What is the approximate thickness of lead-208's neutron skin?

• A. Approximately 0.28 nanometers.
• B. Approximately 2.8 picometers.
• C. Approximately 28 femtometers.
• D. Approximately 0.28 trillionths of a millimeter.

Answer: (D)

The distance scales inside a nucleus are incredibly tiny, with typical nuclear sizes measured in femtometers. The neutron skin thickness of a heavy nucleus like lead-208 is a small extra margin—on the order of a few tenths of a femtometer, not something spanning nanometers or picometers.

Among the options, the one that aligns with a few tenths of a femtometer is the length described as 0.28 trillionths of a millimeter. Converting that to meters: 0.28 × 10^-12 mm equals 0.28 × 10^-15 m, which is 0.28 fm. That matches the expected scale for a neutron skin.

The other choices are off by many orders of magnitude: nanometers or picometers are far larger than a few tenths of a femtometer, and 28 fm would imply a skin much larger than the nucleus itself.