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Chemical Bonds

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The first theory of chemical bonding was given by G.N.Lewis in 1916. According to him a chemical bond is formed by sharing a pair of electrons between the two nuclei. Only a few elements, particularly the noble gases exist as individual atoms; most atoms are joined by chemical bonds to other atoms.

When atoms interact through the sharing, loss or gain of electrons to form molecules and chemical compounds many attain an octet of outer shell electrons. This tendency is the basis of the octet rule of chemical bonding. The placement of electrons in its orbitals determines the chemical behavior of an atom.

Definition

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"A chemical bond results in when the two electrons are paired. For a stable bond formation, it is essential that the two electrons must have opposite spins."

In order for an atom to enter into chemical combination, it must possess one or more unpaired electrons. The valency of a species is thus determined by the number of unpaired electrons which it possesses. Although participation in bond formation by pairing electrons is formally ruled out, such electrons may be involved if they can be unpaired without the expenditure of excessive energy.

Types 

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When two atoms of the same element or different elements, combine together, both change into more stable systems by the redistribution or reorientation of their valence electrons. The electrons in their outer shells. This process is called chemical bonding.

Atoms may achieve stable electronic configurations in either of the following two ways.
  • By the transfer of one or more electrons from the outer shell of one atom to the outer shell of another atom. This phenomenon is called electrovalency, and the bond thus established is known as an electrovalent bond or ionic bond.
  • By sharing electrons in the outer shell in such a manner that the shared electrons produce completed orbitals in each one of the two atoms. When the shared electrons are contributing equally by the two combining atoms, the phenomenon is referred to as covalency and the resulting chemical bond is called a covalent bond. When the shared electrons are contributed entirely by one of the atoms, the phenomenon is referred to as coordinate or dative covalency and the bond established as a result of it is called a coordinate bond.

Formation

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Chemical bonds are formed when sub orbitals of the outermost tier of electrons of adjacent atoms overlap in order to form a shared electron pair. Carbon's four single electrons in each of its four maximally spaced sub orbitals give carbon the ability to form bonds at four maximally and symmetrically spaced angles. Thus the simplest organic molecule is methane formed from single carbon atom with four hydrogen atoms each donating an electron to complete the electron pair in each of carbons four outermost sub orbitals. Oxygen with only two unpaired electrons, binds two hydrogen atoms to fill out electron pairs in all of its outer sub orbitals to form water.

Ionic bonds are formed by the attraction of positive ions to negative ions. When an atom gives up an electron, it becomes positively charged. Negatively charged electrons are acquired when an atom gains an electron. These charged particles are called ions and the bonds formed from these processes are called ionic bonds. The ionic or electrostatic attraction between sodium and chloride atoms are shown below.

Ionic Compound

In contrast to electrostatic or ionic bond formation, covalent bonds are formed when atoms of elements share electrons. Carbon shares electrons with hydrogen atoms when methane (CH4) is formed. The model of carbon and hydrogen elements in a chemical reaction is shown below.

Covalent Bond

Nature 

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The chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species.

The fundamental idea of nature of chemical bond is summarized below.
  1. In order for an atom to enter into chemical combination, it must possess one or more unpaired electrons. The valency of a species is thus determined by the number of unpaired electrons which it possesses.
  2. Although participation in bond formation by paireing electrons is formally ruled out, such electrons may be involved if they can be unpaired without the expenditure of excessive energy.
  3. A chemical bond results in when the two electrons are paired. For a stable bond formation, it is essential that the two electrons must have opposite signs.

Weak Chemical Bonds

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The formation of chemical bond releases energy where bond formation is exothermic. If weak chemical bonds within reactants are replaced by stronger chemical bonds within products, then the overall chemical reaction is exothermic. Weak chemical bonds are less stable and therefore have greater potential energy.

For example, a weak chemical bond formed when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom called hydrogen bond. Among the weak bonds, there is a distinction between polar bonds or hydrophilic bonds and nonpolar or hydrophobic bonds.
  • Weak polar bonds are electrostatic and correspond to simple attractions between dipoles in compounds or molecules with inhomogeneous or polarizable charges.
  • Weak nonpolar bonds or hydrophobic bonds are formed by repulsion. In a polar liquid the molecules try to establish a maximum number of bonds between each other.

Strongest Chemical Bonds

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The physical processes underlying attractive interactions between atoms, elements and molecules are termed chemical bonding.Strong chemical bonds are associated with the sharing or transfer of electrons between bonding atoms and such bonds hold bio molecules together. Bond strength depends on certain factors and so called covalent bonds and ionic bonds are generally categorized as strong bonds.

Carbon-carbon bonds are particularly strong and stable covalent bonds. The major organic elements have standard bonding capabilities C, N and P can form up to four covalent bonds with other atoms O and S can form two and H can only form one. Ionic bonds are the strongest of all chemical bonds and are the basis of ionic lattices composed of alternate anions and cations held firmly in position by the attraction of opposite charges.

Single Covalent Bond

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A single covalent bond is formed by the sharing on one pair of electrons that is 2 electrons between two atoms each atom contributing one electron each for sharing. The shared electron pair is always shown between the two atoms. For example, a hydrogen molecule H2 containing a single covalent bond is represented as H:H, the two dots drawn between the hydrogen atoms represent a pair of shared electrons which forms a single covalent bond.

A single covalent bond is also represented by a short line (-) between the two atoms. Hydrogen molecule can thus also be written as H-H. A hydrogen atom has one electron in its only shell, to attain nearest noble gas stable electronic configuration the two hydrogen atoms share mutually one electron from each of them and form a single covalent bond.

Hydrogen Atom

Double Covalent Bond

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The covalent bond formed by the sharing of two pairs of electrons between two atoms is known as double covalent bond. The oxygen atom gets these electrons by mutually sharing its two electrons with the two electrons of another oxygen atom. So two oxygen atoms share two electrons each and form a double covalent bond.

Oxygen Molecule

Covalent bonds can be viewed as behaving like rubber bands between two atoms. As a result the bonds can undergo stretching, twisting and bending. Like the rubber band a covalent bond can be stretched beyond its elastic limit at which point it breaks and the two atoms are no longer bonded to each other.

Triple Covalent Bond

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When an atom shares three valence electrons with each other or other atom, triple covalent bond is formed. For example, the atomic number of nitrogen is 7 and its electronic configuration is 2, 5. This means nitrogen has 5 electrons in its outermost shell. So nitrogen needs three more electrons to complete the octet and attain configuration of the nearest noble gas and become stable. Thus, two nitrogen atoms combine together by sharing three pairs of electrons to form a triple bond between two nitrogen atoms.

Nitrogen Molecule

Since the nitrogen atoms share three pairs of electrons among themselves, the bond between them is called a triple covalent bond. The strength of a given bond depends on its bond energy. The strength of a bond is proportional to its bond energy value. Thus the strength of various types of bonds will be in the following order.
Triple bond > Double bond > Single bond