The number of neutrons that are in the atom of the element mentioned in the problem is ten (10). The proton number should be equal to the atomic number, then, the mass of the automic is equal to the sum of neutrons and protons. Therefore the answer is 10.
The number of neutrons in the atom of the element with atomic number 10 and an atomic mass of 20 is 10. The number of neutrons can be calculated if the atomic number is subtracted from the atomic mass. Therefore, the number of neutrons = 20 - 10 = 10.
