Neutrons = Mass Number - Atomic Number

Steps:

• Identify A as the mass number (total protons + neutrons).
• Identify Z as the atomic number (number of protons).
• Subtract protons from total nucleons to find neutrons: A - Z.
• For isotope X, this formula applies regardless of specific element.

Why B is correct:

• The number of neutrons is defined as A - Z, where A is mass number and Z is atomic number.

Why the others are wrong:

• A: Mass number includes protons and neutrons, not neutrons alone.
• C: Z - A yields a negative value, impossible for particle count.
• D: Z represents protons, not neutrons.

Final answer: B