Thallium has atomic number 81. How many neutrons are in an atom of thallium-205 (Tl-205)?

• A. $286$
• B. $81$
• C. $124$
• D. $205$

Answer: (C)

Verified step by step guidance

1. Identify the atomic number of thallium, which is given as 81. The atomic number represents the number of protons in the nucleus of the atom.
2. Note the mass number of the thallium isotope, which is 205 in Tl-205. The mass number is the total number of protons and neutrons in the nucleus.
3. Recall the relationship between mass number (A), atomic number (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: \(N = 205 - 81\), which will give the number of neutrons in the Tl-205 atom.