Which term describes the energy needed to remove an electron from an isolated atom?

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

Answer: (C)

Ionization energy is the energy required to remove an electron from a neutral, isolated atom. It measures how strongly the nucleus holds onto its electrons; you have to supply enough energy to overcome the electrostatic attraction between the positively charged nucleus and the electron being removed. In general, atoms with a higher effective nuclear charge and electrons closer to the nucleus require more energy to remove, while electrons further away or shielded by inner electrons are easier to remove.

Across a period, ionization energy tends to increase because the atoms gain protons without a large increase in shielding, so the electrons feel a stronger pull from the nucleus. Down a group, it tends to decrease as the outer electrons occupy higher, farther shells and shielding by inner electrons reduces the net attraction of the nucleus.

Electronegativity, by contrast, describes how strongly an atom attracts electrons in a chemical bond, not how much energy is needed to remove an electron from a free atom. Atomic radius is a size measure of the atom, not an energy quantity. A hydrogen bond is an intermolecular interaction involving hydrogen and an electronegative atom, unrelated to removing an electron from an isolated atom.