Which of the following represents the correct ground-state electron configuration for Ru^{2+} (ruthenium(II))?

• A. [Kr] 4d^5 5s^1
• B. [Kr] 4d^6
• C. [Kr] 4d^4 5s^2
• D. [Kr] 4d^8

Answer: (B)

Verified step by step guidance

1. Step 1: Identify the atomic number of ruthenium (Ru). Ruthenium has an atomic number of 44, meaning a neutral Ru atom has 44 electrons.
2. Step 2: Write the electron configuration for the neutral Ru atom. The full configuration ends with 4d and 5s orbitals, typically written as [Kr] 4d^7 5s^1 for Ru.
3. Step 3: Determine the electron configuration for Ru^{2+} by removing two electrons from the neutral atom. Remember that electrons are removed first from the outermost shell, which is the 5s orbital before the 4d orbital.
4. Step 4: Remove two electrons starting from the 5s orbital and then from the 4d orbital as needed. This will give the electron configuration for Ru^{2+}.
5. Step 5: Express the final electron configuration for Ru^{2+} using the noble gas shorthand notation, starting from [Kr] and then the appropriate number of electrons in the 4d orbital.