What is the approximate energy required to remove a k-shell electron from a tungsten atom?

• A. 69.5 keV
• B. 0.5 keV
• C. 12 keV
• D. 100 keV

Answer: (A)

K-shell binding energy is the energy needed to pull the innermost electron away from the atom. For heavy elements like tungsten (Z = 74), those inner electrons are held extremely tightly, so the energy required sits in the tens of keV range. A simple hydrogen-like estimate uses E ≈ 13.6 eV × (Z_eff)^2, and with Z_eff for the K-shell around Z−1, you get E on the order of 13.6 eV × 73^2 ≈ 70 keV. The measured K-shell binding energy for tungsten is about 69.5 keV, so removing a K-shell electron requires roughly 69.5 keV. That’s why this value is the best approximation—the energy needed climbs steeply with nuclear charge, placing tungsten’s K-edge squarely in the tens of keV, not at fractions of a keV or near 100 keV.