What is true about the densities of atomic nuclei?

• A. All nuclei have nearly the same density.
• B. Nuclei are denser as mass increases.
• C. Density varies significantly between different nuclei.
• D. Density depends on electric charge only.

Answer: (A)

Nuclear matter has a nearly constant density because the nucleus grows in size roughly as radius ∝ A⅓, so its volume ∝ A while its mass ∝ A. The density is mass divided by volume, which cancels the A dependence and leaves a nearly same value for all nuclei. In numbers, this common nuclear density is about 2.3 × 10^17 kg/m^3 (roughly 2 × 10^14 g/cm^3). That means both light and heavy nuclei share a similar density.

This also helps explain why the other ideas don’t fit: density doesn’t rise with mass since larger nuclei just have proportionally larger volume; there isn’t large variation in density from one nucleus to another; and density isn’t set by electric charge alone because neutrons contribute to mass without adding charge, while the strong force sets the typical spacing and packing of nucleons.