Final answer:
The untrue statement about isotopes is that their physical properties are similar; while their chemical properties are indeed similar due to the same electron arrangement, the physical properties can differ because of variations in mass.
Step-by-step explanation:
The statement that is untrue with regard to isotopes is 'D. An element's isotopes have similar chemical and physical properties'. Although isotopes of an element have the same number of protons and therefore similar chemical properties, their physical properties can vary significantly due to the difference in mass resulting from the varying number of neutrons.
Isotopes: Differing Numbers of Neutrons
Isotopes are atoms of the same element that have the same number of protons (and hence, the same atomic number) but a different number of neutrons. Consequently, isotopes have different mass numbers. The chemical properties of isotopes are similar because they have the same number of protons and electrons. However, the physical properties, such as melting and boiling points, can vary because of the differences in mass.
Radioisotopes, on the other hand, have unstable nuclei that release radiation. These are a specific type of isotope that can change into a different element through radioactive decay. An example of a radioisotope is Carbon-14 which decays over time, eventually transforming into Nitrogen-14.