Video Transcript
Beryllium-15 is an isotope of beryllium that is highly unstable. It does not occur naturally, but it can be created synthetically. Beryllium-15 decays via neutron emission. This is shown on the graph. What isotope does beryllium-15 decay to?
Okay, so on this graph, we can see that we’ve got the number of protons on the horizontal axis and the number of neutrons on the vertical axis. Now initially, we start out at this point over here. And then we finish at this point over here.
Now initially before decay, we can see that the beryllium has 11 neutrons. And we can see that it has four protons. And then after decay, we can see that it still has four protons. But now it has 10 neutrons. Therefore, in this process, our isotope of beryllium-15 has now lost a neutron. So it’s no longer beryllium-15. But what is it?
Well, we can see that in the process, the number of protons have not changed. And it’s the number of protons in a nucleus that define what element we’re talking about. So if the number of protons have not changed, then it must be the case that we’re still looking at beryllium. It’s not changed to another element. And this is even hinted to us on the graph.
We see that we’ve got Be, the symbol for beryllium, before the decay. And after the decay, we still have Be. So we’ve been given a clue that we’re still looking at beryllium. But it can no longer be beryllium-15. So what is it?
Well, the 15 in beryllium-15 refers to the sum of the number of protons and neutrons in the nucleus of the atom. In other words, before the decay, it had four protons as we’ve said already. And it had 11 neutrons. 11 plus four is equal to 15. That’s where the 15 comes from.
But then after the reaction, the beryllium still has four protons but only 10 neutrons. So this time, instead of 11 plus four, we’re now going to have 10 plus four, which is equal to 14. In other words, we’ve still got beryllium. But it’s now lost a neutron, which makes sense because we’ve been told that it decays via neutron emission.
In other words, we initially have an atom of beryllium which has 11 neutrons and four protons. And then one of those neutrons is emitted from the nucleus. So it’s no longer in the nucleus. Hence, we’ve only got 10 neutrons left. And nothing happens to the number of protons.
So anyway after the reaction, there are now 10 neutrons and four protons. When we add them together by the way, this is known as the mass number. And the mass number of our product of this decay is 14 now. So as we said already, it’s still beryllium. And it has a mass number of 14.
Therefore, we can say that the isotope that the beryllium-15 decays to is beryllium-14.
