What is the energy required to remove an electron from the outermost shell called?

• A. Electronegativity
• B. Ionization energy
• C. Atomic radius
• D. Valence electrons

Answer: (B)

The energy needed to remove an electron from the outermost shell is called ionization energy. This quantity measures how strongly the nucleus holds onto the outer, most loosely bound electron and is defined for a neutral atom in the gaseous state. When you remove that outer electron, you’re paying the energy to overcome the electrostatic attraction between the positively charged nucleus and the negative electron.

A key nuance is that the first ionization energy specifically refers to removing one electron from the valence (outermost) shell. Factors that raise this energy include a higher effective nuclear charge and a shorter distance between the nucleus and the outer electron; factors that lower it include greater shielding and larger atomic size. Across a period, ionization energy tends to increase as atoms pull electrons more tightly, while down a group it tends to decrease due to increased distance and shielding.

The other terms don’t describe this energy cost: electronegativity is about an atom’s tendency to attract electrons in a bond, atomic radius is a size measure of the atom, and valence electrons are the electrons in the outer shell itself.