The inhibition of the enzyme by very low concentrations of lanthanum ion is probably the strongest known biological effect of rare earth salts.

An Arrhenius acid is a substance that increases the concentration of the hydronium ion, H < sub > 3 </ sub > O < sup >+</ sup >, when dissolved in water.

An Arrhenius base is a molecule which increases the concentration of the hydroxide ion when dissolved in water.

As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed.

A proton is transferred from an unspecified Brønsted acid to ammonia, a Brønsted base ; alternatively, ammonia acts as a Lewis base and transfers a lone pair of electrons to form a bond with a hydrogen ion.

Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created.

A positively or negatively charged atom is known as an ion.

At pH values greater than the pKa of the carboxylic acid group ( mean for the 20 common amino acids is about 2. 2, see the table of amino acid structures above ), the negative carboxylate ion predominates.

Also, mass spectrometry is categorized by approaches of mass analyzers: magnetic-sector, quadrupole mass analyzer, quadrupole ion trap, time-of-flight, Fourier transform ion cyclotron resonance, and so on.

Sodium is a " water-holding " ion, so water is also retained, which leads to increased blood volume, hence an increase in blood pressure.

After introduction into the vacuum system, the sample is reduced to cryogenic temperatures ( typically 20-100 K ) and manipulated such that the needle's point is aimed towards an ion detector.

In field ion and atom probe methods, the polarity of the electric field is reversed, with a high positive field applied to the sample.

For field ion microscopy a gas is introduced, known as the imaging gas, into the chamber at very low pressures.

Subsequently, the accelerative force on any given ion is controlled by the electrostatic equation, where n is the ionisation state of the ion, and e is the fundamental electric charge.

Assuming that the ion is accelerated during a very short interval, the ion can be assumed to be travelling at constant velocity.

However, the hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons.

The samples used in atom probe are usually a metallic or semi-conducting material, with the needle geometry produced by electropolishing, or focused ion beam methods.

Whilst the magnification of both the field ion and atom probe microscopes is extremely high, the exact magnification is dependant upon conditions specific to the examined specimen, so unlike for conventional electron microscopes, there is often little direct control on magnification, and furthermore, obtained images may have strongly variable magnifications due to fluctuations in the shape of the electric field at the surface.

Initial field ion microscopes, precursors to modern atom probes, were usually glass blown devices developed by individual research laboratories.

An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons.

In chemistry, a coordination complex or metal complex, consists of an atom or ion ( usually metallic ), and a surrounding array of bound molecules or anions, that are in turn known as ligands or complexing agents.

The atom within a ligand that is bonded to the central atom or ion is called the donor atom.

Polydentate ( multiple bonded ) ligands consist of several donor atoms, several of which are bound to the central atom or ion.

The central atom or ion, together with all ligands comprise the coordination sphere.

In some cases where the radius of the substitutional atom ( ion ) is substantially smaller than that of the atom ( ion ) it is replacing, its equilibrium position can be shifted away from the lattice site.

Dislocations can be observed using transmission electron microscopy, field ion microscopy and atom probe techniques.

An atom or ion that gives up an electron to another atom or ion has its oxidation state increase, and the recipient of the negatively charged electron has its oxidation state decrease.

In the half-cell which is undergoing oxidation, the closer the equilibrium lies to the ion / atom with the more positive oxidation state the more potential this reaction will provide.

Similarly, in the reduction reaction, the further the equilibrium lies to the ion / atom with the more negative oxidation state the higher the potential.

Below pH 2. 2, the predominant form will have a neutral carboxylic acid group and a positive α-ammonium ion ( net charge + 1 ), and above pH 9. 4, a negative carboxylate and neutral α-amino group ( net charge − 1 ).

The heme group is a highly-conjugated ring system ( which allows its electrons to be very mobile ) surrounding a metal ion, which readily interconverts between the oxidation states.

When the group of covalently bound atoms bears a net charge, the group is referred to more properly as a polyatomic ion or a complex ion.

A heme group consists of an iron ( Fe ) ion ( charged atom ) held in a heterocyclic ring, known as a porphyrin.

Their hydrolysis occurs when the nucleophile ( a nucleus-seeking agent, e. g., water or hydroxyl ion ) attacks the carbon of the carbonyl group of the ester or amide.

The carboxylic acid has a hydroxyl group derived from a water molecule and the amine ( or ammonia ) gains the hydrogen ion.

In this reaction, a base, usually triethylamine, removes the acidic proton alpha to the carbonyl group, inducing the formation of the carbon-carbon double bond and the loss of a chloride ion.

Finally, the amphiboles are usually hydrated — that is, the have a hydroxyl group (< sup >-</ sup >), although it can be replaced by a fluoride, chloride, or an oxide ion.

Loss of a positive hydrogen ion ( H < sup >+</ sup >) from the hydroxyl group of a phenol forms a corresponding negative phenolate ion or phenoxide ion, and the corresponding salts are called phenolates or phenoxides, although the term aryloxides is preferred according to the IUPAC Gold Book.

In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group.

* Chemistry: Molecular theory — Kinetic theory of gases — Molecular orbital theory — Valence bond theory — Transition state theory — RRKM theory — Chemical graph theory — Flory-Huggins solution theory — Marcus theory — Lewis theory ( successor to Brønsted – Lowry acid – base theory ) — HSAB theory — Debye – Hückel theory — Thermodynamic theory of polymer elasticity — Reptation theory — Polymer field theory — Møller – Plesset perturbation theory — Density Functional Theory — Frontier molecular orbital theory — Polyhedral skeletal electron pair theory — Baeyer strain theory — Quantum theory of atoms in molecules — Collision theory — Ligand field theory ( successor to Crystal field theory ) — Variational Transition State Theory — Benson group increment theory — Specific ion interaction theory

X-Ray diffraction studies on the hydrogen urate ion in crystals of ammomium hydrogen urate, formed in vivo as gouty deposits, reveal the keto-oxygen in the 2 position of a tautomer of the purine structure exists as a hydroxyl group and the two flanking nitrogen atoms at the 1 and 3 positions share the ionic charge in the six membered pi-resonance-stabilized ring.

Thus, while most organic acids are deprotonated by the ionization of a polar hydrogen-to-oxygen bond, usually accompanied by some form of resonance stabilization ( resulting in a carboxylate ion ), uric acid is deprotonated at a nitrogen atom and uses a tautomeric keto / hydroxy group as an electron-withdrawing group to increase the pK < sub > 1 </ sub > value.

In this reaction the CN < sup >−</ sup > ion is the nucleophile which attacks the partially positive carbon atom of the carbonyl group.

This intermediate ion rapidly reacts with H < sup >+</ sup >, such as from the HCN molecule, to form the alcohol group of the cyanohydrin.

In a given group, polarizability is more important in the determination of the nucleophilicity: The easier it is to distort the electron cloud around an atom or molecule the more readily it will react ; e. g., the iodide ion ( I < sup >−</ sup >) is more nucleophilic than the fluoride ion ( F < sup >−</ sup >).