Calcium has an atomic number of 20 and its most common isotope has a mass number of 40. How many neutrons are present in this isotope of calcium?

• A. 20
• B. 40
• C. 18
• D. 20

Answer: (A)

Verified step by step guidance

1. Identify the atomic number of calcium, which is given as 20. The atomic number represents the number of protons in the nucleus of an atom.
2. Note the mass number of the most common isotope of calcium, which is given as 40. The mass number is the total number of protons and neutrons in the nucleus.
3. Recall the relationship between mass number (A), number of protons (Z), and number of neutrons (N): \(A = Z + N\).
4. Rearrange the formula to solve for the number of neutrons: \(N = A - Z\).
5. Substitute the known values into the equation: \(N = 40 - 20\), which gives the number of neutrons in the isotope.