Definition - What does Anion mean?

An anion is an ionic species having a negative charge. They are atoms that have gained electrons.

Anions are one of the two types of ions. The other type is called a cation. Anions are attracted to the anode, while cations are attracted to the cathode.

The concept of an anion is important in the corrosion industry because certain anions facilitate corrosion. When metals, such as mild steel, corrode in an aqueous solution, it is commonly accepted that certain electrolytes, notably chlorides, sulphates, etc., are corrosive. Exposure of reinforced concrete to chloride ions is the primary cause of premature corrosion of steel reinforcement. The risk of corrosion increases as the chloride content of concrete increases. Pitting occurs in stainless steels in neutral or acid solutions containing halides, primarily chlorides (CI- ), for example seawater.

Corrosionpedia explains Anion

Anions are ions with a net negative charge. Examples of anions include:

  • Hydroxide anion: OH-
  • Oxide anion: O2-
  • Sulfate anion: SO42-

Since they have more electrons than protons, anions have a negative charge. When the chloride anion is represented with Cl-, the "-" charge indicates that it has one less proton than the total number of electrons.

Electrons, due to their smaller mass and thus larger space-filling properties as matter waves, determine the size of atoms and molecules that possess any electrons at all. Thus, anions are larger than the parent molecule or atom, as the excess electron(s) repel each other, and add to the physical size of the ion, because its size is determined by its electron cloud. As such, in general, cations are smaller than the corresponding parent atom or molecule due to the smaller size of its electron cloud.

The electrostatic attraction between the positives and negatives brings the particles together and creates an ionic compound, such as sodium chloride. An anion is formed through the gain of electrons, called ionic bonding. Metals lose electrons, making them positive cations. Nonmetals gain electrons, making them negative anions. Nonmetals generally are more electronegative than metals, meaning that they have a stronger pull on their electrons. Thus, when they form ions, metals give up electrons and nonmetals take up electrons.

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