Isotopes - Online Tutor, Practice Problems & Exam Prep

Isotopes are variations of elements with the same atomic number (number of protons) but different mass numbers (total of protons and neutrons). The atomic number, denoted as z, identifies the element, while the mass number, a, is calculated as a=z+n, where n is the number of neutrons. In a neutral atom, the number of protons equals the number of electrons, ensuring electrical neutrality. Understanding these concepts is crucial for exploring chemical properties and reactions.

Isotopes

Isotopes are elements with the same number of protons but a different number of neutrons. Now, with this whole idea of isotopes, we're going to be introduced to some new terms. The first term is "atomic number," which uses the variable z. It provides the number of protons. So basically, if I have an unknown element, but I know its atomic number, I can go to the periodic table, look up that atomic number, and it will give me the identity of that unknown element. Because of this, we're going to say it helps us determine the identity and chemical properties of an element. The whole aspect of chemical properties of an element, we'll discuss much later on in later chapters.

Besides atomic number, we have "mass number," which uses the variable a. This one provides the number of protons and neutrons. Now, we're going to say to calculate the number of neutrons, we can just take the mass number, which is a, and it's subtracted by the atomic number, which is z. That'll tell us the number of neutrons for any given isotope or element.

If we take an atomic view of an atom, remember, an atom is composed of four primary parts. We have our nucleus, which houses the neutrons and protons, and we have our electrons, which spin around the nucleus. If we take a look here at this example, we know that our neutrons, colored as red, have 6 for this particular atom. Protons, which we show as blue, we have 5 in this example. The electrons are the green spheres that are rotating or orbiting the nucleus. We have 5 electrons. Remember, mass number, which we said is the number of protons (5) plus the number of neutrons (6) when added together, gives us the mass number, which comes out to be 11. So just remember, you have blue and red mixed together. It gives us the color purple. So, we're using this purple a to designate our mass number for this particular atom.

We also said earlier that the atomic number gives us the number of protons. So, if you know the number of protons, you know your atomic number is 5, and because we know the atomic number is 5, we know the element's identity. Here, we don't show an example of a periodic table. We'll go into a greater discussion on it later on. But if we were to look at a periodic table and look for the atomic number 5, we'd see that it belongs to the element of boron. So, this atom is an illustration of the element boron.

Speaking of this atom, we're going to say, for a neutral element, which is oftentimes called an atom, the number of protons and electrons are equal. This makes sense because, remember, protons are positively charged, electrons are negatively charged. If you have an equal number of positive and negative, they're going to cancel one another out, and you'll be left with a neutral element. So just remember, when we're dealing with a neutral element, that's another way of saying atom. In an atom, the number of protons and electrons are equal. Now later on, we'll talk about other species called ions. In ions, the number of protons and electrons are not equal. But for right now, just realize that they're talking about the words neutral element or atom, which means we're dealing with a species or an element that has the same number of protons and electrons.

Now that we've looked at the atomic view of the atom and incorporated these new terms of mass number and atomic number, let's continue our discussion and exploration of the atom itself.

Isotopes Example 1

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.

Which of the following answers give the correct number of subatomic particles for ?

• A. 38 protons, 38 neutrons and 38 electrons
• B. 88 protons, 50 neutrons and 50 electrons
• C. 38 protons, 50 neutrons and 38 electrons
• D. 38 protons, 27 neutrons and 13 electrons

Which of the following answers give the correct number of subatomic particles for ?

• A. 34 protons, 34 neutrons and 34 electrons
• B. 50 protons, 18 neutrons and 18 electrons
• C. 16 protons, 18 neutrons and 16 electrons
• D. 17 protons, 17 neutrons and 17 electrons

Fill in the gaps for the following table of atoms.

So here in this practice question, it says, fill in the gaps for the following table of atoms. Before we even start, because it says the word "atoms," that means we're dealing with neutral elements, and with neutral elements, that means that the number of protons equals the number of electrons. Now if we take a look here, we have our isotope name, as well as the number associated with it. When we talk about our mass number, remember that it is just represented by the variable a. The atomic number is represented by the variable z. The number of neutrons. Remember, the number of neutrons equals your mass number minus your atomic number. For protons, we can say that the number of protons equals your atomic number, or, in this case, because we're dealing with atoms, we can also say the number of protons equals the number of electrons. And then finally, with electrons, because we're dealing with atoms, that means the number of protons is equal to the number of electrons. Let's start out with the first one. In the first one, we have zirconium 94. Zirconium represents the element's name, and remember the number after it would represent its mass number. So here our mass number is 94. Next, the atomic number is z, but remember z is equal to the number of protons. Because we know we have 40 protons in this example, our atomic number has to be 40. And because we know the number of protons, we know the number of electrons is also 40. So this would be the filled-in row for zirconium 94. Next, we have atomic number, which is 33, and we have neutrons, which is 42. Well, let's see. We can say here that the atomic number tells us the number of protons, which is also 33. Because we're dealing with atoms, the number of protons and electrons are the same, so this would also be 33. Then we're going to say, remember your mass number, the mass number tells us the number of protons plus neutrons together. So, add up protons, add up neutrons together, that total there gives us 75. With 75, that means we're dealing with arsenic 75. Next, we have aluminum 27. 27, of course, is the mass number. The atomic number is equal to the number of protons. The number of protons is 13, so the atomic number is 13. Because we have an atom, the number of protons and electrons are the same. And then finally remember, the number of neutrons equals mass number minus atomic number, so that'd be 27 minus 13, so that'd be 14 neutrons. And then finally, we have the last row. So we have a mass number of 59. We know the isotope element is 59. Its atomic number is 27. 27 gives us a number of protons, and because it's neutral, it also has 27 electrons. The number of neutrons, again, is equal to mass number minus atomic number, so that's going to equal 59 minus 27, which is 32. Then finally, we need to determine the element name. If you look on the periodic table and you find the atomic number of 27, you'd see the only element that's connected to that atomic number of 27 would have to be cobalt. So this last isotope would be cobalt 59. Just remember that our atomic view of the atom is composed of 4 parts. We have our nucleus, which houses the protons and neutrons, and then spinning around it are the electrons. Connected to these ideas are your atomic number and mass number. Knowing that we're dealing with these subatomic particles, remember the relationships they share with one another. When we're dealing with atoms, we're dealing with an equal number of protons and electrons. And remember that the mass number itself is connected to the number of protons and neutrons. Knowing and understanding these connections is a way of identifying correctly the type of isotope that you're dealing with. Now that we've taken a look at isotopes, we'll continue on to further discussions of elements of the periodic table.

Here’s what students ask on this topic:

What are isotopes and how do they differ from each other?

Isotopes are variations of the same element that have the same number of protons (atomic number, Z) but different numbers of neutrons, resulting in different mass numbers (A). The mass number is calculated as the sum of protons and neutrons: $A=Z+n$. For example, Carbon-12 and Carbon-14 are isotopes of carbon; both have 6 protons, but Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons. This difference in neutron number leads to variations in their physical properties and stability, but they share the same chemical properties.

How do you calculate the number of neutrons in an isotope?

To calculate the number of neutrons in an isotope, you subtract the atomic number (Z) from the mass number (A). The formula is: $n=A-Z$. For example, if an isotope has a mass number of 14 and an atomic number of 6, the number of neutrons is: $14-6=8$. Therefore, this isotope has 8 neutrons.

What is the significance of the atomic number in identifying an element?

The atomic number (Z) is crucial for identifying an element because it represents the number of protons in the nucleus of an atom. Each element has a unique atomic number, which determines its position on the periodic table and its chemical properties. For instance, an atomic number of 5 corresponds to the element boron. Knowing the atomic number allows you to identify the element and predict its behavior in chemical reactions.

Why do isotopes of the same element have similar chemical properties?

Isotopes of the same element have similar chemical properties because they have the same number of protons and electrons. The chemical behavior of an element is primarily determined by its electron configuration, which is dictated by the number of protons (atomic number). Since isotopes have identical atomic numbers, their electron configurations are the same, leading to similar chemical properties. Differences in neutron numbers do not significantly affect chemical behavior.

What is the difference between a neutral atom and an ion?

A neutral atom has an equal number of protons and electrons, resulting in no overall charge. In contrast, an ion is an atom or molecule that has gained or lost one or more electrons, giving it a net positive or negative charge. For example, if an atom loses an electron, it becomes a positively charged ion (cation), and if it gains an electron, it becomes a negatively charged ion (anion). The number of protons remains unchanged in both cases.