An alkalide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

in which

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

atoms are

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s (negative ions) with a charge or

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

of −1. Until the first discovery of alkalides in the 1970s, alkali metals were known to appear in

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s only as

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s (positive ions) with a charge or oxidation state of +1. These types of compounds are of theoretical interest due to their unusual

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

and low

ionization potential In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule. The first ionization energy is quantitatively expressed as :X(g) ...

s. Alkalide compounds are chemically related to the electrides, salts in which trapped electrons are effectively the anions.

"Normal" alkali metal compounds

Alkali metals form many well-known stable salts.

Sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

(common table salt), , illustrates the usual role of an alkali metal such as sodium. In the empirical formula for this

ionic compound In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions (Cation, cations) and negatively charged ions (Anion, anions), which results in a compound with no net electric charge (electrica ...

, the positively charged

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

ion is balanced by a negatively charged

chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

ion. The traditional explanation for stable is that the loss of one

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

from elemental sodium to produce a cation with charge of +1 produces a stable closed-shell

electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon ato ...

Nomenclature and known cases

There are known alkalides for some of the alkali metals: *Sodide or natride, *Potasside or kalide, *Rubidide, *Caeside, Alkalides of the other alkali metals have not yet been discovered: *Lithide, *Francide,

Examples

Normally, alkalides are thermally labile due to the high reactivity of the alkalide anion, which is theoretically able to break most

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s including the carbon–oxygen bonds in a typical

cryptand In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for ...

. The introduction of a special cryptand ligand containing amines instead of ether linkages has allowed the isolation of kalides and natrides that are stable at room temperature. Several alkalides have been synthesized: *A compound in which hydrogen ions are encapsulated by adamanzane, known as hydrogen natride or "inverse sodium hydride" (hydrogen sodide or hydrogen natride ), has been observed. *Sodium-crypt natride, cryptand[2.2.2">2.2.2-Cryptand.html" ;"title="a(2.2.2-Cryptand">cryptand[2.2.2]⁺Na⁻, has been observed. This salt contains both and . The

isolates and stabilizes the , preventing it from being reduced by the . *

Barium Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

azacryptand-sodide, Ba²⁺ ₅Azacryptand[2.2.2⁻Na⁻⋅2CH₃NH₂, has been synthesized. *Dimer (chemistry)">Dimers of cationic and anionic sodium have been observed.

References

{{Caesium compounds Sodium compounds Potassium compounds Rubidium compounds Caesium compounds Anions Alkali metals