# Video: Applying Knowledge of Isotopes, Their Atomic Number, and Their Composition

For statements I and II, state for each if they are true or false. I) Two isotopes of the same element differ in their atomic numbers. II) Isotopes of the same element differ in the number of neutrons they have. If both are true, state if II is a correct explanation for I.

03:12

### Video Transcript

For statements I and II, state for each if they are true or false. I) Two isotopes of the same element differ in their atomic numbers. II) Isotopes of the same element differ in the number of neutrons they have. If both are true, state if II is a correct explanation for I.

The atomic number of an element is equal to the number of protons that are in the nucleus of the atom. You can determine the atomic number of an element using the periodic table. For example, the atomic number of carbon is six. Because all atoms of an element have the same number of protons in their nucleus, the atomic number for all atoms of an element will be the same. For example, the atomic number of all carbon atoms is six because all carbon atoms contained six protons in their nucleus. Protons aren’t the only thing in the nucleus of an atom. There’s also neutrons. So each atom also has a mass number, which tells us the total number of protons and neutrons that are in the nucleus.

For example, carbon-12 has a mass number of 12. Since all carbon atoms have six protons, this means that carbon-12 must have six neutrons, since six protons and six neutrons give you a total mass number of 12. Carbon can also occur with the mass number of 14. Like all carbon atoms, carbon-14 contains six protons. Carbon-14 also contains eight neutrons. So while all atoms of an element have the same number of protons, which means that they have the same atomic number, they don’t all have the same mass number. We call atoms of an element that differ in their mass number isotopes. Like carbon-12 and carbon-14, isotopes have different number of neutrons in their nucleus.

Statement I of this question said that two isotopes of the same element differ in their atomic numbers. This statement is false. As we’ve discussed, all atoms of the same element have the same number of protons in their nucleus, which means they have the same atomic number. Isotopes of the same element differ in their mass numbers, not their atomic numbers. Statement II of this question says that isotopes of the same element differ in the number of neutrons that they have. This statement is true. Like we’ve discussed with two of the isotopes of carbon, both had the same number of protons, but carbon-12 has a smaller mass number than carbon-14 because it has fewer neutrons in its nucleus. Since statement one is false, we don’t have to state if two is a correct explanation for one.