How many neutrons are present in an atom of lead-207 (Pb-207)?

• A. 207
• B. 125
• C. 82
• D. 126

Answer: (B)

Verified step by step guidance

1. Identify the atomic number (Z) of lead (Pb), which is the number of protons in the nucleus. For lead, Z = 82.
2. Recognize that the number 207 in lead-207 represents 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: \(N = 207 - 82\), which gives the number of neutrons in lead-207.