Valence Bond Theory
Actually speaking treatment of the covalent bond is far more difficult than treatment of the electrovalent bond. Because here we have to understand what type of forces exist which keep the electron pair in contact with the tow atoms. What is the bond energy in a covalent bond and what is the geometry of the covalent compound? The answer of such questions may be given by: (i) Valency Bond Theory as formulated by Heitler and London (1931) and developed by Pauling and Slater (1931) and (ii) Molecular Orbit Theory as postulated by Hund and...
read moreResonance
Resonance There are a number of the molecules and ions in which basic modes of bond formation are fully adequate to explain the properties of the molecules or ions. E.g. Carbon dioxide is represented as O = C = O, the carbon – oxygen bond length in while the expected value for double bond is and for triple is . It means that only double bond formula is inadequate for carbon dioxide. Similarly, hydrogen molecule is represented as H – H. if it is so then the theoretical bond energy is quite low in comparison to the experimental value. It...
read moreBond lenght, angle and energy
Bond length, Bond angle and Bond energy Since atoms in a molecule are always vibrating with respect to each other, hence the question of any fixed distance between the nuclei of the atoms does not arise. Even then the average distance between the two nuclei of two bonded atoms is called bond length. In a covalent molecule of the same atoms, bond length is double the covalent radius of the atom. While in the case of ionic compounds, bond length is sum of the ionic radii of cation and anion. Bond lengths of some common diatomic molecules are...
read moreConcept of Polarization
Application of the concept of Polarization Polarization power of a cation is generally represented by and is known as ionic potential or charge density. It can be represented as: = charge on cation / radius of cation. The important applications of this concept are given below: (i) Character of a cation : The larger the value of , the greater is the degree of covalence and its tendency to form complex compounds. (ii) Salvation energy : When a substance is introduced into a solvent the inter action that takes place is called salvation, and the...
read moreFajan’s Rule
Fajan’s rule When two oppositely charged ions approach each other closely, the positively charged cation attracts the outermost electrons of the anions and repels its positively charged nucleus. This results in the distortion or polarization of the anion following some sharing of electrons between the two ions, i.e., the bond becomes partly covalent in character. This indicates how a transition between ionic and covalent bonding occurs. This clearly explains that greater the distortion of the anion, larger the tendency of sharing and hence...
read moreDipole moment
Dipole moment The degree of polarity is expressed in terms of dipole moment, i.e., the product of charge and distance between the tow charged ends, i.e., . Since charge and distance are of the order of respectively hence . In S.I. units e and d are of the order of respectively. Thus (meter coulomb). Dipole moment of non-polar diatomic molecule is zero. But in the case of tri-atomic, tetra-atomic etc. molecules, it is not so. In such cases the value of also depends upon the geometry of the molecule e.g. the value of for and is zero...
read moreOdd Electron Bond
Odd Electron Bond It may be defined as, “The bonds in which odd numbers of electrons are used in its formation are known as odd-electron bonds and the molecules are called odd-electron molecules. These molecules are quite stable and paramagnetic in nature”. Some examples of such molecules and ions are given below: Characteristics and Conditions of the Formation of Three Electrons Bond: If has been found that three electrons bond can never be formed if electronegativity difference exceed 0.5. Three electrons bond must...
read moreExceptions of Octet Rule
Exceptions of Octet Rule Some compounds such as etc. do not follow octet rule, because in these compounds Be, B, P, S, I etc. possess 4, 6, 10, 12, 14 electrons, respectively. According to Sidgwick, the maximum covalence of an element is limited according to the period, in the periodic table, in which it occurs, e.g. it is 2 for hydrogen, 4 for the elements of the second period (Li to F), 6 for those of the third and fourth periods and 8 for the rest. Sidgwick’s covalence rule is purely empirical. This rule explains satisfactorily the...
read moreCovalent Bond
Covalent bond: Such bonds are formed by mutual sharing of equal number of electrons by the atoms of the molecule. The compounds having covalent bond are called covalent compounds. Covalency can be established between similar as well as in different atoms. For example, Characteristics of Covalent Compounds (i) These compounds are slightly soluble in water but highly soluble in organic solvents. Solubility is fit far fat concept, i.e., polar covalent compounds dissolve in polar solvent and non-polar covalent dissolve in non-polar...
read moreVariable Electrovalency
The elements which change their valency or show more than one electrovalency are said to possess variable electrovalency. This is due to following two reasons. (i) Unstable configuration of the core: since in transition metals, core possesses unstable arrangement of electrons hence they show variable valency. For example, lose an electron to change into . Now due to increase of effective nuclear change do not lose more electron. Similarly, we can explain the formation of ions etc. (ii) Inert pair effect: it is observed that...
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