When an unstable nucleus emits a neutron, by how much does the atomic number change?
To answer this question, we first need to know what atomic number means. The atomic number simply means the number of protons in a nucleus. So let’s say we’ve got a nucleus with, for example, five protons. We can label these in red. And let’s say that same nucleus has six neutrons in it. We’ll label these in green.
Now this can be our unstable nucleus. It wants to emit a neutron. So it does exactly that. Here’s the neutron. So what’s left over? Well, we had five protons earlier. And the remaining nucleus will also have five protons. The protons aren’t going anywhere.
However, the nucleus will now have one fewer neutrons. That is, it will only have five. So originally, we had a nucleus with five protons and six neutrons. And now we’ve got a nucleus with five protons and five neutrons, as well as this extra little neutron.
What we’re asked to do is to find out how much the atomic number changes by from here to here. And the atomic number is a measure of the number of protons. So the atomic number before the decay is five. And the atomic number after the decay is also five, which means that, during this process, the atomic number changes by exactly zero. And that is our final answer.