# Video: Applying Knowledge of the Physical Properties of Isotopes and the Number of Electrons per Atom

For statements I and II, state for each if they are true or false. I) The isotopes of an element have identical physical properties. II) Atoms of isotopes of the same element have the same number of electrons. If both are true, state if II is a correct explanation for I.

02:10

### Video Transcript

For statements 1 and 2, state for each if they are true or false. 1) The isotopes of an element have identical physical properties. 2) Atoms of isotopes of the same element have the same number of electrons. If both are true, state if 2 is a correct explanation for 1.

Isotopes are atoms of an element that have different numbers of neutrons. For example, there are three naturally occurring isotopes of hydrogen, protium, deuterium, and tritium. Because these are all atoms of hydrogen, they all have one proton. But protium has no neutrons, deuterium has one, and tritium has two. Because these atoms of hydrogen only vary by the number of neutrons that they have, it’s tempting to think that they might have identical physical properties.

It’s true that isotopes of an element are largely similar, but they do vary in a couple of ways. Let’s take a look at the masses of these isotopes. Protons and neutrons each have a relative mass of about one. Electrons are considerably lighter with a relative mass of 0.00055. This means that protium will have a mass of about one. Deuterium will have a mass of about two. And tritium will have a mass of about three. This means that the isotope of hydrogen called tritium is three times heavier than protium. So, it’s false that isotopes of an element have identical physical properties.

Statement 2 says that atoms of an isotope have the same number of electrons. Atoms are neutral overall, so they must have the same number of protons as they do electrons. We’ve discussed how each isotope of hydrogen has one proton because it’s an atom of hydrogen. Since these are atoms of hydrogen, they must be neutral overall. So they must have one electron each as well. So, statement 2 is true. Since statement 1 was false, we don’t have to answer the last part of this question.