**Core Concept**
Isotopes are atoms of the same chemical element that have the same number of protons in their atomic nuclei but differ in the number of neutrons. This variation in neutron number affects the atomic mass of the isotopes but not their chemical properties.

**Why the Correct Answer is Right**
Isotopes are characterized by their atomic mass, which is the sum of protons and neutrons in the nucleus. Since isotopes of the same element have the same number of protons, their chemical behavior remains unchanged, but their physical properties, such as mass and density, can differ significantly. For example, carbon-12, carbon-13, and carbon-14 are isotopes of carbon, with 6 protons and varying numbers of neutrons.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because it doesn't provide a definition related to isotopes. Isotopes are not defined by their location or mobility in a solution.
* **Option B:** This option is incorrect because it refers to a different concept in chemistry. Isomers are molecules with the same molecular formula but a different structural arrangement of atoms.
* **Option C:** This option is not a definition of isotopes and may be related to a different concept in chemistry or physics.

**Clinical Pearl / High-Yield Fact**
For exam purposes, remember that isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This concept is essential in understanding radioactivity and nuclear reactions.

**Correct Answer: D. Atoms of the same chemical element with the same number of protons but different numbers of neutrons.**