What is the total number of neutrons in an atom of potassium-42 (K-42)?

• A. 21
• B. 19
• C. 42
• D. 23

Answer: (A)

Verified step by step guidance

1. Identify the atomic number (Z) of potassium (K), which is the number of protons in the atom. For potassium, Z = 19.
2. Recognize that the number 42 in potassium-42 (K-42) is the mass number (A), which is the total number of protons and neutrons in the nucleus.
3. Use the relationship between mass number, protons, and neutrons: \(A = Z + N\), where \(N\) is the number of neutrons.
4. Rearrange the formula to solve for neutrons: \(N = A - Z\).
5. Substitute the known values: \(N = 42 - 19\), which gives the total number of neutrons in potassium-42.