Covalent Bond Definition
The elements of Group-13 to Group-17 when permitted to react with each other, they form a chemical bond by mutual sharing of their valence shell electrons. This type of bond, which is formed due to the mutual sharing of electrons, is called a covalent bond.
The energy modifications during the covalent bond development are of substantial worth. When two atoms approach each other, attractive forces develop in between electrons of one atom and the nucleus of the other atom. All at once, repulsive forces between electrons of the two atoms as well as between their nuclei are also developed.
When the attractive forces dominate due to a decrease in distance between those two atoms, a chemical bond is formed in between them. The formation of hydrogen, chlorine, nitrogen, and oxygen gases are a few examples of this kind of bonding.
Types of covalent bonds
The covalent bond is formed by the shared of electrons between two atoms. The electrons that pair to form a chemical bond are called ‘bond pair’ electrons. Depending upon the number of bond pairs, a covalent bond is classified into the following three types:
Single Covalent bond
When one electron is contributed by each bonded atom, one bond pair is formed and it forms a single covalent bond. While drawing the structure of such molecules the single bond pair is shown by a line between those two atoms (___). A few examples of molecules with single covalent bonds are hydrogen (H2), chlorine (cl2), hydrochloric acid (HCl), etc.
Double Covalent bond
When each bonded atom contributes two electrons, two bond pairs are shared and a double covalent bond is formed. These bond pairs are indicated as a double line between those atoms in the structure of such molecules. The particles like oxygen (O2)gas and ethene (C2H4) reveal such type of double covalent bonds.
Triple Covalent Bond
When each bonded atom contributes three electrons, three bond pairs are associated with bond development. This type is called a triple covalent bond. Three small lines are utilized to show these three pairs of electrons in between those atoms in the molecules of such compounds. The examples of molecules having triple covalent bonds are nitrogen (N2) and ethyne (C2H2).
By this mutual sharing of valence shell electrons, each of the contributing atoms attains the ‘Octet’ or nearest noble gas electronic configuration.
Dative Covalent or Coordinate Covalent Bond
Coordinate covalent or dative covalent bonding is a type of covalent bonding in which the bond set of electrons is donated by one bonded atom only. The atom which contributes the electron pair is called the donor and the atom which accepts the electron pair is called the acceptor. A little arrow is generally used to suggest the atom and set of electrons being donated. The head of the arrow is towards the acceptor atom.
The non-bonded electron pair available on an atom, like the one available on nitrogen in ammonia, (NH3) is called a lone pair. When a proton (H+) approaches a molecule with an only set of electrons, that only set is contributed to H+, and a coordinate covalent bond is formed, e.g. formation of ammonium radical (NH4+).
In the formation of BF3 (boron trifluoride) molecule, three valence electrons of boron atom (Z= 5) pair with three electrons, one from each 3 fluorine atoms. The boron atom even after this sharing of electrons (covalent bond formation), remains short or deficit of 2 electrons in its outer shell. Now if a molecule with a lone pair approaches this molecule, it accepts a lone set from that donor and forms a coordinate covalent bond. The only pair on the nitrogen of the ammonia molecule makes it a good donor molecule to form a coordinate covalent bond.
Polar and Non-polar Covalent Bond
If a covalent bond is formed between two similar atoms (homo-atoms), the shared pair of electrons is attracted by both the atoms equally. Such kind of bond is called a nonpolar covalent bond. These bonds are formed by equal sharing of electron set between the two bonding atoms. This type of bond is called a pure covalent bond. For instance, bond formation in H2 and CI2.
If the covalent bond is formed between two different kinds of atoms (heteroatoms) then the bond pair of electrons will not be brought in similarly by the bonded atoms. One of the atoms will attract the bond pair of electrons more strongly than the other one. This atom(element) will be called as more electronegative.
When there is a difference of electronegativity between two covalently bonded atoms, there will be an unequal attraction for the bond pair of electrons between such atoms.
It will result in the development of a polar covalent bond. The distinction between the electronegativities of hydrogen and chlorine is 1.0. As the electronegativity of chlorine is more, it draws in the shared pair of electrons towards itself with a greater force. A partial negative charge is therefore produced on chlorine and in turn a partial positive charge on hydrogen due to electronegativity difference. It develops polarity in the bond and is called polar covalent bond.
The delta () sign shows partial positive or partial negative charge that is developed due to unequal sharing of shared pairs or bonded pairs of electrons. The substances resulting from polar covalent bonds are called polar compounds. For example: water, hydrogen fluoride, and hydrogen chloride.
By utilizing electronegativity values, it is possible to anticipate whether a chemical bond will be ionic or covalent in nature. A bond formed between elements of high electronegativity (halogen group) and components of low electronegativity (alkali metals) is ionic in nature.
There is a complete transfer of electrons between them. The bond between elements of similar electronegativities will be covalent in nature as the bond between carbon and hydrogen in methane, or nitrogen and hydrogen in ammonia. If the difference of electronegativities between two elements is more than 1.7 the bond between them will be primarily ionic bond and if it is less than 1.7, the bond between two atoms will be mainly covalent.