Packing fraction

Aston's mass spectrograph determined accurate masses of isotopes, revealing slight discrepancies from integers. These were explained by the way that subatomic particles were arranged in a nucleus. If one kind of nucleus was converted into a more tickly packed one, mass would be lost and large amounts of energy produced.
 Francis William Aston England 1925 Packing fraction

Aston's mass spectrograph determined accurate masses of isotopes, revealing slight discrepancies from integers. These were explained by the way that subatomic particles were arranged in a nucleus. If one kind of nucleus was converted into a more tickly packed one, mass would be lost and large amounts of energy produced.