Alright. So we've discussed isotopes as elements that have the same number of protons but a different number of neutrons. The way we represent these isotopes is through what we call isotope notation. This is just the representation for an isotope and it includes its atomic number, which, remember, is \(Z\), its mass number, which is \(A\), and its element symbol \(X\). Remember, our periodic table of the elements has the display of element symbol \(X\). In this example below, we have calcium, which is represented by Ca. Typically, when we talk about isotope notation we have our mass number presented on top. On the bottom, we typically have our atomic number \(Z\) on the bottom, and then the symbol for the element here. So if we take a look at this question, it's telling us that we have calcium 43 and it represents one of the isotopes for the calcium atom. If the number 43 represents its mass number, determine the correct numbers for its subatomic particles.

Alright. So, in the example question, 43 represents its mass number, which is this number right here. Here its mass number is 43, its atomic number is 20, and the element symbol is Ca, representing calcium. Remember that the atomic number \(Z\) is equal to the number of protons. Here, the atomic number is 20, so that means we have 20 protons. So that means our answer at this point can either be A or D. So B and C are out. Next, let's determine the number of neutrons because that's the next subatomic particle that's shown. We're going to say here that, remember, up above we said that the number of neutrons equals your mass number \(A\), minus your atomic number \(Z\). So if we look at this isotopic symbol for calcium, we see that the mass number is 43 minus 20, so that's going to give us 23 neutrons. We have 23 neutrons. A says we have 43 neutrons, so we know that cannot be the answer. It looks like D is the correct choice.

Now let's talk about the number of electrons. How do we determine the number of electrons? Well, they say the word atom within the question. Remember, if we're talking about the word atom, we're dealing with a neutral element. For a neutral element, the number of electrons equals the number of protons. So, since the number of protons is 20, the number of electrons must also be 20. So the answer here would be D. Remember, with atoms, we're dealing with neutral elements; the number of protons and electrons are equal. If they had mentioned the word ion, then, with the word ion, that would mean that the number of electrons and protons would be different. We haven't quite gotten there, but just remember that we're eventually going to get to a point where the number of electrons and protons will not be the same. In those cases, we will no longer have a neutral element; it'll have a charge.

But for this example, because we're dealing with an atom, let's just focus on the fact that we have the same number of protons and electrons.