Lesson Video: Isotopes Physics • 9th Grade

In this lesson, we will learn how to work out the number of protons, neutrons, and electrons an atom of an isotope has from its chemical symbol.

12:02

Video Transcript

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 particular element.

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 atom.

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 nitrogen.

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 nitrogen.

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 seven.

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 nitrogen-14.

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 atom have?

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 example.

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.

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