Video: Identifying the Statement Incompatible with the Concept of an Isotope

Which statement is inconsistent with the concept of isotopes of the same element? [A] Isotopes have the same number of protons. [B] Isotopes have the same nuclear charge. [C] Isotopes have the same number of neutrons. [D] Isotopes have the same atomic number. [E] Isotopes differ in mass number.

04:20

Video Transcript

Which statement is inconsistent with the concept of isotopes of the same element? A) Isotopes have the same number of protons. B) Isotopes have the same nuclear charge. C) Isotopes have the same number of neutrons. D) Isotopes have the same atomic number. Or E) Isotopes differ in mass number.

An element is a type of atom, like hydrogen, carbon, or fluorine. The type of an atom or simple ion is fixed, based on the atomic number. The atomic number of an atom or a simple ion is simply the number of protons in its nucleus. The atomic number for all atoms or ions of hydrogen is one. So if you have a nucleus containing only one proton, then that’s a hydrogen nucleus. Isotopes are variants of an element, and they’re defined by their mass number. The mass number is the number of protons and neutrons in a nucleus. There are three well-known isotopes of hydrogen: hydrogen-1, hydrogen-2, and hydrogen-3.

The number at the end of the name of an isotope is its mass number. If a nucleus has one proton and no neutrons, it’s a nucleus of hydrogen-1. But if it has one neutron, it’s a nucleus of hydrogen-2 because one plus one equals two. A hydrogen-3 nucleus has a single proton because it’s still hydrogen but it has two neutrons. A helium-3 nucleus has instead two protons and one neutron. But isotopes are only ever isotopes of the same element. So all the types of nucleus with one proton are hydrogen isotopes. And all the types of nucleus with two protons are helium isotopes. Now, let’s have a look at the statements and see which one is inconsistent with the concept of isotopes of the same element.

The question is looking for an inconsistent statement. So we expect four statements that agree with our description of isotopes and one that doesn’t. Statement A is that isotopes have the same number of protons. As we’ve shown, isotopes share atomic number but have different mass numbers. And atomic number is the number of protons in a nucleus. So it’s true. Isotopes have the same number of protons. So option A is not a correct answer.

Statement B is that isotopes have the same nuclear charge. You’re only ever find protons and neutrons in the nucleus. Protons are positively charged and neutrons are neutral. So if a nucleus has the same charge as another nucleus, they have the same number of protons in them. So it’s definitely true that isotopes have the same nuclear charge.

Statement C is that isotopes have the same number of neutrons. We’ve shown this to be false. All the isotopes of hydrogen have the same atomic number, but different mass numbers, meaning that they have different numbers of neutrons in their nuclei. Since this statement is false, it’s inconsistent with the concept of isotopes of the same element. So that’s our correct answer. But let’s check the other two just to be safe.

Statement D isotopes have the same atomic number is pretty much identical to statement A. The atomic number of a nucleus is interchangeable with the number of protons of a nucleus. So statement D is true and is therefore not a correct answer. Statement E is that isotopes differ in mass number. This is exactly what we’ve just discussed. So statement E is true. Statement E is not a correct answer.

And of the five statements, the only one inconsistent with the concept of isotopes of the same element is that isotopes have the same number of neutrons.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.