Final answer:
The sodium atom 23Na11 has 12 neutrons, which is determined by subtracting the atomic number (11) from the atomic mass number (23). The correct option is A.
Step-by-step explanation:
The mass of the atom 23Na11 is 22.9898 amu. To determine the number of neutrons in this sodium atom, we subtract the atomic number (the number of protons, which is 11 for sodium) from its atomic mass (rounded to the nearest whole number).
The atomic mass approximately equals the sum of protons and neutrons, known as the mass number. In this case, 23 (the mass number) minus 11 (the number of protons) equals 12, which represents the number of neutrons. Therefore, the correct answer is (a) 12 neutrons.