Which of the following are allotropes? A) NO and NO₂. B) ¹⁴N and ¹⁵N. C) CH₃CO₂K and CH₃CO₂Na. D) CH₃CH₂OCH₃ and CH₃CH₂CH₂OH. Or E) S₂ and S₈.
Allotropes are different structural forms of the same chemical element. For example, there’re two common allotropes of carbon. One is diamond and the other is graphite, which is the substance that’s found in pencil lead. Though both of these substances have incredibly different properties, they’re both entirely composed of carbon atoms. The only difference is that, in graphite, we have each carbon atom attached to three other carbon atoms. And in diamond, we have each carbon atom attached to four other carbon atoms.
So in this question, we need to identify which of our answer choices are allotropes. That is, which of them are different forms of the same element. Of our answer choices, A, C, and D are clearly not different forms of the same chemical element, which leaves B and E. Answer choice B, nitrogen-14 and nitrogen-15, are actually isotopes, which are atoms of the same element that have a different number of neutrons. Allotropes, on the other hand, have to do with how atoms are bonded or arranged. Nitrogen-14 has seven neutrons and nitrogen-15 has eight. But these forms of nitrogen are both atoms of nitrogen. They aren’t different structural forms of nitrogen like we’re looking for.
So that leaves us with answer choice E, S₂ and S₈, or sulphur two and sulphur eight. Sulphur two is a violet gas that’s composed of two sulphur atoms double-bonded together. Sulphur eight, on the other hand, is a yellow solid that’s composed of eight sulphur atoms bonded in a ring. Sulphur eight is the most common and stable allotrope of sulphur. So just like our example with carbon, where both diamond and graphite are composed entirely of carbon atoms, but they have a different structure, the same is true for sulphur two and sulphur eight. So they are allotropes.