Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses.
From the options provided:
A. Uranium-235 and uranium-238 - These are isotopes of uranium because they have the same number of protons (92) but different numbers of neutrons (143 and 146, respectively).
B. Uranium-235 and plutonium-239 - These are not isotopes because they are different elements (uranium and plutonium).
C. Sulfur-32 and phosphorus-32 - These are not isotopes because sulfur and phosphorus are different elements.
D. Hydrogen-1 and helium-4 - These are not isotopes because they are different elements (hydrogen and helium).
The correct answer is A. Uranium-235 and uranium-238.
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