The atomic mass of an element represents the total number of protons and neutrons in its nucleus. The atomic number, on the other hand, corresponds to the number of protons in an atom. To determine the number of neutrons, you can subtract the atomic number from the atomic mass.
In this case, the atomic mass is 23 and the atomic number is 11. Therefore, the number of neutrons can be calculated as follows:
Number of neutrons = Atomic mass - Atomic number = 23 - 11 = 12
So, an atom with an atomic mass of 23 and an atomic number of 11 contains 12 neutrons.