What is the energy required to remove an electron from an atom?

• A. Atomic Radius
• B. Electronegativity
• C. Hydrogen Bond
• D. Ionization Energy

Answer: (D)

Ionization energy is the energy needed to pull an electron completely away from a neutral atom in the gas phase. It reflects the strength of the electrostatic attraction between the nucleus and its electrons—the more tightly bound the electrons, the higher the energy required. This quantity is typically reported per mole of atoms (kJ/mol) or per atom (electronvolts). The energy needed to remove an electron varies across the periodic table: it generally increases across a period as the effective nuclear charge grows, and it decreases down a group as electrons are farther from the nucleus and more shielded. The first ionization energy refers to removing the first electron; removing subsequent electrons requires progressively more energy because the remaining electrons feel a stronger pull from the now more highly charged nucleus. The other terms listed describe different concepts: atomic radius is about the size of the atom, electronegativity is about an atom’s tendency to attract electrons in a bond, and a hydrogen bond is an intermolecular interaction, not the energy to remove an electron.