Which of the following is the approximate effective nuclear charge (Z_eff) experienced by a valence electron in a bromine (Br) atom?

• A. 17.0
• B. 5.3
• C. 35.0
• D. 7.8

Answer: (D)

Verified step by step guidance

1. Identify the atomic number (Z) of bromine (Br). Bromine has an atomic number of 35, meaning it has 35 protons in its nucleus.
2. Determine the number of core (inner) electrons that shield the valence electrons. Bromine's electron configuration is [Ar] 3d^{10} 4s^{2} 4p^{5}, so the core electrons include all electrons except the 7 valence electrons in the 4s and 4p orbitals.
3. Calculate the shielding constant (S) by counting the number of core electrons that effectively shield the valence electrons. For bromine, this is typically the total electrons minus the valence electrons, so S ≈ 28.
4. Use Slater's rules or the approximate formula for effective nuclear charge: \(Z_{\text{eff}} = Z - S\), where \(Z\) is the atomic number and \(S\) is the shielding constant.
5. Subtract the shielding constant from the atomic number to find the effective nuclear charge experienced by a valence electron: \(Z_{\text{eff}} = 35 - 28\) (or the value obtained from Slater's rules), which will give an approximate value close to 7.8.