Cracking the SAT Physics Subject Test

Binding Energy

The masses of the proton and neutron are listed below.

proton: mp = 1.6726 × 10−27kg

neutron: mn = 1.6749 × 10−27kg

Because these masses are so tiny, a much smaller mass unit is used. With the most
abundant isotope of carbon (carbon-12) as a reference, the atomic mass unit

(abbreviated amu or simply u) is defined as 1/12 the mass of a^12 C atom. The

conversion between kg and u is 1 u = 1.6605 × 10−27 kg. In terms of atomic mass
units

proton: mp = 1.00728 u

neutron: mn = 1.00867 u

Now consider the deuteron, the nucleus of deuterium, an isotope of hydrogen that
contains 1 proton and 1 neutron. The mass of a deuteron is 2.01356 u, which is a
little less than the sum of the individual masses of the proton and neutron. The
difference between the mass of any bound nucleus and the sum of the masses of its
constituent nucleons is called the mass defect, ∆m. In the case of the deuteron
(symbolized d), the mass defect is