Let us define first the unit of fm which means femtometer. It is equivalent to 10⁻¹⁵ of a meter. If you convert this to centimeter, the conversion would be 1 fm = 10⁻¹³ cm. Let's solve for the volume of the sphere of the nucleus:
V = 1/4*πd² = 1/4*π*[2 fm*(10⁻¹³ cm/1 fm)]²
V = 3.1416×10⁻²⁶ cm³
The mass of the nucleus should be known to determine the density. According to literature, the mass of a Helium nucleus is 6.64 × 10⁻²⁴ g.
Density = 6.64 × 10⁻²⁴ g/3.1416×10⁻²⁶ cm³
Density = 211.36 g/cm³
