Chemistry Revision of Chemical Bonding For NDA Part-4
Please Wait... While Loading Full Video

Revision of Chemical Bonding For NDA Part-4

Click for Only Video

Coordinate or Dative Bond

This is defined as a covalent bond in which one of the atoms donates both electrons.

In this, one atom is deficient in atleast two electrons, while the other atom has already acquired stable noble gas configuration.

The atom which donates the electrons is called the donor while the other which only accepts the electrons pair is known as acceptor. This bond is usually represented by an arrow (`->`) pointing from donor to the acceptor atom.

e.g. a hydrogen ion can combine with water molecules by coordinate covalent bond to form hydronium ion `(H_3 O^+)`.

Coordinate Covalent Bonds in Complex Compounds

In complex compounds, ligands donate a pair of electron to the central metal atom. Thus, in a complex, ligands join with central metallic atom by a coordinate covalent bond. e.g. `K_4 [Fe (CN)_6]`.


`=>` In complex salts, central metal atom acts as Lewis acid while ligand acts as Lewis base.

Characteristics of Coordinate Covalent Compounds

The main characteristics are as following

(i) These exist as solids, liquids, and gases under normal conditions.

(ii) These are sparingly soluble in polar solvent like water but readily soluble in non-polar solvents.

(iii) These are bad conductor of heat and electricity.

(iv) The melting and boiling points are higher than purely covalent compounds.

(v) It is rigid and directional. Thus, these compounds show isomerism.

(vi) Compounds which have coordinate covalent bond possess high values of dielectric constant.


Note : According to octet rule, an atom has a tendency to occupy 8 electrons (`2` in `H` like and `He`) in its valence shell, which is stable
arrangement, but a number of molecules violates octet rule.

e.g. (i) `BeF_2, BF_3` (incomplete octet)

(ii) `NO, NO_2` (odd number electrons)

(iii) `PCI_5, SF_6` (expanded octet)

Hydrogen Bond

The attractive interaction of a hydrogen atom with an electronegative atom such as nitrogen (`N`), oxygen (`O`) or fluorine (`F`) that comes from another molecule or chemical group, is called hydrogen bond.

It is stronger than van der Waals interaction and has energy `5` to `30` kJ/ mol. It occurs in both inorganic molecules such as water and organic molecules such as DNA.

There are two types of `H`-bond :

(i) lntermolecular H-bonding : It occurs between different molecules of a substance and increases solubility in water and also increases boiling point.

e.g. `HF, H_2O, C_2H_5OH` etc.

`H- F ...... H - F ...... H - F`

(ii) Intramolecular H-bonding : It is found within different parts of a single molecule and decreases solubility in water and also decreases boiling point. e.g. o-nitrophenol.

`=>` Metals forms metallic bond.

`=>` The atoms of transition metals are held together by strong metallic bonds because of large number of unpaired electrons in their atom. Therefore, all transition metals are much harder and less volatile.

`=>` Soft metals like `Na`, `K` possess weak metallic bond and hence can be cut with knife easily.
 
SiteLock