Question

What is the mass defect of lithium-7? Assume the following: Atomic number of lithium = 3 Atomic mass of lithium = 7.016003 atomic mass units. Mass of 1 proton = 1.007276 atomic mass units. Mass of 1 neutron = 1.008665 atomic mass units

Answer 1

Mass defect is the difference of the mass of the constituent particles (protons and neutrons) of the actual mass of the nucleus. The mass of a nucleus is actually less than the mass of the protons and neutrons.

This is how you calculate it for this problem.

lithium - 7 => 3 protons and 4 neutrons

mass of 3 protons = 3 * 1.007276 amu = 3.021828 amu

mass of 4 neutrons = 4 * 1.008665 amu = 4.03466

=> mass of 3 protons + 4 neutrons = 3.021828 amu + 4.03466 amu = 7.056488

Mass defect = mass of 3 protons and 4 neutrons - atomic mass of lithium =

Mass defect = 7.056488 amu - 7.016003 amu = 0.040485 amu