Electron transfer between neutral atoms forms ions: the donor becomes a positive ion (cation), and the receiver becomes a negative ion (anion). This transfer depends on the atom’s ionisation energy (the energy required to remove an electron) and electron affinity (the energy released when an electron is gained).
Key Concepts
- Cation: A positively charged ion formed when an atom loses an electron.
- Anion: A negatively charged ion formed when an atom gains an electron.
- Ionisation Energy: The minimum energy required to remove an electron from a neutral atom.
- Electron Affinity: The energy change that occurs when an electron is added to a neutral atom.
- Ionic Crystals: Organised 3D structures (lattices) of alternating positive and negative ions.
The Energy Problem
Atoms exchange electrons to become stable. However, removing an electron (ionisation energy) always requires more energy than what is released when another atom captures it (electron affinity).
Think of this like carrying a heavy rock to the top of a hill. The process takes more effort than the tiny amount of energy released if the rock rolls down the other side. Even in ideal pairs like Sodium and Chlorine, the raw transfer of a single electron takes more energy than it creates.
Crystal Lattices: The transfer happens because the newly formed ions pull together into rigid, repeating patterns called structured lattices, or ionic crystals. When these positive and negative ions group together into a crystal, they release a massive amount of energy, called lattice energy. This release of energy makes the entire reaction possible. The energy gained by forming the lattice easily pays for the energy “debt” created when the electrons were first moved.