- Formation of an anion
- Formal charges and fewer links
- Reductions
- Physical
- characteristics
- Types
- Monatomic
- Oxoanions
- Organic
- Polyatomic
- Molecular or complex
- References
An anion is any chemical species with a negative charge, in addition to being one of the two types of existing ions. Its negative charge originates from the fact that it has an excess of electrons compared to the neutral form of the species; For each additional electron, its negative charge increases by one.
The negative charge can be located on one or more atoms, as well as its influence on a molecule as a whole. For simplicity, regardless of where the (-) charge is, the entire species, compound, or molecule is considered an anion.
Anions. Source: Gabriel Bolívar.
If a neutral species X gains an electron, the excess negative charges will manifest itself in the formation of the anion X -, which will also lead to an increase in its atomic radius (top image, with green spheres). X and X - differ greatly in their properties and in the way they interact with their environment.
If X is now assumed to be the H atom, for example, a cation or anion may arise from it: H + or H -, respectively. The cation H + is the hydrogen ion, also called a proton; and H - is the hydride anion, the "simplest" of all known anions.
Formation of an anion
The formation of an anion can be easily explained within theory; although, experimentally, it can be a challenge, especially if you want it pure, without positive charges attracted by its negative charges.
Formal charges and fewer links
An anion will form when there is an excess or gain of electrons in an atom. Said gain can be determined by making use of the formal charges in a Lewis structure. Also, by means of the previous method it is possible to know exactly from which atom or group of them the negative charge comes.
When the atoms form covalent bonds, even if the distribution of electrons is equal, there may be a partial loss of electrons. In this sense, the fewer bonds the more electronegative atoms form, the more free pairs of electrons they will have and therefore exhibit negative charges.
Consider for example the molecule of ammonia, NH 3. NH 3 is neutral and therefore has no electrical charges. If an H were removed, that is, an NH bond was broken, the anion NH 2 - would be obtained. By drawing its Lewis structure and calculating the formal charge on N, this can be verified.
Following the breaking of more NH bonds, we now have the anion NH 2-; and eliminating the last H, the anion N 3- is finally obtained, called nitride anion. Nitrogen no longer has a way to gain more electrons, and its -3 charge is the most negative it can reach; their orbitals do not give for more.
Reductions
An anion can be formed as a result of a reduction: it gains electrons by oxidizing another species, which loses them. Oxygen, for example, represents this type of chemical reaction very well.
When oxygen is reduced, it oxidizes to another species, and it becomes the oxide anion, O 2-; present in innumerable minerals and inorganic compounds.
Physical
An atom can gain electrons if it is in the gas phase:
X (g) + e - => X - (g)
This way of forming an anion implies a wide knowledge of physical techniques, while gaseous anions are not easy to study, nor are all species easily volatilized or atomized to the gaseous phase.
characteristics
In general terms, the typical characteristics of an anion will be mentioned below before discussing their types and examples:
-It is more voluminous than the neutral atom from which it derives.
-It can be more or less stable, despite the increasing electronic repulsion between its own electrons.
-If the anion comes from a low electronegative atom, such as carbon, it is very reactive.
-Establish strong dipole moments.
-It increases its interaction with polar solvents even more.
-The monatomic anion is isoelectronic to the noble gas of its period; that is, it has the same number of electrons in its valence shell.
-It can polarize the electronic cloud of a neighboring atom, repelling its external electrons.
Types
Monatomic
As its name suggests, it is an anion consisting of a single atom: the negative charge is well localized. Each group in the periodic table has characteristic negative charges; and because they are anions, they are the non-metals located in the p block. Some examples and their names are below:
-Cl -, chloride.
-I -, iodide.
-F - fluoride.
-Br -, bromide.
-O 2-, rust.
-S 2-, sulfide.
-I know 2-, selenide.
-Te 2-, telluride.
-Po 2-, poloniuro.
-N 3-, nitride.
-P 3-, phosphide.
-As 3-, arsenide.
-Sb 3-, antimoniuro.
-C 4-, carbide.
-If 4-, silicide.
-B 3-, boride.
Oxoanions
Oxoanions are characterized by having an X = O bond, where X can be any non-metallic element (except fluorine), or a metal (chromium, manganese, ect.). They can also have one or more XO simple links.
Some oxoanions with their respective names are:
-ClO -, hypochlorite.
-BrO -, hypobromite.
-IO -, hypoiodite.
-ClO 2 -, chlorite.
-ClO 3 -, chlorate.
-IO 3 -, iodate.
-ClO 4 -, perchlorate.
-PO 4 3-, phosphate.
-CO 3 2-, carbonate.
-CrO 4 2-, chromate.
-Cr 2 O 7 2-, dichromate.
-SO 4 2-, sulfate.
-S 2 O 3 2-, thiosulfate.
-NO 3 -, nitrate.
-NO 2 -, nitrite.
-BO 3 3-, borate.
-AsO 4 3-, arsenate.
-PO 3 3-, phosphite.
-MnO 4 -, permanganate.
Organic
Organic molecules have functional groups that can become electrically charged. How? By forming or breaking covalent bonds, very similar to the example of the NH 3 molecule.
Some organic anions are:
-CH 3 COO -, acetate.
-HCOO -, format.
-C 2 O 4 2-, oxalate.
-RCOO -, carboxylate.
-CH 3 CONH -, amidate.
-RO -, alkoxide.
-R 3 C -, carbanion.
-CH 3 O -, methoxide.
Polyatomic
Oxoanions are also polyatomic anions, that is, they consist of more than one atom. The same is true for organic anions. However, polyatomics do not fall into any of the above classifications. Some of them are:
-CN -, cyanide (has a triple bond, C≡N).
-OCN -, cyanate.
-SCN -, thiocyanate.
-NH 2 -, Amide.
-OH -, hydroxyl, hydroxide or hydroxyl.
-O 2 -, superoxide.
-O 2 2-, peroxide.
Molecular or complex
In organic anions some negatively charged functional groups were mentioned. These groups can be part of a large molecule, and thus the anion can be a whole robust compound with many bonds. The simplest of this type of anions is the hypothetical H 2 - molecule.
Another example of these anions are polysulfides, S n 2-, which consist of chains with multiple SS bonds. Also, negatively charged metal coordination compounds such as - and 2- can be counted.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Wikipedia. (2019). Anion. Recovered from: es.wikipedia.org
- Helmenstine, Anne Marie, Ph.D. (March 21, 2019). Common Anions Table and Formulas List. Recovered from: thoughtco.com
- CK-12 Foundation. (June 29, 2016). Anion formation. Chemistry LibreTexts. Recovered from: chem.libretexts.org
- Francis E. (2002). Anions. Clackamas Community College. Recovered from: dl.clackamas.edu
- American Physical Society. (November 3, 2011). Synopsis: The Simplest Molecular Anion. Recovered from: physics.aps.org