The mass defect is the difference between the mass of an atom and the sum of the masses of its constituent particles (protons, neutrons, and electrons). It arises due to the conversion of mass to energy during the formation of the nucleus.
To calculate the mass defect of an atom of 75As, we need to know the number of protons and neutrons in its nucleus. Since arsenic has an atomic number of 33, we know that it has 33 protons in its nucleus. To find the number of neutrons, we subtract the atomic number from the mass number:
Number of neutrons = mass number - atomic number = 75 - 33 = 42
The mass of the protons and neutrons in the nucleus is:
Mass of protons and neutrons = (33 x 1.007825 amu) + (42 x 1.008665 amu)
= 33.358725 amu + 42.36393 amu
= 75.722655 amu
The mass of the atom is given as 74.921597 amu. Therefore, the mass defect is:
Mass defect = 75.722655 amu - 74.921597 amu
= 0.801058 amu/atom
So the mass defect of an atom of 75As is 0.801058 amu/atom.