Video: Identifying the Pair of Isotopes in a Set of Pairs of Chemicals

Which of the following are isotopes? [A] NO and NO₂ [B] ¹⁴N and ¹⁵N [C] CH₃CO₂K and CH₃CO₂NA [D] CH₃CH₂OCH₃ and CH₃CH₂CH₂OH [E] S₂ and S₈

03:28

Video Transcript

Which of the following are isotopes? A) NO and NO₂, B) ¹⁴N and ¹⁵N, C) CH₃CO₂K and CH₃CO₂NA, D) CH₃CH₂OCH₃ and CH₃CH₂CH₂OH, or E) S₂ and S₈.

Isotopes are atoms of the same element that have different numbers of neutrons. For example, there are three naturally occurring isotopes of hydrogen: protium, deuterium, and tritium. All three of these isotopes have one proton in their nucleus. If they didn’t have one proton, then it wouldn’t be a hydrogen atom. But they differ in the number of neutrons that they have. Protium, has no neutrons in its nucleus. Deuterium has one. And tritium has two.

We have a special notation to distinguish different isotopes of the same element. This notation consists of the chemical symbol for the element with two numbers on the left-hand side. The bottom number is the atomic number for the element, which is always equal to the number of protons in the nucleus. The top number is called the mass number, which is equal to the number of protons plus the number of neutrons that are in the nucleus.

Sometimes you’ll see this notation with just the top number. That is the mass number written as the atomic number is implied from the chemical symbol for the element. So, using this notation, protium would be H with two ones because there’s only one proton in the nucleus and no neutron. Deuterium’s mass number is two because it has one proton and one neutron in the nucleus. Finally, tritium has a mass number of three because it has one proton and two neutrons, which is reflected in the mass number in this notation. Again, the atomic numbers for all of these isotopes is one because each of them has one proton in the nucleus because they’re all hydrogen.

So, now we want to determine which of our answer choices are isotopes. We’ll be looking for two atoms of the same element that have different numbers of neutrons, which will most likely be indicated using the notation that we’ve discussed. The only answer choice that matches this is answer choice B, which is nitrogen-14 and nitrogen-15, two different isotopes of the element nitrogen. Since nitrogen’s atomic number is seven, which means it has seven protons in its nucleus, nitrogen-14 must have seven neutrons and nitrogen-15 must have eight neutrons in their nucleus.

If you’re curious, nitrogen-14 makes up 99.63 percent of the Earth’s total nitrogen, and nitrogen-15 makes up the remaining 0.37 percent. All other isotopes of nitrogen are not stable. So, of our list of chemical species, answer choice B or nitrogen-14 and nitrogen-15 are isotopes.

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