Covalent bond –Lewis Langmuir concept :
Langmuir , in 1919 improved the Lewis concept by suggesting that when both the atoms taking part in a chemical combination are short of electrons than the nearest noble gas configuration , they can share their electrons in order to complete their octet.
The bond formed between the two atoms of mutual sharing of electrons between than so as to complete their octet or duplet in case of elements having only one shell is called covalent bond.
Examples:
Bond pair: The shared pair of electrons present between the atoms are called bond pair.
Lone pair: The valence electrons not involve in bonding are called Ione pairs.
Formal charge:
Formal Charge = No. of Valence electrons in an atom – no. of electrons represented by lewis structure
Formal charge on an atom = (Total no. of valence e– in free atom) – ½ (Total no. of bonded e–) – (Total no. of non-bonded e–)
Significance of formal charge:
Formal charge help in the selection of the lowest energy structure from a number of possible Lewis structures for a given species. Structure having lowest charge is the most stable structure.
Limitation of Octet Rule:
(1) Formation of compounds involving hydrogen:- The compounds containing hydrogen complete their duplet only not octet because they contain only one electron in their shell.
(2) If the number of electrons surrounding the central atom is less than eight : Elements having less than four valence electrons. Example are LiCl , BeH2 and BF3
(3) Formation of compounds like PCl5, SF6, IF7 In each of these molecules the central atom has more than 8 electrons. These compounds are called hypervalent compounds.
(4) Nobel gases which have already complete octet form their compounds like XeF2, XeF4, XeF6
(5) Odd – electron molecules: The molecules with an odd number of electrons like nitric oxide , NO, NO2 the octet rule is not satisfied for all the atoms.
Limitations of Lewis-Langmuir concept of covalent bond:-
(a) It could not explain the shapes of molecules.
(b) It could not explain the release of energy during the formation of a covalent bond.
Characteristics of Covalent Compounds:
1. Physical state:
Under the normal conditions of temperature and pressure, these exist as gases or liquids of low boiling points. This due to the fact that very weak forces of attraction (van der Waals’ forces) exist between discrete molecules.Some exist as soft solids if their molecular masses are high. Sulphur, phosphorus, iodine are soft solids.
2. Melting and boiling points:
With the exception of few which have giant three dimensional structures such a diamond, carborundum (SiC), silica (SiO2), others have relatively low melting and boiling points. This happens due to the presence of weak attractive forces between the molecules.
3. Crystal structure: These are of three types:
(a) First type: These are the crystals in which the unit is molecules. These molecules are held by van der Waals’ forces. These are readily fusible and volatile. Examples are sulphur, iodine phosphorus pentoxide etc.
(b) Second type: This includes crystals which have separate lattice layers. The best example of this type is graphite. Each atom has three neighbours. It is bonded to two of them by single covalent bonds and to the third by double bond. The bonds tie the atoms very tightly together into a layer. These layers can slide on one another. This accounts for the softness of graphite.
(c) Third type: This includes crystals in which every atom is bounded to four other atoms by single covalent bonds resulting in the formation of giant structure. Examples are diamond, silicon carbide (SiC), aluminium nitride (AIN). These crystals are very hard and possess high melting points.
4. Electrical conductivity:
In general, covalent substances are bad conductors of electricity. Polar molecules like HCl in solution, can conduct electricity. Covalent solids having giant molecules are bad conductors since they do not contain charged particle or free electrons. The graphite can conduct electricity since electrons can pass from one layer to the other.
5. Solubility:
In general, covalent substances are insoluble in polar solvents like water but soluble in non-polar solvents like benzene, carbon tetrachloride, ether, etc. This happens according to the principle, “Like dissolves like”. Compounds like alcohols, amines which are covalent dissolve in water due to hydrogen bonding. Covalent solids having giant molecules are practically insoluble in all solvents.
6. Isomerism:
The covalent bond is rigid and directional. On account of this, there is a possibility of different arrangements of atoms in space. Covalent compounds can, thus, show isomerism (structural and space).