What Is Ionization Energy? Periodic Trends Explained
Why does sodium throw away an electron at the slightest provocation, while neon — right next door — clings to every one it has? Chemistry has a number that measures exactly this stinginess, and it's one of the most predictable patterns on the whole periodic table. The short answer: ionization energy (IE) is the energy required to remove the most loosely held electron from a neutral atom in the gas phase, forming a positive ion. It's always an energy input (endothermic), it's usually quoted in kJ/mol, and it generally increases across a period and decreases down a group . What ionization energy actually means The definition in equation form, using sodium: Na(g) + 496 kJ/mol → Na⁺(g) + e⁻ Three details worth underlining: Gas phase. The atom must be isolated, so no neighbors interfere. That's why the definition says Na(g). It always costs energy. Electrons are attracted to the nucleus; prying one off is never free. Every ionization energy is positive. "F...