Salt (chemistry)

In chemistry, a salt or ionic compound is a special kind of chemical compound made up of tiny particles called ions. Ions can have a positive charge or a negative charge, and when they come together, they balance each other out so the whole compound has no charge at all. These ions stick together because of strong forces called electrostatic forces, which we call ionic bonds.

The ions in a salt can be simple or complex. Some are inorganic, like chloride (Cl⁻), while others are organic, like acetate (CH₃COO⁻). They can also be single atoms, like sodium (Na⁺) and chloride (Cl⁻) in regular table salt, sodium chloride, or they can be groups of atoms, like ammonium (NH₄⁺) and carbonate (CO₃²⁻) in ammonium carbonate.

When salts are solid, they usually form neat, repeating patterns called crystalline structures. Many salts, especially those with small ions, have high melting and boiling points, and they feel hard and brittle. Even though they don’t conduct electricity when solid, they can conduct very well when melted or dissolved in water because their ions are free to move.

History of discovery

X-ray spectrometer developed by W. H. Bragg

In 1913, scientists William Henry Bragg and his son William Lawrence Bragg studied the structure of sodium chloride. They discovered that the atoms were arranged in a special pattern, forming a network rather than small groups. This helped scientists understand that many inorganic compounds are made of tiny particles called ions.

Later, in the mid-1920s, experiments using X-ray reflection provided more proof that these compounds are made of ions. Important scientists like Max Born, Fritz Haber, Alfred Landé, Erwin Madelung, Paul Peter Ewald, and Kazimierz Fajans helped explain how these structures work.

Formation

Halite, the mineral form of sodium chloride, forms when salty water evaporates leaving the ions behind.

Many metals, like the alkali metals, can react with halogens gases to create salts. Salts can also form when water dries up from a solution that has too much salt. This happens in nature and makes minerals.

Salts can also be made by mixing two solutions together. One solution has the positive part, and the other has the negative part. If water is used, the salt might keep some water in its structure, which changes how it behaves. Salts can be created in different ways, like mixing a base and an acid, or a metal and an acid.

Bonding

Main article: Ionic bonding

A schematic electron shell diagram of sodium and fluorine atoms undergoing a redox reaction to form sodium fluoride. Sodium loses its outer electron to give it a stable electron configuration, and this electron enters the fluorine atom exothermically. The oppositely charged ions – typically a great many of them – are then attracted to each other to form a solid.

In salts, tiny particles called ions stick together mostly because of a strong pull between them. Positive ions are attracted to negative ions, which keeps the salt together. There can also be a tiny extra pull from other forces, but it only adds a little bit to the overall stickiness.

When ions get very close, their outer layers of electrons can push each other away, which stops them from getting too close. This balance of pulling and pushing makes the ions settle at a certain distance apart. Sometimes, the way electrons share can make the bond a bit more than just this pull, adding a little extra connection. Even in salts that seem to only have this pull, there is often a small bit of sharing happening too.

Structure

In a salt, the tiny parts called ions stick together because they have opposite charges. This makes the salt stay together in a certain pattern, like how building blocks fit into each other. Scientists can use special math to guess how strong this sticking together is.

The unit cell of the zinc blende structure

Sometimes, these ions don’t form perfect patterns. Instead, they mix in ways that don’t make a clear shape. This can happen when the salt is cooled very quickly.

Defects

Even in perfect patterns, there can be small mistakes or gaps. These gaps happen in pairs to keep the salt balanced. One type of gap happens when a small part is in the wrong place, and another happens when there’s an empty spot. These gaps can move around and change how the salt behaves, like letting it carry electricity.

Common ionic compound structures with close-packed anions
Stoichiometry	Cation:anion coordination	Interstitial sites	Cubic close packing of anions	Hexagonal close packing of anions
Occupancy	Critical radius ratio	Name	Madelung constant	Name	Madelung constant
MX	6:6	all octahedral	0.4142	sodium chloride	1.747565	nickeline
	4:4	alternate tetrahedral	0.2247	zinc blende	1.6381	wurtzite	1.641
MX₂	8:4	all tetrahedral	0.2247	fluorite	5.03878
	6:3	half octahedral (alternate layers fully occupied)	0.4142	cadmium chloride	5.61	cadmium iodide	4.71
MX₃	6:2	one-third octahedral	0.4142	rhodium(III) bromide	6.67	bismuth iodide	8.26
M₂X₃	6:4	two-thirds octahedral	0.4142			corundum	25.0312
ABO₃		two-thirds octahedral	0.4142			ilmenite	Depends on charges and structure
AB₂O₄		one-eighth tetrahedral and one-half octahedral	r_A/r_O = 0.2247, r_B/r_O = 0.4142	spinel, inverse spinel	Depends on cation site distributions	olivine	Depends on cation site distributions

Common ionic compound structures with simple cubic packed anions
Stoichiometry	Cation:anion coordination	Interstitial sites occupied	Example structure
Name	Critical radius ratio	Madelung constant
MX	8:8	entirely filled	cesium chloride	0.7321	1.762675
MX₂	8:4	half filled	calcium fluoride
M₂X	4:8	half filled	lithium oxide

Properties

Salts are special kinds of chemicals made up of positively charged particles called cations and negatively charged particles called anions. These particles stick together because of strong forces, forming a compound that has no overall charge.

Salts can be made from different kinds of particles. Some salts have simple particles like chloride (Cl−), while others have more complex particles like acetate (CH3COO−). When salts dissolve in water, they break apart into their separate particles, which spread out through the water. This is different from most other compounds, which stay together when dissolved.

Uses

Salts have been used in many ways for thousands of years. People have used common salt, which is sodium chloride, for over 8000 years to add flavor to food and keep it from spoiling. Today, salts are also used in factories, farming, treating water, and even to help melt ice on roads.

When salts dissolve in water, they can change how the water behaves. For example, they can make it harder for water to freeze, which is why we put salt on icy roads. Salts can also help make other liquids carry electricity better. Some salts, like those that contain fluoride, are added to drinking water to help protect teeth.

Nomenclature

According to the rules of IUPAC, salts are named based on what they are made of, not how their atoms are arranged. For a simple salt with two parts, we name it using two words. The first word is the name of the positive part (cation), and the second word is the name of the negative part (anion). For example, MgCl₂ is called magnesium chloride, and Na₂SO₄ is called sodium sulfate.

When a salt has more than one type of positive or negative part, we use special prefixes like di- or tri- to show how many of each part there are. The parts are listed in alphabetical order. For example, KMgCl₃ is named magnesium potassium trichloride. If a part already has a prefix in its name, we use different prefixes like bis- or tris- instead.

Non-salt

Zwitterion

Zwitterions have both a negative and a positive charge in the same molecule, but they are not considered salts. Examples of zwitterions include amino acids, many metabolites, peptides, and proteins.