Anion

In order to answer the question, what is an anion, it is necessary to understand what is an ion first. In simple terms, an ion is a particle, atom or molecule or compound that has a net electrical charge. This electrical charge can be either positive or negative. Now, any particle, atom or molecule or compound that has a positive electrical charge is known as a cation. Similarly, any particle, atom or molecule or compound that has a negative electrical charge is called an anion. These electrical charges play a very important role in the physical and chemical properties of a particle, atom/molecule or compound. Hence, according to the anion definition chemistry of a particular substance is largely influenced by the negative electrical charge that it carries with itself.

Determination of Anion Charge

Having understood what is an anion, it is clear the charge of an anion is negative. By convention, the charge of an electron is considered negative. Hence, the anion charge is the property of the charge of an electron. In the case of a neutral atom or molecule or compound, the total number of negative and positive charges is the same because of the equal number of electrons and protons. But in the case of an ion, the net charge is non-zero because of the difference in the total number of electrons and protons. Thus, as per the anion definition, the total number of electrons in an anionic atom/molecule or a compound is more than the total number of protons. This ability of negative charge of anions leads to an attractive force in-between the anions and cations which also leads to the formation of ionic compounds. 

From the given explanation, it is easy to understand the question: how are anions formed. The anion charge is negative which is due to the excess of electrons as compared to the protons. Thus, for an anion to form one or more electrons are to be gained. They have typically pulled away from an atom, a molecule or a compound having a weaker affinity for them. In certain reactions, when electrons are released by atoms the free electrons can be readily absorbed by the atoms, molecules or compounds that are known to have a strong affinity for electrons. Once gained, the charge is represented as a negative symbol and the number of electrons gained becomes the value of the negative charge of the anion. An image given below explains how are anions formed:

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The primary reason for an atom or molecule or a compound accepting an electron and gaining a negative charge is to attain a stable electron configuration. Typically for an atom, the number of electrons required to complete an octet configuration i.e. 8 electrons in the outermost orbit is the reason why an atom becomes an anion by accepting electrons. This same concept applies to any of the formations of the anions and is the same even for the coordination compounds. This is also the primary reason for the formation of any of the ionic compounds. 

Properties of Anions and Differences With Cations

The anion is the ion that has more electrons than the protons. Most of the properties of anions are determined by the negative charge that it carries. 

Metallic atoms are the ones that hold some of their electrons that are relatively loose. As a result, they usually lose electrons and lead to the formation of cations. Opposite to this most of the non-metallic atoms typically attract the electrons more strongly than the metallic atoms and thus in the process gain a negative charge and become an anion. An example of anion formed from a non-metallic atom is that of chlorine anion. It is represented as Cl-. Along with the sodium cation, Na+, it forms an ionic bond and the product obtained is NaCl. This is the common table salt that is found in daily use. This happens as sodium loses one electron to get its octet configuration and this electron is accepted by the chlorine atom to complete the octet configuration with 8 electrons from the 7 electrons. 

The formation of negative ions is periodic. Based on the position in the periodic table, the electron configuration can help in determining the formation of the anion. Elements present in group 17 of the periodic table, known as halogens, have seven valence electrons in the outermost shell. These are the most readily forming anions in the periodic table as it is easier for them to gain an electron rather than lose an electron because of the attraction with the nucleus. Instead of losing electrons, they accept one electron and form an anion with one negative charge. Other non-metals such as oxygen, carbon and sulphur are also known to form an anion and out of the three oxygen and sulphur belong to group 16 and carbon belongs to group 12. Thus, as one goes from left to right in the periodic table the tendency to form anions generally increases because of the increase in the number of electrons in the valence shell. As the number of valence electrons increases, the electron shielding increases and the atomic size also generally decreases because of the increased attraction between the nucleus and the outermost electrons. These properties influence the formation and also the characteristics of anions as an atom, molecule or within a compound. 

Generally, the anions have opposite properties when compared to the cations. The cations are the ones that lose one or more electrons and gain positive electrical charge. This affects the general properties of the cations to be different and in some cases to be opposite of the anions. For example, during the reaction of an electrochemical cell, the anions are attracted to and are deposited on the electrode with positive charges known as the anode while the cations are attracted to the negative electrode known as cathode. Also, the cations are usually formed by metallic atoms whereas it is known that non-metallic atoms are the ones that transform into anions on accepting electrons. These differences later translate into the differences in their physical properties. In the end, it is common knowledge that the cations and anions are attracted towards each other owing to their net opposite electric charge. 

Thus, the conclusion from the given article is that an anion is a particle, atom, molecule or compound that has a net negative electric charge because of electrons or negatively charged particles being more in number as compared to protons or positively charged particles. And this aspect of an anion is more important in influencing its various properties and chemical reactions.