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What Is Ionization Energy?

Ionization energy is the amount of energy required to remove an electron from an atom and create an ion. It reflects how tightly an electron is bound to the nucleus and is a fundamental property that links atomic structure to chemical reactivity.

Key Points

• First ionization energy removes the most loosely bound electron.
• Subsequent ionization energies (second, third, etc.) remove progressively more tightly bound electrons.
• Measured in kJ mol⁻¹ for bulk chemistry or eV for single‑atom physics.

Factors Influencing Ionization Energy

Two primary factors determine an element’s ionization energy:

1. Effective Nuclear Charge: More protons in the nucleus increase the attraction to electrons, raising ionization energy.
2. : A filled outer shell (e.g., helium’s 1s²) is more stable and harder to ionize than a partially filled shell. Electrons in inner shells shield outer electrons from the nucleus, reducing the energy needed to remove them.

Trends Across the Periodic Table

Generally, ionization energy rises from left to right across a period due to increasing nuclear charge and falls from top to bottom within a group because additional electron shells shield outer electrons more effectively. Deviations occur for elements with half‑filled or fully filled subshells (e.g., nitrogen, noble gases), so always refer to reliable tables for specific values.

What Happens When an Atom Is Ionized?

Removing an electron produces a cation—an atom with a net positive charge. For example, sodium loses one electron to form Na⁺ in sodium chloride, an ionic compound. Conversely, gaining an electron creates an anion (e.g., chloride ion Cl⁻). Ionization energies thus underpin the formation of ionic bonds and the behavior of atoms in chemical reactions.

For authoritative data on ionization energies, consult the National Institute of Standards and Technology (NIST) database: NIST Atomic Spectra Database.