### Video Transcript

The diagram shows the nuclei of the three isotopes of hydrogen. Red circles represent protons and grey circles represent neutrons.

Now let’s go through each one of the questions below and answer them one by one.

Firstly, isotope (b) is called deuterium. How many protons does deuterium have?

Well, we’ve been told earlier in the question that red circles represent protons. And in diagram b, there’s only one red circle. Therefore, deuterium has one proton.

So the next question asks us how many neutrons does deuterium have?

Again, we know that grey circles represent neutrons. And diagram b only has one grey circle. Therefore, deuterium has one neutron.

Now the last question in this little chunk of questions as a little bit more involved. It says which of the following is the correct element symbol of deuterium? And then it lists five different symbols. And we need to figure out which one is the right one.

In order to do this, we need to first know how element symbols are written. And they are written as follows. Let’s say we’ve got an element 𝑥, whatever that 𝑥 maybe. It could be hydrogen, which is H, or could be helium, which is He, so on and so forth. Then we write the element symbol in the following way. We write two numbers to its left, one above and one below. The one above is the mass number. And the one below is the atomic number.

The atomic number simply represents the number of protons in the nucleus of that atom. Coincidentally, that also defines the element that we’re looking at. So the number of protons in the nucleus of an atom defines what element it is. If it’s got one proton, it’s hydrogen. If it’s got two protons, it’s helium, and so on and so forth. That’s why, by the way, in this case we’ve got three diagrams, each with one proton in the nucleus. And because each one only has one proton in the nucleus, they are isotopes of hydrogen. Anyway, so that’s what the atomic number is. The atomic number is the number of protons in the nucleus.

So what’s the mass number? Well, the mass number is the total number of nucleons in the nucleus of the atom. Nucleons are basically protons and neutrons. So we need to add together the total number of protons and neutrons in the nucleus in order to give us the mass number.

What we can do is to label each one of these diagrams such that orange represents the atomic number and pink represents the mass number. So let’s look at diagram a. It’s got one proton in it. Therefore, its atomic number is one. Makes sense, the number of protons is the atomic number. So for the mass number, well, the total number of protons and neutrons is one proton plus zero neutrons which is just one. Hence, diagram a has an atomic number of one and a mass number of one.

Let’s move on to diagram b. Again, the atomic number measures the number of protons which has to be one because we’re looking at hydrogen. And there’s only one proton in the nucleus as we can see. So what’s the mass number? Well, the mass number is the total number of protons and neutrons once again. So that’s one proton plus one neutron. And hence, the mass number is two.

Finally, for diagram c, once again, the atomic number is one because there’s one proton. And the mass number is one proton plus two neutrons. And so, the total mass number is three. And under this knowledge, we can go back to the question we were looking at.

Which of the following is the correct element symbol of deuterium?

Remember that deuterium is diagram b. So we need something that shows an atomic number of one and a mass number of two. Now the atomic number is written at the bottom. So we’ve got all of these that show the correct atomic number. The mass number is written on top. And we need something that shows two. That’s this one. Therefore, that’s the correct element symbol of deuterium.

Moving on, we’re then told that isotope (c) is called tritium. We’re asked how many protons does tritium have. Well, once again, we’re looking at isotopes of hydrogen. So clearly, it can only have one proton. And again, the number of red circles in diagram c is just one. So the answer is one.

The next question asks us; how many neutrons does tritium have?

Well, we can see that there are two grey circles in diagram c. So the answer is two.

And finally, we once again need to find the correct element symbol, this time for tritium.

And once again, we need something that shows an atomic number of one. That’s just these two. And we need something that shows a mass number of three. That’s this one. And hence, this is the correct element symbol of tritium.