In this video, we will learn about
atomic number and mass number and see how they apply to elements, isotopes, atoms,
Ever since we discovered that atoms
existed, it’s been important to know what they were made of. We now know that every atom or
simple ion has a nucleus. And in that nucleus are protons,
which have a positive charge, and neutrons, which are neutral. Collectively, protons and neutrons
are known as nucleons, because they are particles commonly found inside a
Around the nucleus, we find
electrons in the electron cloud. Protons and neutrons have roughly
the same mass. However, electrons have much less
mass. So most of the mass of an atom or
simple ion will be because of the nucleus. So this raises a question. How do we best describe nuclei and
compare one to another?
If you count up the protons in a
nucleus, you’ll get that nucleus’s atomic number. For instance, a nucleus of carbon
contains one, two, three, four, five, six protons. So this nucleus and all nuclei of
carbon have an atomic number of six. However, the atomic number can mean
slightly different things depending on what you’re referring to. At its simplest, atomic number of a
nucleus is simply the number of protons in that nucleus. You might use this in a phrase like
this. A carbon nucleus has an atomic
number of six.
But what if we’re not looking
directly at the nucleus? Instead, we’re looking at an atom
or an ion. If we’re talking about the atomic
number of an atom or a simple ion, then the atomic number is the number of protons
in the nucleus of that atom or ion. Therefore, for a carbon atom, we’d
say a carbon atom has an atomic number of six.
Lastly, how do we talk about atomic
number when we’re dealing with an isotope or an element? The atomic number of an elemental
isotope is simply the number of protons in any nucleus of any atom or ion of that
element or isotope. So you might say that the element
carbon has an atomic number of six. The atomic number is unique to each
element. In some ways, giving the atomic
number of an element is the same as giving the name of the element.
So in practice, if you know the
element, you therefore know the atomic number because you can look it up on the
periodic table. And therefore, you know the number
of protons in the nuclei. For example, if you know that
you’re dealing with the element carbon or you know that you’ve got an atomic number
of six or there are six protons in the nuclei. You can figure out the other parts
of the circle.
Now, so far, we’ve understood
different nuclei based on the number of protons they contain. But they also contain neutrons. How do we account for that? For almost every nucleus, protons
are only a part of the story. About half of the mass of a carbon
atom is because of the neutrons. The mass of our carbon atom is
about the mass of six protons plus the mass of six neutrons. The electrons don’t have enough
mass for us to worry about at the moment.
If you add up the number of protons
and the number of neutrons in a nucleus, you’ll get what’s called the mass
number. This particular atom has six
protons and six neutrons, so it has a mass number of 12. There is something a little
confusing about the mass number. It is not a mass. It’s just a number, like the number
of cats and dogs at your house. So that begs the question, why is
it called mass number at all?
The mass number is called mass
number because it’s very close to the relative atomic mass for that atom or ion. This is usually good enough for a
rough calculation. Uniquely, for this type of carbon
atom, the mass number and the relative atomic mass are identical. And the average atomic mass for
this type of carbon atom is 12 unified atomic mass units.
But let’s see what happens when we
have one more neutron in our carbon atom. The first thing we notice on our
magic scales is that the mass of the atom has increased by about one unified atomic
mass unit. This is roughly the mass of six
protons plus the mass of seven neutrons. But we still have an atom of carbon
because there are six protons. But we have a different mass number
this time. Six plus seven is equal to 13.
So the relative atomic mass of this
particular atom is about 13, and the average atomic mass is about 13 unified atomic
mass units. These variants of carbon are what
we call carbon isotopes. This is why the mass number is
useful. It allows us to distinguish between
isotopes of the same element. We use the mass number in the name
of the isotope because it’s a nice round number that uniquely identifies the
Now, let’s have a look at the
definition of mass number on each level. The mass number of a nucleus is
simply the number of protons plus the number of neutrons in that nucleus. For example, a carbon-12 nucleus
has a mass number of 12. So what about atoms and simple
Mass number of an atom or ion is
the number of protons plus the number of neutrons in the nucleus of that atom or
ion. For example, a carbon-12 atom has a
mass number of 12. Lastly, we need to think about mass
number in the context of isotopes and elements. The mass number of an isotope is
the number of protons and neutrons found in every nucleus of atoms or ions of that
isotope. For example, carbon-12, the isotope
of carbon, has a mass number of 12.
You need to be careful when you use
the term mass number and element because mass numbers apply to isotopes of an
element and not necessarily the element itself. Elements can have more than one
isotope, and therefore there isn’t a distinct mass number for those elements.
Now, let’s look at the most common
scenario where you’ll find atomic number, the periodic table. You can use the periodic table to
find the atomic number of any element. You can also use it to make a good
guess at the mass number of the most common isotope.
We can pick out carbon on the
periodic table. Inside the cell for carbon, you
might see two numbers. The smaller of these is the atomic
number. The bigger number is the average
atomic mass without the units of unified atomic mass units. The average atomic mass is
calculated based on the abundance of the isotopes on Earth. It just so happens that most of the
carbon on Earth is carbon-12. So it works out that the average
atomic mass for carbon is quite close to the mass of the most abundant isotope.
So we could’ve actually worked in
reverse and looked at the average atomic mass and predicted that the most common
isotope for carbon would be carbon-12. However, that doesn’t work for
every element. For instance, bromine is an element
that could be found in group 17 of the periodic table. And its average atomic mass is
79.904 unified atomic mass units. So you might think that the most
abundant isotope of bromine on Earth is bromine-80, which would have 35 protons and
45 neutrons per nucleus.
In actual fact, of the bromine
nuclei on Earth, about 50 percent are bromine-79 and about 50 percent are
bromine-81. And it just so happens that the
average atomic mass averages out at about 80 unified atomic mass units. So just bear in mind that finding
the mass number of the most common isotope using the average atomic mass doesn’t
The last thing we’re going to look
at is how to represent atoms, ions, isotopes, and elements using nuclide
notation. With nuclide notation, we start
with the element’s symbol. We put the mass number, if there is
one, to the top left. And if you want to, you can put the
atomic number to the bottom left.
So let’s imagine we start off with
an element. In this example, our element is
carbon. We look up carbon on the periodic
table. And we see that the element’s
symbol involved is C. That starts off the nuclide
notation for our element. If you wanted to, you could include
the atomic number of the element, six, in the bottom left. However, this isn’t necessary
because you could use the letter C to look up the atomic number on the periodic
table. But what if you had a particular
Let’s imagine we’ve got
carbon-12. We can look up the element carbon
on the periodic table and get the symbol and the atomic number for that isotope. And then we can take the mass
number of that isotope and put it in the top left to uniquely identify it. If you wanted to have the symbol
for an atom of the element, you might just use the element’s symbol on its own or
the element’s symbol with the atomic number.
If you wanted to be really clear,
you could add the number one before the symbol, meaning one carbon atom. And you could do the same thing
with an atom of a specific isotope, with or without the atomic number. For the equivalent ion, all you
need to do is add charge in the usual place, in the top right.
After all that, I think it’s time
for some practice.
Neon has an atomic number of
10. How many protons are there in an
atom of neon?
Neon is the name of an element to
be found on the periodic table of elements. It’s in group 18, otherwise known
as group eight, and period two. An atom is a nucleus surrounded by
an electron cloud. Inside the nucleus, we can find
protons and neutrons. So when the question asks, how many
protons are there in an atom of neon, we’re looking inside the nucleus.
The question also tells us that
neon, the element, has an atomic number of 10. We can see that in its cell on the
periodic table. The other number in the box,
20.180, is the atomic mass of neon. We don’t need that for this
question. The atomic number of an element is
the number of protons we can find in every atom or simple ion of that element. So all neon atoms or ions contain
10 protons in their nuclei. And we can even flip that around
and say all atoms or ions with 10 protons in their nuclei are neon. Therefore, we can say with
confidence that the number of protons in an atom of neon is 10.
Our next question looks at using
both mass number and atomic number.
An atom of calcium has a mass
number of 42 and an atomic number of 20. How many neutrons are there in its
An atom consists of a nucleus
surrounded by an electron cloud. In the nucleus, we can find protons
and neutrons. And electrons occupy the electron
cloud. We won’t need it for this question,
but it’s important to remember that atoms are neutral. So they have the same number of
positive protons and negative electrons. The next thing we need to ask
ourselves is, what do we mean by an atom of calcium?
Calcium is the name of an
element. And we can find information about
calcium on the periodic table of elements. The first thing you might notice is
that the number in the box for calcium is the same as the atomic number of the atom
of calcium. The atomic number is simply the
number of protons in an atom or ion of a particular element. So the first thing we realize is
that we have 20 protons in our atom of calcium. And while it doesn’t make a
difference in this question, we also have 20 electrons.
The other piece of information in
the question is that this atom has a mass number of 42. The mass number is the number of
protons and neutrons in the atom or ion. So this tells us that if we take 20
and add the number of neutrons, we’ll get 42. All this sets us up nicely to
answer the question. How many neutrons are there in the
nucleus of the atom of calcium? The number of neutrons must be 42
minus the number of protons, which is 20. Therefore, the number of neutrons
in an atom of calcium with a mass number of 42 is 22.
And here with the key points, we
have atomic number, the number of protons in the nucleus of an atom or ion of an
element. The mass number is the number of
protons and neutrons in the nucleus of an atom or ion of an isotope of an
element. Elements are identified by atomic
number, and isotopes are identified by atomic number and mass number.