Ionization Energy Calculator

Two notable exceptions: (1) Group 13 has lower IE than Group 2 because the new electron enters a higher-energy p orbital (easier to remove than s). (2) Group 16 has lower IE than Group 15 because the fourth p electron must pair with an existing electron, experiencing electron-electron repulsion that makes it easier to remove. These exceptions occur in every period.

Ionization energy measures the energy to remove an electron (always positive, endothermic). Electron affinity measures the energy change when adding an electron (usually exothermic for nonmetals). Together, they define the Mulliken electronegativity: EN = (IE + EA) / 2. Elements with high IE and high EA (like fluorine) are the most electronegative.

Each electron removed reduces shielding, increasing Z_eff for the remaining electrons. Also, removing electrons from a positive ion requires overcoming additional electrostatic attraction. The largest jumps occur when penetrating into a lower shell (e.g., removing a 2p electron after all 3s/3p electrons), because the electron is much closer to the nucleus.

Photoelectron spectroscopy (PES) is the primary method. Atoms are irradiated with high-energy photons (UV or X-ray), and the kinetic energy of ejected electrons is measured. IE = photon energy - kinetic energy. PES can determine the ionization energies of all electron shells, providing a complete energy level diagram for the atom.

The threshold wavelength is the maximum (longest) wavelength of light that has enough energy per photon to ionize the atom. Photons with longer wavelength (lower energy) cannot cause ionization. For most atoms, the threshold falls in the UV region (10-200 nm). Only alkali metals like cesium (IE = 3.89 eV, threshold = 319 nm) can be ionized by visible light.