While no single person can be credited with discovering this fact, the concept of the atom's mass being concentrated in the nucleus was developed through the work of many scientists, including:

* Ernest Rutherford: His gold foil experiment in 1911 provided strong evidence that the atom had a small, dense, positively charged core (the nucleus). This led to the understanding that the nucleus contained most of the atom's mass.

* James Chadwick: He discovered the neutron in 1932. This discovery helped to complete the picture of the nucleus, showing that it contained both protons and neutrons, which accounted for almost all of the atom's mass.

So, it's more accurate to say that the understanding of the nucleus containing most of the atom's mass was a result of collective scientific progress rather than a single person's statement.