All right, so we've discussed isotopes being elements that have the same number of protons but different number of neutrons. The way we represent these isotopes is through what we call isotope notation. Now this is just the representation for an isotope and it includes its atomic number, which remember is Z, it's mass number which is A, and now it's element symbol X. Remember our periodic table of the elements has the display of all the elements that we know of with their elemental symbols. For example, carbon is represented by the element symbol C.
In this example down below we have Calcium which is represented by CA. Typically when we talk about isotope notation, we have our mass number presented here 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.
All right, so we're going to say Calcium 43. Again, I say it within the example question, 43 represents its mass number. Which is this number right here? Here it's mass number is 43. It's atomic number is 20, and the element symbol is CA, representing Calcium. All right, 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&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, which is A minus your atomic number, which is Z. So if we look at this isotopic symbol for calcium, we see that the mass number is 43 - 20. So that's going to give us 23 neutrons. So we have 23 neutrons. A says we have 43 neutrons. We know that cannot be the answer, so 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 the neutral element. For neutral element, the number of electrons equals the number of protons. Since my number of protons is 20, my number of electrons also must be 20. So the answer here would be D.
Now remember, with atoms we're dealing with neutral elements. Number of protons and electrons are equal. If they had said 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. OK, but for this example, because we're dealing with atom, let's just focus on the fact that we have the same number of protons and electrons.