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Chemical Bonds

All known elements do not exist in free nature due to their reactivity with various reagents. Only few elements which is placed in last group of periodic table has completely filled octet configuration and also termed as noble gases.

All other elements have tendency to attain octet configuration by formation of compounds. Electrons, especially those of the outermost valence electronic shell, play a fundamental role in chemical bonding. In some cases chemical bonding results from the transfer of one or more electrons from one atom to another. This leads to the formation of positive and negative ions.

The bond is known as ionic bond. In other cases chemical bonding results from a mutual sharing of electrons between atoms. This leads to the formation of molecules having a bond called a covalent bond.
The transfer or sharing of electrons occurs to the extent that each atom involved acquires an especially stable electron configuration. Often this configuration is that of a noble gas (ns2np6), that is, involving eight outer-shell electrons called an octet.

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What is a Chemical Bond?

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"The process of combination involves the reunion of two or more atoms through redistribution of electrons in their outer shells either by the process of sharing of electrons amongst themselves so that all the atoms acquire the stable noble gas configuration of minimum energy, is called as chemical bond."

Types of Chemical Bond

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On the basis of transfer of electrons; chemical bonds can be different types.
  1. Ionic bond
  2. Covalent bond
  3. Metallic bond
  4. Van der wall force
  5. Coordinate bond

Ionic bond

An ionic bond is formed by the complete transfer of one or more electrons from the valence shell of an atom to the valence shell of the other atom. Hence in ionic bond there is complete transfer of electron from one atom to another. The atom which loses the electron and gets positive charge is known as cation, while another anion termed as anion, getting negative charge.

The electrostatic force of attraction between oppositely charged ions results in the formation of ionic bond or electrovalent bond. Compounds involve in ionic bond are known as ionic compound or electrovalent compound. Generally those atoms involve in ionic bond formation, which have either high electronegativity or high electro positivity.

For example: Sodium atom lose its valence shell electron and form sodium ion (Na+), while chlorine accept one electron to attain octet, and form chloride ion (Cl-). The electrostatic force of attraction between sodium ion and chloride ion is known as ionic bond.

NaCl Ionic Bond
Ionic compounds have high melting point and boiling point due to strong attraction of force between ions. They are hard and good conductor of electricity.

Covalent bond

This type of bond formed between two non-metals. When two same or different atoms shared their valence electron to attain the noble gas configuration, it’s known as covalent bond. For example; for attaining the nearest noble gas configuration, each hydrogen atom share its valence electron with other hydrogen atom and form a covalent bond in hydrogen molecule.
Hydrogen Bonding

Two hydrogen atoms come close to each other, the nuclei of one will attracted by electrons of other and vice. Formation of any chemical bond involves decreasing in energy level. When two atoms approach towards each other, new attraction and repulsion forces set in. If the net result is attraction force; the total energy of the system decreases and chemical bond formed. If the net result is repulsion force, the total energy of system will increases and no chemical bond is possible.

For example: when the two hydrogen atoms approaches towards each other, the potential energy of the system decreases because of the force of attraction. At a particular distance between atom nuclei, the value of potential energy reaches a minimum. When the forces of attraction and repulsion balance each other and the potential energy reached to minimum, covalent formed in molecule.

Due to decreasing energy the covalent bond formed in molecule. The decrease in potential energy continues till a certain minimum value and the formation of molecule takes places at minimum energy value in potential energy curve. At the minimum energy level, there is a minimum distance between nuclei and this minimum distance is called as bond length.

Potential Energy Curve H2 Molecule

Hence the 1s orbital of two hydrogen atoms overlapped two form covalent bond. When a covalent bond forms due to the overlapping of orbitals along the same axis, it’s known as sigma bond. This type of overlapping takes place to substantial extent and the resulting bond is very strong covalent bond.

Covalent bond can be formed by overlapping along same axis and by the side way overlapping of orbitals. The overlapping along axis forms sigma bond whiles the sidewise overlapping of half filled orbitals form pi bond. Out of these two bonds, sigma bond is stronger than pi bond, hence pi bond always exist with sigma bond.

Single Covalent Bond

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In a compound all atoms bonded with sigma covalent bond only, than these bonds are termed as single covalent bond.
For example: Ethane (C2H6), in which each carbon form one sigma covalent bond with another carbon atom and three sigma covalent bonds with three hydrogen atoms.

All are sigma covalent bond formed by axial overlapping of sp3 hybridized orbitals of carbon atom. If a carbon atom forms only single covalent bond, it’s hybridization will be sp3. Four sp3 hybridized orbital arranged in tetrahedral manner with 109°28’ bond angel and overlap with 1s orbital of three hydrogen atom and one sp3 orbital of another carbon atom.

Double Covalent Bond

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  1. When two pairs of electrons are shared between the atoms rather than just one pair, it formed double covalent bond.
  2. There is one sigma and one pi bond in a double covalent bond. The double covalent bond is shown by two lines joining the bonding atoms where each line represents one pair of shared electrons.

Double Covalent Bond Examples

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The simplest example of double covalent bond is oxygen molecule (O2) where two oxygen atoms attains the octet configuration by sharing two pairs of electrons with each other.

Double Covalent Bonding

Another example of double covalent bond is carbon dioxide in which center carbon atom is sp hybridized and form two double covalent bond with two oxygen atoms.

CO2 Covalent Bonding
Apart from carbon dioxide; ethene is another example of double covalent bond with carbon atom.
Ethene contains one carbon –carbon double covalent bond and four single covalent bonds between carbon and hydrogen atoms.

Carbon atom with 1s2, 2s2, 2px1, 2py1 electronic configuration needs to promote one of the 2s2 pair into the empty 2pz orbital for making four unpaired electron required for the formation of four covalent bonds.

In ethene each carbon is joined with three other atoms rather than four. Hence the carbon atoms hybridize by using their valence orbitals before forming bonds but only hybridize three of the orbitals rather than all four. So each carbon atom use the 2s electron and two of the 2p electrons and leave the other 2p electron unchanged.

Ethene Bonding

These new sp2 hybrids arrange themselves in trigonal planer geometry with 120° bond angle. The unhybridized p- orbitals on both carbon atoms exist at right angles to the hybridized orbitals.

These unhybridized orbitals overlapped in sideways manner and form a molecular orbital, but of a different from sp2 orbitals. In this one the electrons are held above and below the plane of the molecule results in the formation of a pi bond.

Ethene Molecule

Triple Covalent Bond

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A covalent bond formed with three pairs of electron is called as triple covalent bond. It represented by three straight lines between bonded atoms. Out of three bonds, one is sigma and another two are pi bonds. Since the presence of pi bonds make the molecule unsaturated, hence molecules with triple bonds show additional reactions. They show positive test with Bayer reagent and decolorize the bromine water solution.

Triple Covalent Bond Example

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There are many molecule consist triple bond between two bonding atoms. For example; nitrogen molecule, in which each nitrogen atom has five electrons in its valence shell and required three electrons to complete the octet. Hence each nitrogen atom can form three bonds results in the formation of nitrogen molecule with one triple covalent bond.

Nitrogen Molecule Triple Covalent Bond
Another example of triple covalent bond is ethyne molecule which consist a triple bond between two carbon atoms. In this molecule, each carbon atom is Sp-hybridized and generates two sp-hybrid orbitals. These hybrid sp orbitals are arranged in linear geometry and with 180o bond angle.

Remaining two py and pz un hybridised orbitals of each carbon atom which lie perpendicular to the plane of sp-orbitals involve in the formation of pi-bond. Out of two sp-hybrid orbital of each carbon atoms , one overlaps with 1s orbital of hydrogen to produce one sigma bond between Carbon and Hydrogen atoms.

While the other sp-orbital overlaps with another sp orbital of next carbon atom to form sigma bond with another carbon atom. The triple covalent bond is quite shorter than single and double covalent bonds with 1.09Å of carbon –hydrogen bond length and 1.2Ao C-C bond length. The bond angle is 180° with linear geometry.
Ethyne Molecule

Single Double and Triple Covalent Bonds

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Covalent bonds can be three type depends upon the number of electrons involve on bond formation. Each type of covalent bond shows unique characteristics.
  • Single covalent bond - 1 shared pair or two electrons. For example; fluorine molecule (F:F or F-F or F2) , other examples are H2, F2, I2 and other diatomic molecules.
  • Double covalent bond - 2 shared pairs or four electrons. For example; oxygen molecule ( O::O or O=O or O2), other examples are O2, CO2 and HCHO.
  • Triple covalent bond - 3 shared pairs or six electrons. For example; Nitrogen molecule ( :N:::N:, N=N or N2) , ethyne (C2H2 ).
  • As the number of bond increases, bond energy increases. Hence the increasing order of bond energy is, Single < Double < Triple covalent bond
  • However the bond length decreases with increasing the number of bonds. Hence the decreasing order of bond length is, Single > Double >Triple covalent bond

Three Types of Covalent Bonding
More topics in Chemical Bonds
Polar Covalent Bond Carbon Carbon Bond Length
Carbon Bonding Bond Angles
Bond Length Tetrahedral Bond Angles
Ionic and Covalent Compounds Types of chemical Bonds
Nonpolar Covalent Bond
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