of elements helps us understand isotopes. In fact, by looking at the periodic
table itself, we can start to unearth clues about isotopes.
Now looking at a table, there are
many many elements and lots of information presented. So, let’s focus in on just one
element out of this whole table. Let’s say that we pick this one
right here. If we look at a close-up view of
this particular element, we’ll start to see some more information about it. Depending on the periodic table
we’re using, more or less information might be in this little box about this
In general though, there are four
bits of information we see. In this case, we see a whole
number, seven, the symbol representing this element, capital N, the name of the
element, and then a number below that name. We see then this element we’ve
picked is the element nitrogen. And it’s symbolized using a capital
N. And more than that, each of the
numbers in this little tile tell us something about this element.
The whole number at the top of the
panel, in this case seven, indicates what’s called the atomic number of this
element. The atomic number tells us how many
protons are in the nucleus of a nitrogen atom. This number is very important. It’s what identifies the
element. And actually, looking at the
periodic table as a whole, we see that it’s the atomic number that orders the
table. In the upper left, we start out
with one. And then, in the upper right, we go
to two. And then, three, four, five, six,
seven, eight, nine, 10, and so on.
All this means that if we know the
atomic number of an element, we can easily find it on the table. That’s how the table is set up. Now it makes sense that the atomic
number is a whole number, an integer. In any given atomic nucleus, we can
only have a whole number of protons. But then, what about this number
here at the bottom of this panel, 14.01, in the case of nitrogen. This number is called the average
atomic mass of the element.
There are a couple of things worth
noticing about this number. First, notice that it’s a mass
value. But then, that raises the question,
what are the units of this mass value? 14.01 what, kilograms or some other
unit? Given how small and lightweight
individual atoms are, if we wrote out the mass of those atoms using units of
kilograms, that number would be very very small. After all, the mass of a proton is
approximately 1.67 times 10 to the negative 27th kilograms.
In order to be able to write out
atomic masses without using very very small numbers, a measurement unit was
developed called the amu. It stands for atomic mass unit. In this system of units, the mass
of a single proton is approximately one amu and, in fact, same thing with a neutron,
which is the other constituent that atomic nuclei are made of.
Now if you’re wandering about the
other constituent of atoms electrons, the mass of an electron in this unit is
approximately one two-thousandth of an atomic mass unit. Since this mass is so small
compared to the mass of a proton and neutron, often when we calculate the total mass
of an atom, we neglect the mass of the electrons. Practically speaking then, this
number, average atomic mass, comes down to the number of protons and neutrons in an
But notice this interesting word,
it’s an average mass. And we also see that this isn’t a
whole number, 14.01 rather than, say, 14 exactly. These are clues from the periodic
table, which are telling us that it’s possible for an atom of nitrogen, an atom with
seven protons in its nuclear core, to have a different number of neutrons in that
nucleus. Here’s what this can look like for
What we have shown here is the
nucleus of a nitrogen atom. How would we know it’s a nitrogen
atom? Well, if we count up the protons,
the blue dots in this nucleus, we find one, two, three, four, five, six, and then,
seven, almost hidden away there. Since it’s the number of protons in
the nucleus that determines the atomic element, when we count seven, then we look
that up in the periodic table, and we see that corresponds to the element
And then, if we count the number of
neutrons, those are the green dots, we find that there are one, two, three, four,
five, six, and, again, seven of theses. So, this particular atom of
nitrogen has seven protons, which it must, since it’s nitrogen, and it has seven
neutrons. Now if we add all this up, we find
that the total mass of this nucleus is approximately 14 atomic mass units. One atomic mass unit for each
proton and one for each neutron.
Now if every nitrogen atom in the
world where exactly like this, having seven protons and seven neutrons, then we
would expect the average atomic mass, here, to be 14. But it’s not. Notice that it’s a little bit
bigger than 14. The reason for this is it’s also
possible to have an atom of nitrogen, that is an atom with a nucleus that has seven
protons, but when it comes to neutrons in the nucleus, it has one, two, three, four,
five, six, seven, eight.
So, this atom, which is also a
nitrogen atom because it has seven protons, has eight neutrons. Which means if we add up the masses
of these particles, we find a result of about 15 amu. So, these are two different atoms
of nitrogen. They’re different because they have
different numbers of neutrons. But they’re both atoms of nitrogen
because they both have seven protons. Whenever we find this going on with
any particular element, where there are the same number of protons but a different
number of neutrons in the nucleus, we say that these are isotopes of one
another. In this situation, we might simply
refer to them as nitrogen isotopes.
This word isotopes actually refers
back to the periodic table. The prefix iso means same. And then, topes refers to
place. That is, these two different atoms
of nitrogen are both found in the same place on the periodic table, element number
This idea of isotopes helps us
start to understand this notion of average atomic mass. The reason it’s an average is
sometimes nitrogen has a mass of about 15 amu, while sometimes its mass about 14
amu. By the way, the reason that this
particular number is so close to 14 indicates the relative abundance of this type of
nitrogen atom over this type. In other words, nitrogen atoms with
seven neutrons are much more common than nitrogen atoms with eight neutrons.
Outside of the periodic table, when
we write down an isotope, say as part of a nuclear reaction, we often include three
pieces of information. We write the symbol of the isotope,
in this case the symbol of nitrogen is capital N. We also write the atomic number of
that element in the lower left. That’s seven for nitrogen, the
number of protons in its core. And then, to the upper left to this
symbol, we write what is called the mass number. This is the number of protons plus
the number of neutrons in that nucleus.
So, for the isotope of nitrogen
that has seven neutrons, this would be 14. And we can write that over here by
that particular atom. But for the nitrogen atom that has
eight neutrons in its core, the mass number isn’t 14, but 15. The way that we would refer to
these different isotopes is we would say that the isotope with eight neutrons is
called nitrogen-15, whereas the one with seven neutrons we will call
One reason this concept of isotope
is important is because isotopes of a given element can have different physical and
chemical properties. Take carbon for example. Carbon is an element that has three
common isotopes, carbon-12, carbon-13, and carbon-14.If we were to sketch out the
nuclei of these three isotopes, they would all have six protons. That’s what makes them carbon. But carbon-12 would have six
neutrons. Carbon 13 would have seven. And carbon-14 would have eight. That’s what gives us the mass
numbers of 12, 13, and 14, respectively.
Two of these carbon isotopes,
carbon-12 and carbon-13, are stable, while one, carbon-14, is unstable. It’s fairly radioactive. So, something about the addition of
this extra neutron beyond the neutrons in carbon-13 makes carbon-14 unstable. Now that we know what isotopes are,
let’s get some practice with these ideas through an example.
A platinum atom has the chemical
symbol platinum-195. How many protons does the platinum
atom have? How many neutrons does the platinum
Taking a look at this chemical
symbol, we see that capital Pt is the abbreviation for platinum. In order to answer these questions
about the number of protons and neutrons the platinum atom has, we’ll need to
understand these numbers to the left of this abbreviation.
We see this number to the lower
left 78. This is known as the atomic number
of the element. It tells us the number of protons
in the nucleus. It’s this number that gives the
element its identity and helps us find it on the periodic table. Since the atomic number is the
number of protons in the nucleus, that gives us the answer to our first
question. The answer is 78.
To answer the second question about
the number of neutrons in this atom, we’ll need to look at the number to the top
left. In general, this is referred to as
the mass number of an isotope. Just as the atomic number was equal
to something else, it was equal to the number of protons in the nucleus, so the mass
number represents something else as well. It’s equals to the number of
protons plus the number of neutrons in the nucleus.
In order to find the number of
neutrons in this platinum atom then, we can perform some subtraction. If we take the mass number of this
atom and subtract from it the atomic number, then that will give us the number of
protons plus the number of neutrons minus the number of protons. Altogether then, that will yield
the number of neutrons, the answer to the second part of our question.
We see that the mass number of this
atom is 195. And then, atomic number, the number
of protons, is 78. 195 minus 78 is equal to 117. This then is the number of neutrons
in this platinum atom.
Let’s look now at a second
Which of the following is another
name for atomic number? a) Nucleon number. b) Mass number. c) Electron number. d)
Proton number. e) Neutron number.
Now this name, atomic number, may
ring a bell because whenever we write down the symbol for an atomic isotope, we
include this number. Let’s say that we had an atom of a
made-up element we’ll call element A. When we write down in symbolic form
a particular isotope of this element, it’s normal to write the atomic number at the
lower left of the symbol for it and the mass number at the upper left of the symbol
for this element. We can recall that these two terms
indicate something about this isotope. The atomic number tells us the
number of protons in the nucleus. And the mass number tells us the
number of protons plus the number of neutrons.
By the way, because protons and
neutrons are both elements of the nucleus, sometimes they’re called nucleons. Knowing all this, let’s look at our
answer options once more. Since a nucleon is either a proton
or a neutron, that means nucleon number is the same thing as mass number, the number
of protons and the number of neutrons in the nucleus. We’ve seen that mass number and
atomic number refer to different things. This means that option a, as well
as option b, doesn’t describe atomic number.
Option c talks about electron
number. Those are parts of an atom that
aren’t in the nuclear core. The atomic number doesn’t refer to
electrons, so we’ll cross that choice off as well. Then, we get to option d proton
number. We’ve seen that atomic number is an
indication of this number. And then, looking ahead at option e
neutron number, well, that’s included in the mass number, but it’s not referred to
by atomic number. Option e then is off our list. We find that indeed it’s option d
proton number, which is another name for atomic number.
Let’s take a moment to summarize
what we’ve learnt about isotopes in this lesson. We’ve learnt that isotopes are
atoms with the same number of protons but different numbers of neutrons. For example, we saw that the
element nitrogen has two different isotopes. One isotope has seven neutrons in
its nucleus and the other has eight.
We learned further that isotopes of
an element occupy the same spot on the periodic table of elements. If we were to look up our two
isotopes of nitrogen, we would look for element number seven, that is nitrogen, and
find that both of these isotopes are represented on that same tile. And lastly, isotopes of an element
may have different physical, as well as chemical, properties. For example, of the three isotopes
of carbon that we saw, two were stable, while one was unstable, radioactive.