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.