Bonding and molecular structure

Syllabus : 

Valency electrons, the octet rule. Electrovalent and covalent bonds with examples. Properties of electrovalent and covalent compounds. Limitation of octet rule (examples), coordinate covalent bonds (examples).

Directionality of covalent bonds, shapes of polyatomic molecules (examples), concept of hybridization of atomic orbitals (qualitative pictorial approach): sp³, sp² and sp hybridizations with typical examples. Tetrahedral space model of-carbon atom, single-bond, double- bond and triple - bond involving carbon atom with examples a and 7t bonds.

Valence shell Electron Pair Repulsion (VSEPR) concept (elementary idea) - shapes of H₂0, H₂S,
CH₄, NH₃, C0₂, N0₂ and S0₂ molecules. Concept of resonance (elementary idea), resonance structures (examples). Elementary idea about electronegativity, bond polarity and dipole moment. Hydrogen bonding (inter - & intra molecular structures) and its effects on physical properties (mp, gp, and solubility).

Double salts and complex salts, and coordination compounds (examples only), coordination number (examples with C.N 4 and 6 only).

Valency Electrons and the Octet rule

When details of the electronic configurations of the elements came to be known, it was found tha the arrangement of energy levels in different orbits round the nucleus was different for different elements. Chemical union between atoms to form molecules are energetically favoured only if the chemical combination leads to lowering of energy of the system i.e, if the energy of the combined atoms i.e., the product molecule is less than the sum of the energies of the reactant molecules.

Chemical combination of atoms involves electrons of the two atoms and nuclei take no active part in chemical combination. In each individual atom, the outermost electrons have the highest energy amongst all its electrons. So the lowering of energy must be through the interactions of the outermost electrons of the two interacting atoms. Thus chemical combination in all probabilities, should involve only the outermost electrons of the participating atoms and the interactions should be such that it causes lowering of energy w.r.t. to the initial condition when the atoms lay separated from each other.

A close study of chemical properties of the different classes of elements led us to the fact that the noble gases were the most chemically inactive species amongest all the elements. They had no tendency to combine with themselves or with other elements. They were so inert that they even did not like to form molecules by the combination of two atoms. Inert gas molecules are monatomic — or in other words their atoms do not form molecules. They are devoid of any chemical affinity. mSo it may be seen in the light of our discussion above that their outer electronic configurations are most stable amongest all the elements and no further stabilization is possible by further interaction among themselves or with electrons in other elements.

The extra nuclear electrons present in n successive inert gases are 2,10,18, 36,54 and 86. These numbers were termed magic numbers as the presence of electrons in any one of these numbers in an atom gives special stability to the atom. This stability is lost if this number is changed even by 1 unit on either side.

The arrangement of the electrons in the inert gases can be described as follows :
He — 2
Ne — 2-8
Ar — 2-8-8
Kr —2-8-18-8
Ne —2-8-18-18-8