Atoms that gain or lose electrons are called ions. When an atom loses one or more electrons, it ends up with more protons than electrons and carries a positive charge; this positively charged ion is called a cation. When an atom gains one or more electrons, it has more electrons than protons and carries a negative charge; this negatively charged ion is called an anion. The distinction sounds simple, but ions show up in an enormous range of settings, from table salt dissolving in a glass of water to the soft X-rays streaming out of the solar wind, and their behavior changes depending on where they are and how much charge they carry.
Cations, Anions, and How They Form
The names cation and anion trace back to early electrochemistry experiments. In those setups, ions migrated toward electrodes: cations moved toward the negatively charged cathode, and anions moved toward the positively charged anode. The naming convention stuck. A balanced atom has equal numbers of protons and electrons, so its overall charge is zero. Remove an electron and the balance tips positive; add one and it tips negative.1Technology Networks. Cation vs Anion: Definition, Chart and the Periodic Table
Some atoms form ions more readily than others. Metals on the left side of the periodic table, such as sodium and potassium, hold their outermost electrons loosely. They shed one or two electrons with little provocation, becoming cations like Na⁺ or K⁺. Nonmetals on the right side, such as chlorine and oxygen, hold their electrons tightly and tend to grab extras from nearby atoms, becoming anions like Cl⁻ or O²⁻. This tendency is what makes the periodic table useful for predicting which atoms will form which kind of ion: the further left and lower an element sits, the easier it gives up electrons, while elements in the upper right corner (excluding noble gases) are electron-grabbers.
Polyatomic Ions
Not every ion is a single atom. Groups of atoms bonded together can carry a net charge as a unit, and these are called polyatomic ions. Hydroxide (OH⁻), ammonium (NH₄⁺), and nitrate (NO₃⁻) are everyday examples. The atoms within each group are held together by covalent bonds, meaning they share electrons internally. But the group as a whole has an unbalanced electron count, so it behaves like a single charged particle when it interacts with other ions.2ScienceDirect. Polyatomic Ion Compounds built from polyatomic ions, such as sodium hydroxide (NaOH) or ammonium chloride (NH₄Cl), are still classified as ionic compounds even though covalent bonding exists inside the polyatomic piece.
This matters practically because polyatomic ions are everywhere in chemistry and biology. Phosphate ions (PO₄³⁻) are the backbone of DNA. Bicarbonate ions (HCO₃⁻) buffer your blood’s pH. Sulfate ions (SO₄²⁻) show up in everything from fertilizer to drywall. Thinking of ions only as single charged atoms misses a huge portion of the ionic world.
What Happens When Ions Meet Water
Drop a pinch of table salt into water and the crystal lattice of sodium and chloride ions falls apart. Water molecules crowd around each freed ion, forming a shell called a hydration shell or solvation shell. The water molecules orient themselves so that the oxygen end (which carries a slight negative charge) faces a cation, while the hydrogen end (slightly positive) faces an anion. The structure and thickness of that shell depend on how much charge the ion packs into its size.
Small, highly charged ions like lithium or magnesium have very high charge density. They grab water molecules tightly and form compact, well-ordered hydration shells. These ions are sometimes called kosmotropes because of their strong ordering effect on nearby water. Large ions with low charge density, like cesium, barely disturb the water around them; their surrounding water molecules remain largely hydrogen-bonded to each other rather than reoriented by the ion. These are sometimes called chaotropes.3PubMed Central. How ions affect the structure of water Research modeling the relationship between ionic charge and water structure has shown a sharp transformation in the hydration shell: below a critical charge density the ion sits inside a thick, loosely organized shell of a dozen or more water molecules, but above that threshold the shell contracts to a tight arrangement of roughly four to eight molecules.4Nature Communications. Impact of hierarchical water dipole orderings on the dynamics of aqueous salt solutions
This hydration behavior is not just a laboratory curiosity. It influences how fast ions move through solutions, how well electrolytes conduct electricity, how biological membranes sort one ion from another, and even how proteins fold. Molecular dynamics simulations of a sodium chloride crystal dissolving into its own solution have shown that the hydration structure of Na⁺ and Cl⁻ ions changes measurably right at the crystal-solution boundary but returns to normal just a layer or two away from the surface.5PubMed Central. Ion Distribution and Hydration Structure at Solid–Liquid Interface between NaCl Crystal and Its Solution
Ionic Bonding and Crystal Lattices
When a cation and an anion are drawn together by their opposite charges, they form an ionic bond. In a solid like table salt, this does not happen in isolated pairs. Instead, every sodium ion is surrounded by chloride ions and vice versa, creating a repeating three-dimensional grid called a crystal lattice. The energy holding that lattice together is called lattice energy, and it is a measure of how tightly the ions are packed. Higher lattice energy means a harder, higher-melting compound.
Calculating lattice energy is not straightforward because the real bonding in most ionic crystals is not purely electrostatic. There is always some degree of covalent character mixed in, particularly for ions that are large or highly charged. Research into lattice-energy estimation has shown that each bond’s contribution can be separated into ionic and covalent parts, and that doing so gives results that match experimentally measured lattice energies across a wide variety of crystals.6PubMed. Lattice energy estimation for inorganic ionic crystals In practice, this means the old textbook picture of purely electrostatic ionic bonds is a useful simplification but not the whole story.
Ions in Your Body
Your cells run on ions. The electrical voltage across a cell membrane, which is what allows nerves to fire and muscles to contract, arises from an uneven distribution of ions on either side of that membrane. Three mechanisms contribute. One is the natural tendency of ions to diffuse from areas of high concentration to low concentration. Another is an electrically neutral pump, such as the sodium-potassium pump found in many cells, which moves Na⁺ out and K⁺ in. A third is an electrogenic pump that directly transfers net charge across the membrane, adding to the voltage.7American Zoologist. The Origin of Bioelectrical Potentials in Plant and Animal Cells
Getting the right ions through the membrane at the right time requires exquisitely selective channels. Potassium channels, for example, must allow K⁺ through while blocking the slightly smaller Na⁺ ion. They accomplish this at a narrow stretch called the selectivity filter, which contains specific binding sites for potassium ions.8PubMed Central. Mechanism of potassium-channel selectivity revealed by Na(+) and Li(+) binding sites within the KcsA pore Detailed simulations of these channels have revealed that the ion conductance is actually controlled at the selectivity filter, and that the wider opening at the bottom of the channel mainly transmits a mechanical signal that adjusts the filter’s conformation.9Nature Communications. Molecular mechanism of a potassium channel gating through activation gate-selectivity filter coupling In plain terms, the channel acts like a lock that checks the ion’s size and charge before letting it pass. This selectivity is the reason a nerve impulse can travel reliably along a fiber: sodium rushes in through one set of channels, potassium flows out through another, and the voltage wave propagates down the nerve.
Beyond nerve signaling, ions are central to muscle contraction (calcium ions trigger the process), bone formation (calcium and phosphate ions build hydroxyapatite crystals), and even how proteins fold into their functional shapes (zinc ions stabilize many enzyme structures). Your body is, in a meaningful sense, an electrochemical system, and its working parts are ions.
Ions in Batteries
A lithium-ion battery works by shuttling Li⁺ ions back and forth between two electrodes through a liquid electrolyte. During discharge, lithium atoms at the anode give up an electron (becoming Li⁺ cations), travel through the electrolyte to the cathode, and recombine with electrons that have done useful work in an external circuit. Charging reverses the process. The entire technology hinges on how smoothly those lithium ions can move and how evenly they deposit.
One of the biggest problems in next-generation lithium metal batteries is uneven deposition. If Li⁺ ions run out at certain spots on the electrode surface faster than fresh ions arrive, the depositing metal grows in spiky, tree-like formations called dendrites. Modeling work has shown that rapid depletion of Li⁺ at the electrode surface, caused by low temperatures, dilute electrolytes, or high charging currents, is the fundamental driving force behind this non-uniform growth.10Advanced Energy Materials. Role of Li‐Ion Depletion on Electrode Surface: Underlying Mechanism for Electrodeposition Behavior of Lithium Metal Anode The practical takeaway: keeping lithium ions abundant and mobile at the electrode surface is key to preventing dendrites and making fast-charging batteries safer.
Complicating things further, a thin layer called the solid electrolyte interphase (SEI) forms on the lithium surface. Research has shown that at low charging currents, lithium plates underneath this layer, but at high currents the SEI breaks down and lithium plates directly onto fresh metal surfaces. Above a critical current density, this breakdown leads to problematic growth patterns and poor battery life.11PubMed. Resolving Current-Dependent Regimes of Electroplating Mechanisms for Fast Charging Lithium Metal Anodes Simulations have further shown that grain boundaries within the SEI actually help lithium ions pass through more quickly than the crystalline regions do, which is an important clue for designing better protective layers.12PubMed Central. SEI Formation and Lithium-Ion Electrodeposition Dynamics in Lithium Metal Batteries via First-Principles Kinetic Monte Carlo Modeling
Room-Temperature Ionic Liquids
Most ionic compounds are solids at room temperature because the electrostatic forces holding their lattice together are strong. But certain combinations of large, oddly shaped cations and anions have such weak lattice energies that the substance is a liquid even without any solvent. These are called room-temperature ionic liquids (RTILs), and they are essentially molten salt at comfortable temperatures.
RTILs have unusual properties: they conduct electricity, have negligible vapor pressure, and resist catching fire. These traits make them attractive as electrolytes, solvents for chemical reactions, and lubricants. Research into their transport behavior has shown that the effective ionic concentration in an RTIL, a measure of how many ions are actually contributing to conductivity versus how many are paired up and electrically silent, correlates well with properties like boiling point and distillation rate.13PubMed. How ionic are room-temperature ionic liquids? An indicator of the physicochemical properties In many RTILs, the cation diffuses faster than the anion, but the combined diffusion rate of both ions exceeds what the conductivity alone would predict, indicating that a significant fraction of the ions travel as neutral pairs rather than as free charges.14Electrochimica Acta. Combined analysis of self-diffusion, conductivity, and viscosity data on room temperature ionic liquids Understanding that pairing dynamic is one of the active problems in designing better RTILs for battery and industrial use.
Using Ions to Identify Molecules
Mass spectrometry, one of the most powerful tools in analytical chemistry, works by converting molecules into ions and then sorting them by mass. If you can turn a molecule into a charged particle, you can steer it with electric and magnetic fields, measure how it responds, and figure out its mass with extraordinary precision.
Two widely used methods for making those ions are electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). In ESI, a solution of the molecule is sprayed through a charged needle, producing a fine mist of tiny charged droplets. As the solvent evaporates, the ions emerge intact in the gas phase.15PubMed. Unraveling the mechanism of electrospray ionization In MALDI, the molecule is embedded in a crystalline matrix that absorbs laser light. A pulse from the laser ejects the molecule into the gas phase as an ion, typically carrying a single charge.16PubMed Central. Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry: Mechanistic Studies and Methods for Improving the Structural Identification of Carbohydrates A newer variant called laserspray ionization bridges the two approaches: it uses a MALDI-style laser ablation step but produces multiply charged ions resembling those from ESI, expanding what the technique can analyze.17Molecular & Cellular Proteomics. Laserspray Ionization, a New Atmospheric Pressure MALDI Method for Producing Highly Charged Gas-phase Ions of Peptides and Proteins Directly from Solid Solutions
These techniques are used to identify everything from drug metabolites in a patient’s blood to trace contaminants in drinking water. The common thread is the same: turn a neutral molecule into an ion so you can manipulate and measure it.
Radical Ions
A standard ion forms when an atom or molecule gains or loses electrons in a straightforward sense. But there is a more exotic category: radical ions. These are species that carry both a charge and an unpaired electron, making them simultaneously an ion and a free radical. Cation radicals (positively charged radicals) are especially common in organic chemistry. They form when a neutral molecule loses one electron, often during an oxidation reaction, and can be surprisingly reactive. The dimerization of cation radicals from simple benzene derivatives, for instance, can produce biphenyl-type compounds through pathways involving either radical-radical coupling or radical-substrate coupling.18Advances in Physical Organic Chemistry. Kinetics and Mechanisms of Reactions of Organic Cation Radicals in Solution Radical ions play important roles in photochemistry, electrochemistry, and biological processes like the electron-transfer reactions inside mitochondria.
Highly Charged Ions in Space
Earth-bound chemistry typically deals with ions carrying one, two, or maybe three extra or missing electrons. In space, things get more extreme. The solar wind is a stream of plasma blowing off the sun, and it contains ions of carbon, nitrogen, oxygen, and neon that have been stripped of most or all of their electrons. These highly charged ions, sometimes missing six or seven electrons, are exotic by everyday standards but ordinary in astrophysics.
When these ions collide with neutral atoms or molecules, such as those in Earth’s outer atmosphere or in a comet’s gas cloud, they capture electrons through a process called charge exchange. That capture releases energy as soft X-ray photons. This emission has been observed coming from cometary comas, planetary atmospheres, and even the general glow of the heliosphere.19AIP Conference Proceedings. Solar Wind Charge Exchange Studies Of Highly Charged Ions On Atomic Hydrogen The X-ray glow from Earth’s exosphere, for instance, is produced by highly charged solar-wind ions colliding with neutral particles in the outermost reaches of our atmosphere.20Geophysical Research Letters. Empirical Functions for Highly Charged Ion Abundances in Solar Wind Charge Exchange Models: Addressing Post‐2011 ACE Data Limitations
Researchers study these interactions not only with space telescopes but also in the lab. Extreme ultraviolet emission lines from highly charged iron ions in the solar corona have been reproduced in devices like the Large Helical Device and compact electron beam ion traps, allowing scientists to calibrate their models against controlled experiments.21Journal of Fusion Energy. Spectroscopic Diagnostics for Highly Charged Iron Ions Observed in Solar Corona and LHD Understanding how these ions behave is essential for interpreting X-ray observations of distant objects: without accurate charge-exchange cross-sections, astronomers cannot separate the foreground glow of our own solar system from the signals they are trying to detect from far-off galaxies.
Ions and Cloud Formation
Ions even play a role in the atmosphere above your head. Cosmic rays constantly slam into air molecules, knocking out electrons and creating clusters of ions in the lower atmosphere. One hypothesis, called ion-induced nucleation, proposes that these atmospheric ions help tiny aerosol particles form and grow, which in turn seed cloud droplets. A review of the field noted that while evidence for ion-induced nucleation is accumulating, the exact mechanism remains under investigation.22Atmospheric Chemistry and Physics. The role of atmospheric ions in aerosol nucleation – a review
More recent modeling work has described a feedback loop in which a small fraction of ultrafine aerosols, activated with the help of charges on haze droplets, grow into cloud droplets. Those droplets then sweep up more ultrafine particles, producing larger residual aerosols that are more efficient at seeding further clouds. The result is a runaway increase in cloud-droplet concentration over time.23Oxford Open Climate Change. Ion-induced cloud modulation through new particle formation and runaway cloud condensation nuclei production Whether this mechanism is strong enough to meaningfully influence climate is still debated, but it is a vivid example of how ions connect the subatomic world to something as large-scale as weather.
The question “what do you call atoms that gain or lose electrons?” has a two-word answer: they are ions. But that two-word answer opens a door to nearly every corner of science. The same basic idea, an imbalance of electrons producing a charged particle, underpins the firing of your neurons, the charging of your phone, the identification of unknown chemicals, the glow of comets, and the formation of clouds. Ions are arguably the most versatile actors in chemistry and physics, all because of what happens when an atom ends up with the wrong number of electrons.