Divalent refers to an atom or ion with valence two, meaning it has two valence electrons to form bonds. Divalent ions are important in biological systems and can be found in group 2 of the periodic table. Calcium and magnesium cations play important roles in biological processes.
In chemistry, divalent is an older term for bivalent, meaning an atom or ion with valence two. Valence refers to the number of covalent bonds the atom or ion can form with other atoms. Covalent bonds form through the sharing of electrons in the outermost valence shell or orbit of an atom, so a divalent atom or ion has, by definition, two valence electrons with which to form bonds. Divalent anions and cations – ions with a negative and positive charge, respectively – are important in many biological systems, including human metabolism.
Electrons can be visualized as orbiting the nucleus of an atom in a series of shells, each of which can hold only a certain number of electrons before being filled. Shells closest to the core are filled first. When atoms react with each other, they have a tendency to bond in such a way that their outermost electron shell, or valence, is filled. In almost all cases, a complete valence shell consists of eight electrons, the same valence configuration as elements in the noble gas family, the last vertical column of the periodic table of elements. The tendency of atoms to gain or lose electrons from other atoms to achieve this stable valence octet is known as the octet rule.
Divalent elements have two valence electrons, so the easiest way to get a full octet valence shell is to lose those two outer electrons, leaving the underlying shell full instead. For example, when magnesium, which has two valence electrons, bonds with oxygen, which has six valence electrons, the oxygen atom gains the two electrons from the magnesium to fill its octet. This forms the compound MgO, magnesium oxide.
In the periodic table, chemical elements with the same valence electron configuration are displayed in the same group or vertical column. Members of each group tend to react chemically in a similar way due to their common number of valence electrons. Elements with two valence electrons are found in group 2 and include beryllium, magnesium, calcium, strontium, and barium. Some transition metals, such as iron and manganese, can exist in this form as well.
Divalent ions are also often important in chemistry. When an atom loses its two valence electrons but does not bond covalently, it becomes a positively charged ion, or cation. Similarly, an atom that gains electrons from its neutral state is called an anion. For example, sulfide, a divalent anion, has a negative charge of two: it has two more electrons than protons. Magnesium and calcium, which commonly form divalent cations, each have a positive charge of two.
Calcium and magnesium cations are often found in hard waters, waters with a high mineral content. They also play an important role in biological processes. Calcium cations, for example, bind to proteins in cells and act as messengers in signal transduction, in other words they stimulate events within the cell. They are also critical as a structural component of plant cell walls and in activating muscle contraction in animals.
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