Ion
An ion is an atom or group of atoms that has lost or gained one or more electrons. This produces a charged particle, which can be either positive or negative.
Normally, an atom is neutrally charged. When negatively charged electrons are lost from an atom the resultant ion is positively charged and it is called a cation. An ion that has gained electrons is negatively charged and called an anion. The name ion comes from the Greek ienai, meaning to go. This is related to their behavior during electrolysis where the ions move to the two opposite electrodes. Properties of ions are generally very different from the atoms from which they are derived.
The net charge on an ion is denoted by a superscript showing both the size and charge. A cation can have +1, +2, or +3, these cases are brought about by the loss of one, two, or three electrons respectively. An anion can be -1, - 2, or -3, and these are brought about by the addition of one, two, or three electrons respectively. In general, metal atoms and hydrogen tend to lose electrons and non- metal atoms (except hydrogen) tend to gain electrons. The commonest charge on a cation is +2 and the commonest charge on an anion is -1. Some cations can have different charges due to different numbers of electrons lost. For example, iron can be +2 or +3. This is usually written as iron (II) and iron (III).
When the number of electrons of an atom are altered, it alters the physical shape of the atom. A cation has a smaller size than the atom from which it is created. Normally, the electrons surround the nucleus of an atom and contained within the nucleus are the positively charged protons. In a neutral atom, the charge of the electrons balances the charge of the protons, but when a cation is produced, the charges are in an imbalance. There is a net positive charge--there are more protons in the nucleus than electrons surrounding it. The protons are able to attract the smaller number of electrons towards the nucleus more strongly because of this imbalance in charge. This physically reduces the size of the atom. With an anion, the reverse is true. The anion is physically larger than the atom from which it was created because the electrons are being held less tightly--the attractive forces of the protons are spread over a greater number of electrons.
When atoms lose or gain electrons, they are attempting to become like the noble gas closest to their position in the periodic table. Noble gases are very unreactive and they have a stable electron structure. By losing or gaining electrons, the atoms are trying to achieve the same level of stability.
A single atom that loses or gains electrons produces a simple ion. There are more complex ions possible where several atoms are involved. Polyatomic ions are groups of atoms that are positively or negatively charged. The hydroxyl ion (OH- ) is an example of a polyatomic anion.
When two atoms swap electrons to produce a cation and an anion, the two ions are attracted to each other. This is due to the fact that opposite charges attract each other and the anion and the cation are oppositely charged. The bond that is formed between the two ions is an ionic bond (sometimes also called an electrovalent bond), and the compound produced is an ionic compound. Since metals tend to produce cations and nonmetals tend to produce anions, ionic compounds are usually combinations between a metal and a nonmetal. This is actually an oversimplification of what occurs as there is no discrete ionic compound containing only one cation and one anion. What is actually produced is a three dimensional matrix of cations and anions bound together. These regular patterns have each ion surrounded by examples of the other ion. They are packed together in a regular arrangement called a giant structure or a lattice. This usually leads to a physical structure that is crystalline in nature. These strong interactive forces between the ions give certain characteristics to ionic compounds. Ionic compounds tend to be solids at room temperature; they have high melting and boiling points; they are hard substances; and they cannot conduct electricity as a solid, but they can conduct electricity as a liquid or in aqueous phase. This last characteristic is because as solids the ions are not free to move but as a liquid they are. In solution, the cations move towards the cathode and the anions move to the anode. This is the principal behind electrolysis. Once at their respective electrodes electrons are gained or lost to return the ion to the neutrally charged state. The practical result of this is that the anions are given off at the anode, generally in the form of a gas and the cations are often deposited at the cathode as a metal layer.
When an ionic bond is formed, energy is released to give a stable molecule. This is due to the ionization energy (the energy required to remove an electron from an atom or ion) of the cation being small and the electron affinity (the energy released when an electron is added to a neutral atom) of the anion being large. As a general rule if a cation is from the left-hand side of the periodic table and the anion is from the right-hand side then an ionic bond will be formed. When this occurs, the larger the charge on the ions, or the smaller the size of the ions, the stronger will be the ionic attraction, and hence, the stronger will be the ionic bond produced.
The ionic bond is the name of the bond formed between two oppositely charged ions. The cation and anion bonded together are known as an ion pair. The compound produced when such a bond is formed is an ionic compound. A process called ion exchange exists whereby ions in solution are exchanged for ions that are bound on the surface of a resin. The resin is an ion exchange resin. This technique can be used to change hard water into soft water. An ion can be formed by ionizing radiation. This is an energetic form of radiation which when it strikes an atom can cause the atom to lose an electron, producing an ion. The energy required to remove an electron form an atom is called the ionization energy. Ionic compounds are electrically neutral because the number of cations balances the number of anions and the charges cancel each other out. When the chemical formula of an ionic compound is written down, it is the empirical formula that is used. This is because in reality in an ionic compound the ions are surrounded by ions of the other type, producing a giant structure.
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