Every atom is made up of fundamentals particles electron, proton, and neutrons. Protons and neutrons are present at the center of an atom called a nucleus. Electrons revolve around the nucleus in the shell at a very high speed. The number of valence electrons presents tells what will be the chemical and physical properties of an atom.
The bond formed between two atoms by sharing a pair of electrons provided entirely by one of the combining atoms but shared by both is called a coordinate bond or dative bond.This means that contribution of the electron lone pair is done by only one atom but shared by both. e.g., Ammonium ion (NH4+), Hydronium ion (H3O+).
Condition of Coordinate Covalent Bond(Dative Bond) Formation
- (i) One of the two atoms must have at least one lone pair of electrons, e.g., ammonia (NH3), water(H2O). A pair of electrons that is not shared with any other atom is known as the lone pair of electrons but it is provided to the other atom for the formation of the coordinate Covalent bond.
- (ii) Another atom should be short of at least a lone pair of electrons, e.g., Hydrogen ion (H+). So, A coordinate bond can also be defined as a bond formed between an ion and an atom of a covalent molecule with one or more lone pair of electrons.
A coordinate bond has properties of both covalent and ionic bonds. Therefore, it is also called as co-ionic bond.The atom which provides the electron pair for the formation of a coordinate bond is known as the DONOR and the atom or ion sharing the donated electron pair is known as the ACCEPTOR.
A coordinate bond and is represented by ” → ” whereas a covalent bond is represented by ” ___“. A coordinate covalent bond and covalent bond both have all similar characteristics. Because of the shared pair of electrons, there is no Coulomb force exists between the atoms. So, They are the bad conductor of electricity in the aqueous state. The compounds are not much soluble in water. To understand coordinate bond well let’s take ammonium ion in a discussion.
Ammonium Ion Coordinate Bond(NH3)
The Nitrogen atom has five valence electrons and hydrogen atom has one valence electron. In ammonia, each of the three hydrogen atoms is bonded to the nitrogen atom by a pair of shared electrons. (One electron from the hydrogen atom and one from the nitrogen).
Thus, the nitrogen atom in ammonia is left a lone pair of electrons.
This ammonia molecule combines with a hydrogen ion H+, which has no electrons in its outermost orbit in such a way that the lone pair of electrons on the nitrogen atom is shared by the hydrogen ion. The latter thus acquires the helium configuration. This bond which is formed between the nitrogen atom in ammonia and the hydrogen ion is a coordinate bond.
Lewis Structure of Ammonium ion(NH4+)
The nitrogen atom in an ammonia molecule contains a lone pair of electrons. It is a DONOR. The hydrogen ion (H+) accepts that lone pair of electrons (ACCEPTOR) and forms a coordinate bond. Once this bond is formed, all four N-H bonds become identical. The linkage forms an ammonium ion having a single positive charge.
When ammonium chloride NH4Cl is formed cation NH4+ (having 3 covalent and one coordinate bond) and anion Cl– are attracted towards each other due to electrical charge existing between them ionic bond is formed. Thus ammonium chloride is a good example of a compound having all the three types of bonds i.e., covalent, coordinate and ionic bond.
Coordinate Covalent Bond Examples
When a coordination takes place in atoms each atom acquire stability.The stable configuration of nearest noble gas. For atoms, stability means having the electron arrangement of an inert gas.i.e., Helium has two electrons them together to form molecules. This bond is known (DUPLET) while all other inert gases, i.e., Neon, Argon, Krypton, Xenon and Radon have eight electrons (OCTET) in their outermost shell.
Hydronium Ion Coordinate Bond(H3O+)
Water contains two hydrogen atoms and one oxygen atom. The hydrogen atom has one electron in its valence shell while the oxygen atom has six electrons in its Valence shell.
Two hydrogen atoms share one electron each with the oxygen atom to form a water molecule.The two unshared pairs of electrons known as lone pairs of electrons, do not participate in the bond formation in the water molecule.
The O-H bonds in a water molecule are polarised due to the large difference in the electronegativity values of the oxygen and the hydrogen atoms.(The oxygen atom has an electronegativity value of 3.5, while that of the hydrogen atom is 2.1). Oxygen has higher electronegativity so it attained slightly negative (?-) charge while hydrogen atoms attain slight positive charges (?+). Therefore, a molecule of water due to these charges is a polar molecule and is represented as shown alongside.
When an acid is added to water, strong dipole interaction (attraction between slightly positive and slight negative charge) occurs between the polar water molecules and the polar acid molecules.
As a result of such a dipole interaction, the hydrogen ion gets released from the polar bonds of the acid molecules.
HCl +H2O → H+ + Cl–
The hydrogen ion spontaneously adds on to one of the two lone pairs of electrons of the oxygen atom of the water molecule.
An addition of released H+ ion to a lone pair of electrons of the oxygen atom of the polar water molecule leads to the formation of a hydronium ion.
Proton +Water →Hydronium ion
Hydroxyl ion (OH–)
The hydroxyl ion or hydroxide ion is OH–. It is formed when one hydrogen ion (H+) is removed from the water molecule.
H2O→H+ + OH–
When H+ is removed from the water molecule, the shared pair of electron remains with oxygen. As oxygen is more electronegative and thus hydroxyl ion has the negative charge.
This is Coordinate Covalent Bond.
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