How does isotope mass affect the density of a substance?

The mass of an isotope directly influences the density of a substance through what are known as isotope effects. Generally, for a given spatial distribution of atoms, a substance composed of a heavier isotope will exhibit a higher density than one composed of a lighter isotope.1

This relationship is observable in several chemical contexts:

• Helium: At identical temperature and pressure, a specific volume of the heavier isotope ${}^4\text{He}$ weighs one-third more than the same volume of the lighter ${}^3\text{He}$.1
• Heavy Water: When deuterium is substituted for hydrogen to create heavy water (${}^2{\text{H}}_2\text{O}$), the resulting substance is approximately 10 percent denser than standard water (${\text{H}}_2\text{O}$).1

While mass differences primarily drive these density variations, isotopes may also exhibit differences in physical properties due to their nuclear structure, such as variations in radioactivity, spin, and the spatial distribution of protons.1