Question Video: Understanding the Properties of Atoms Physics

Video Transcript

If an atom has four protons, three neutrons, and four electrons, what is the relative charge of the atom?

Okay. So a relatively short question, but let’s underline all the important bits anyway, just to make sure we don’t miss anything out. We’ve got an atom which has four protons, three neutrons, and four electrons. We need to find the atom’s relative charge. In order to do that, it would probably be quite useful for us to actually know what relative charge means. So the relative charge of a particle is defined as the charge of that particle divided by the charge of a proton. In other words, we’re measuring the charge of that particle relative to the charge of a proton. And that’s why it’s called relative charge. The fact that we’re measuring the charge relative to that of a proton is just something that we have to remember.

So now that we know about relative charge, this is gonna be our game plan. Since we know that the atom is formed of protons, neutrons, and electrons, we’re first going to find the relative charges of each one of those things. In other words, we’ll find the relative charge of a proton, the relative charge of a neutron, and the relative charge of an electron. Then we can combine all of these relative charges to give us the relative charge of the atom.

So let’s start by finding the relative charge of a proton. To do this, we need to know what the actual charge of a proton is. Well, we know that the standard unit of charge is a coulomb. And in coulombs, the charge of a proton is 1.6 times 10 to the power of negative 19 coulombs. But remember, we’re finding the relative charge of a proton. So obviously, in the numerator, the charge of the particle is also going to be 1.6 times 10 to the power of negative 19 coulombs. And so we know that this fraction ends up being one. And more specifically, we’ll call this positive one for now. So the relative charge of a proton is positive one. Doesn’t that make a lot of sense? The charge of a proton relative to the charge of a proton is positive one. Who would’ve guessed?

Anyway, let’s move on to another particle. Let’s work out the relative charge of a neutron. To do this, we need to divide the charge of the neutron in the numerator, which we’ll fill in in a second, by the charge of a proton because, once again, we’re measuring the charges relative to the charge of a proton. So this denominator always stays the same. And now we can fill in the numerator. We need to fill the numerator in with the charge of a neutron. But, it’s a neutron. It’s a neutral particle. So it has a charge of zero. And since we know that zero divided by anything is still zero, the relative charge of a neutron is zero. And so we can put that into our little table. And we can move on to finding the relative charge of an electron.

Once again, we’ve got the charge of a proton in the denominator. And we need to put in the charge of the electron in the numerator. In coulombs, this charge happens to be negative 1.6 times 10 to the power of negative 19 coulombs. In other words, an electron has the same sized charges of proton but the polarity is opposite. It’s a negative charge. Whereas the proton is a positive charge. So the relative charge of an electron turns out to be negative one. In other words, relative to the charge of a proton, the electron has a charge of negative one. And we put that into our little table as well.

So we’ve now found the relative charges of all the three types of particle that make up the atom in our question. Which means that we can move on to finding the relative charge of the atom itself. The relative charge of our atom is simply going to be the combination of relative charges of the protons, neutrons, and electrons. Because, let’s say for example we had an atom that was made up of just two protons. This is basically just a helium nucleus without its electrons or its neutrons. But that, at this point in time, is not relevant.

Let’s say we’ve got an atom with two protons in it. Well, then the relative charge of that atom is the charge of the atom relative to that of one proton. And so the actual charge of that atom would be the charge of two protons, one and two. In other words, it would be 1.6 times 10 to the power of negative 19, the charge of one proton, plus the same again because we’ve got two protons. And that happens to be 3.2 times 10 to the power of negative 19 coulombs. That’s the actual charge on the atom. So the atom’s relative charge is that charge divided by the charge of a proton, 1.6 times 10 to the power of negative 19 coulombs. And this just ends up being two, as we would expect, because two protons have a relative charge of positive two.

But now, let’s say that we still have this atom. But now we’ve got one electron orbiting this nucleus. Well then, the relative charge of this atom changes. The charge of the electron cancels out the charge on one of the protons. And obviously, this doesn’t mean that the proton and electron are disappearing. It’s just that the charges cancel out. And the total remaining charge is just that of one proton. And so the relative charge of this atom now, the two protons and the electron together, is one. Or, if we wanna be more specific, positive one.

But now let’s add a neutron to the mix. Let’s say there’s a neutron in the nucleus of the atom. Well, once again, one of the proton charges cancels out the charge on the electron. And so the resulting charge, the overall remaining charge, is that of one proton and one neutron. But the neutron has a relative charge of zero. So the relative charge of that overall atom is still positive one. And this is an important point to note. The number of neutrons in the atom makes no difference at all to the relative charge of the atom.

Therefore, coming back to our question, we’ve got an atom that consists of four protons, three neutrons, and four electrons. The protons and neutrons are in the nucleus at the center of the atom. And the electrons are orbiting that nucleus. But regardless, this is our atom. Now, we’ve just mentioned that the number of neutrons has nothing to do with the overall relative charge of the atom. So let’s start cancelling protons with electrons. Since we’ve got four protons and four electrons, each one of them cancels out the other. And so our atom has an overall relative charge of zero. We can also calculate this in the following way: the relative charge of the atom is equal to the total relative charge of the four protons plus that of the three neutrons plus that of the four electrons. And this ends up being four plus zero plus negative four which is once again zero.

So our final answer is that the atom which has four protons, three neutrons, and four electrons has a relative charge of zero.