Question

ionization energy is...
a measure of the ability of an atom to attract electrons
the energy required to shield the outer electrons from the nucleus
the energy required to add an electron to a specific atom
how much energy it takes to remove an electron from an atom

Explanation:

Ionization energy is defined as the energy required to remove an electron from an atom. The first option describes electronegativity (ability to attract electrons), the second describes shielding effect (shielding outer electrons from nucleus), and the third describes electron - affinity (energy to add an electron).

Answer:

how much energy it takes to remove an electron from an atom