Q 2.28: The unit of length convenient on the nuclear scale is a fermi : . Nuclear sizes obey roughly the following empirical relation :
where r is the radius of the nucleus, A its mass number, and
is a constant equal to about, 1.2 f. Show that the rule implies that nuclear mass density is nearly constant for different nuclei. Estimate the mass density of sodium nucleus. Compare it with the average mass density of a sodium atom obtained in Exercise. 2.27.
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The equation for the radius of the nucleus is given by,
The volume of the nucleus using the above relation,
We know,
Mass = Mass number× Mass of single Nucleus
=
(given)
The derived density formula contains only one variable,
and is independent of mass number A. Since 
is constant, hence nuclear mass density is nearly constant for different nuclei.
∴ The density of the sodium atom nucleus =
Comparing it with the average mass density of a sodium atom obtained in Q 2.27. (Density of the order
)
Nuclear density is typically
times the atomic density of matter!