Question

Calculate the mass defect in amu for Li-6 with a mass of 6.01348 amu. (Enter your answer with 5 decimal places and remember to use the appropriate sign.)

Answer 1

Mass of Li-6 = 6.01348 amu

Lithium always has 3 protons since its atomic number is 3. Therefore, a lithium atom with neutral charge will have 3 electrons, too.
So, the isotope of lithium (Li-6) has 3 neutrons.

The total mass of protrons in the nucleus = 3 x 1.0072766 amu = 3.02183 amu

The total mass of neutrons in the nucleus = 3 x 1.0086654 amu = 3.02600 amu

Total mass of the nucleons ( protons + neutrons) = 6.04783 amu

Mass defect = Total mass of the nucleons − Mass of the nucleus
​ = 6.04783 amu - 6.01348 amu
   = 0.03435 amu