ions

Ions are atoms or molecules that carry a net electric charge, resulting from the loss or gain of electrons. They form when atoms are ionized by energy input or through electron transfer between species. Cations are positively charged ions formed by removing electrons; common examples include Na+, K+, Ca2+, and NH4+. Anions are negatively charged ions formed by gaining electrons; examples include Cl-, OH-, NO3-, and SO4^2-. Some ions are polyatomic, consisting of several atoms bonded together with an overall charge, such as ammonium (NH4+), carbonate (CO3^2-), and sulfate (SO4^2-).

In solids, many ionic compounds consist of lattices of alternating cations and anions bonded by electrostatic forces, yielding electrically neutral materials. In liquids or solutions, ions dissociate and can conduct electricity; their mobility depends on solvent, temperature, and concentration. Ions also exist in gas phase and plasma, where their motion and reactions drive many physical and chemical processes. The presence and concentration of ions in a solution—its ionic strength or salinity—affect chemical reactivity, solubility, and electrical conductivity.

Ions play key roles in biology, including nerve impulse transmission, muscle contraction, and fluid balance, with ions such as Na+, K+, Ca2+, Cl-, and HCO3- serving as essential signaling and regulatory species. In environmental science and industry, ions determine water quality, electrolysis processes, and the behavior of minerals. Analytical methods such as ion chromatography and mass spectrometry are used to detect and quantify ions in samples.