If a sample has a mass number of 23 and an atomic number of 11, how many neutrons does it have?

If a sample has a mass number of 23 and an atomic number of 11, how many neutrons does it have?

• A. 12
• B. 11
• C. 34
• D. 22

Answer: (A)

Mass number is the total number of protons and neutrons in the nucleus, while atomic number is the number of protons. To find neutrons, subtract the atomic number from the mass number: 23 minus 11 equals 12. So there are 12 neutrons. For context, an atom with 11 protons is sodium, and its common isotope has 11 protons and 12 neutrons, giving a mass number of 23. The other numbers would not keep the same proton count and mass number together.

Mass number is the total number of protons and neutrons in the nucleus, while atomic number is the number of protons. To find neutrons, subtract the atomic number from the mass number: 23 minus 11 equals 12. So there are 12 neutrons. For context, an atom with 11 protons is sodium, and its common isotope has 11 protons and 12 neutrons, giving a mass number of 23. The other numbers would not keep the same proton count and mass number together.