Nuclear sizes are usually expressed in which unit?

The dimensions of atomic nuclei are incredibly small, far beyond what can be measured in conventional units like meters or centimeters. To deal with these tiny scales, physicists use the unit fermi, also known as the femtometer (fm).

One fermi is equal to 10⁻¹⁵ meters, which makes it an ideal unit for expressing nuclear sizes. For example, a typical atomic nucleus has a radius of about 1 to 10 fermi, depending on the number of protons and neutrons it contains.

The unit is named after the Italian physicist Enrico Fermi, who made pioneering contributions to nuclear physics and quantum mechanics. His work in nuclear reactions and neutron scattering helped establish the modern understanding of nuclear structure.

Let’s examine the options:

• 
  

  Tesla (A): A unit of magnetic field strength, unrelated to size measurement.

  
  


• 
  

  Force (B): Not a unit of length, but a physical quantity measured in newtons.

  
  


• 
  

  Fermi (C): Correct answer, a unit equal to 10⁻¹⁵ meters, used for nuclear dimensions.

  
  


• 
  

  Newton (D): A unit of force, not suitable for measuring size.

  
  

Using femtometers (fermi) makes calculations in nuclear physics more practical. For instance:

• 
  

  The radius of a proton is about 0.8 femtometers.

  
  


• 
  

  The radius of a heavy nucleus like uranium can be around 7 femtometers.

  
  


• 
  

  The relationship between nuclear size and mass number (A) can be approximated by the formula:
  R=R0A1/3R = R₀ A^{1/3}R=R0A1/3
  where R0≈1.2R₀ ≈ 1.2R0≈1.2 femtometers.

  
  

This shows that the nuclear radius increases slowly with the cube root of the mass number.

In summary, nuclear sizes are always expressed in fermi (femtometers) because it is the most appropriate unit for describing the extremely small scale of nuclei. Larger units would make the numbers impractically small, while smaller units like angstroms (10⁻¹⁰ m) are too large for nuclear dimensions.

Thus, the correct answer is Fermi (10⁻¹⁵ m).