Ionization energy is approximately how much energy?

• A. About 1 eV
• B. About 4 eV
• C. About 0.4 eV
• D. About 40 keV

Answer: (B)

Ionization energy is the energy required to remove the outermost electron from a neutral atom in its ground state. For many elements, especially those in the middle of the periodic table and the alkali metals, this energy is on the order of a few electron volts. A representative value around four electron volts captures the typical scale of first ionization energies across common elements, which is why the best answer is about 4 eV.

It's helpful to contrast this with the other options. 0.4 eV or 1 eV would be unusually low for the first ionization energy of most elements, making them poor general estimates. 40 keV is vastly higher and corresponds to energies used to ionize inner-shell electrons or to produce X-rays, not to remove the outermost electron.