Which of the following ions has the electron configuration 2,8,8?

Analyze the question: We need the ion whose electrons are arranged 2,8,8, which means a total of 18 electrons (the same as argon).

Concept: Count the electrons in each ion. For a positive ion, subtract the charge from the atomic number. For a negative ion, add the charge.

Solution:

• A. Na⁺: Na has atomic number 11, so Na⁺ has 11 − 1 = 10 electrons. Configuration: 2,8 (neon-like). ✗
• B. Mg²⁺: Mg has atomic number 12, so Mg²⁺ has 12 − 2 = 10 electrons. Configuration: 2,8. ✗
• C. F⁻: F has atomic number 9, so F⁻ has 9 + 1 = 10 electrons. Configuration: 2,8. ✗
• D. Cl⁻: Cl has atomic number 17, so Cl⁻ has 17 + 1 = 18 electrons. Configuration: 2,8,8 (argon-like). ✓

Tip: Group the ions by the noble gas they resemble:

• Na⁺, Mg²⁺, F⁻, O²⁻, Al³⁺ resemble neon (2,8)
• Cl⁻, S²⁻, K⁺, Ca²⁺ resemble argon (2,8,8)

When you see 2,8,8, think of the argon-like ions.