The atomic number of an element represents the number of protons in the nucleus of an atom. In the case of calcium-39, the atomic number is given as 20.
The mass number of an atom represents the sum of its protons and neutrons. In this case, the mass number of calcium-39 is given as 39.
To determine the number of neutrons in an atom, we subtract the atomic number from the mass number.
Therefore, the number of neutrons in one atom of calcium-39 can be calculated as follows:
39 (mass number) - 20 (atomic number) = 19 neutrons
So, the correct answer is option b. There are 19 neutrons in one atom of calcium-39.