- How is a cation formed?
- Formal uploads and more links
- Oxidation
- Differences with anion
- Examples of the most common cations
- Monatomic
- Polyatomic
- References
A cation is a chemical species that has a positive charge. It forms together with the anion the two types of existing ions. Its charge is the product of a deficiency of electrons in the atom, which causes the protons in the nucleus to exert a greater attraction. For every electron that a neutral atom loses, the positive charge increases by one unit.
If an atom loses an electron, and therefore the number of protons is greater than one, then its positive charge will be +1; if you lose two electrons, the charge will be +2, and so on. When a cation has a +1 charge, it is said to be monovalent; on the other hand, if said charge is greater than +1, the cation is said to be polyvalent.
Hydronium ion, one of the simplest cations. Source: Gabriel Bolívar.
The image above shows the cation H 3 O +, called the hydronium ion. As can be seen, it barely has a charge of +1, consequently being a monovalent cation.
Cations are important species, as they exert an electrostatic force on their environment and the molecules around them. They present a high interaction with water, a liquid that hydrates and transports them in humid soils, to later reach the roots of the plants and be used for their physiological functions.
How is a cation formed?
It was mentioned that when an atom loses an electron, its greater number of protons, in relation to electrons, exerts an attractive force that translates into a positive charge. But how can the loss of the electron occur? The answer depends on the transformation that occurs in chemical reactions.
It should be noted that the presence of a positively charged atom does not necessarily imply the formation of a cation. For it to be considered as such, there must not be an atom with a negative formal charge that neutralizes it. Otherwise, there would be attraction and repulsion within the same compound and it would be neutral.
Formal uploads and more links
Electronegative atoms attract electrons from their covalent bonds to them. Even if the electrons are shared equally, there will come a point where they will partially have fewer electrons than in their basal configuration; this is, that of its free atoms without being bound to other elements.
Then, these electronegative atoms will begin to experience a deficiency of electrons, and with it, the protons of their nuclei will exert a greater force of attraction; the positive formal charge is born. If there is only one positive formal charge, the compound will manifest an overall positive ionic charge; thus the cation is born.
The oxygen atom of the cation H 3 O + is a faithful example of the foregoing. By having three OH bonds, one more than in the water molecule (HOH), it experiences the loss of an electron from its basal state. Formal charge calculations allow you to determine when this happens.
If the formation of another OH bond is assumed for a moment, the divalent cation H 4 O 2+ will be obtained. Note that the divalent charge on top of the cation is written as follows: number followed by the '+' symbol; in the same way we proceed with the anions.
Oxidation
Metals are cation formers par excellence. However, not all of them can form covalent (or at least purely covalent) bonds. Instead, they lose electrons to establish ionic bonds: a positive charge attracts a negative one, held together by physical forces.
Therefore, metals lose electrons to go from M to M n +, where n is usually equal to the number of its group on the periodic table; although n can take several integer values, which is especially the case with transition metals. This loss of electrons takes place in a type of chemical reaction called oxidation.
Metals oxidize, lose an electron, the number of protons in their atoms exceeds that of electrons, and consequently exhibit a positive charge. For oxidation to occur, there must be an oxidizing agent, which reduces or gains the electrons lost by metals. Oxygen is the best known oxidizing agent of all.
Differences with anion
Contraction of the atomic radius in a cation. Source: Gabriel Bolívar.
The differences between a cation and anion are listed below:
-The cation in general is smaller than the anion. The image above shows how the atomic radius of Mg is reduced by losing two electrons and becoming the cation Mg 2+; the opposite occurs with anions: they become more voluminous.
-It has more protons than electrons, while the anion has more electrons than protons.
-Being smaller, its charge density is higher, and therefore it has a greater polarizing power; that is, it deforms the electron clouds of neighboring atoms.
-A cation moves in the same direction as the applied electric field, while the anion moves in the opposite direction.
Examples of the most common cations
Monatomic
The monatomic cations come mostly from metals (with certain exceptions, such as H +). Of the rest, it is extremely rare to consider a cation derived from a non-metallic element.
It will be seen that many of them are di- or polyvalent, and that the magnitudes of their charges agree with the number of their groups in the periodic table.
-Li +
-Na +
-K +
-Rb +
-Cs +
-Fr +
-Ag +
They all have in common the charge '1+', which is written without the need to enter the number, and also come from group 1: the alkali metals. Besides, there is the Ag + cation, one of the most common of the transition metals.
-Be 2+
-Mg 2+
-Ca 2+
-Sr 2+
-Ba 2+
-Ra 2+
These divalent cations are derived from their respective metals belonging to group 2: the alkaline earth metals.
-At 3+
-Ga 3+
-In 3+
-Tl 3+
-Nh 3+
Trivalent cations of the boron group.
So far the examples have been characterized as having a single valence or charge. Other cations exhibit more than one valence or positive oxidation state:
-Sn 2+
-Sn 4+ (tin)
-Co 2+
-Co 3+ (cobalt)
-Au +
-Au 3+ (gold)
-Fe 2+
-Fe 3+ (iron)
And other metals, like manganese, can have even more valences:
-Mn 2+
-Mn 3+
-Mn 4+
-Mn 7+
The higher the charge, the smaller and more polarizing the cation.
Polyatomic
Without going into organic chemistry, there are inorganic and polyatomic cations that are very common in daily life; such as:
-H 3 O + (hydronium, already mentioned).
-NH 4 + (ammonium).
-NO 2 + (nitronium, present in nitration processes).
-PH 4 + (phosphonium).
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Helmenstine, Anne Marie, Ph.D. (May 05, 2019). Cation Definition and Examples. Recovered from: thoughtco.com
- Wyman Elizabeth. (2019). Cation: Definition & Examples. Study. Recovered from: study.com
- Dummies. (2019). Positive and negative ions: cations and anions. Recovered from: dummies.com
- Wikipedia. (2019). Cation. Recovered from: es.wikipedia.org