Tiêu đề Med-RM_Chem_SP-1_Ch-4_Chemical Bonding and Molecular Structure.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Chapter Contents Lewis Symbols Ionic or electrovalent bond Fazan’s Rule Lattice Enthalpy Covalent bond Dipole Moment Coordinate or Dative Bond Formal Charge The Valence Shell Electron Pair Repulsion (VSEPR) Theory Valence Bond Theory Hybridisation Hydrogen Bond Resonance Molecular Orbital Theory Chapter 4 Chemical Bonding and Molecular Structure Atoms combine with one another in different ways to form a large number of molelcules. The attractive force which holds the constituents (atoms, molecules or ions) in different chemical species, is called a chemical bond. As the atomic state is regarded as a state of higher energy therefore the atoms combine with one another in a number of ways, i.e., by transference of electrons (ionic bond) or by sharing of electrons (covalent bond), etc. The basic cause of the combination of atoms is :- (i) Tendency to acquire the configuration of nearest noble gas. It is known as octet rule. Though it could successfully explain the formation of many molecules e.g., CH4, H2O, H2S etc. yet it could not provide any answer for forces responsible for formation of molecules. (ii) According to modern views, a bond results between atoms because of their tendency to acquire a state of minimum energy. When the two atoms approach each other, new forces of attraction and repulsion come into play. The net resultant of these forces may be attraction or repulsion. If attractive forces are more, decrease in potential energy takes place and a chemical bond is formed. LEWIS SYMBOLS The electrons in the outer shell are represented by the dots surrounding the symbol. For example, the Lewis symbols for some elements of 3rd period are: Na Mg Al Si P S Cl Ar Types of Bonds There are many different types of bonds, present in different molecules, for example, (a) ionic, (b) covalent, (c) coordinate. In addition to these, weak bonds like hydrogen bonds and Van der Waal’s interactions are also present in molecules.
76 Chemical Bonding and Molecular Structure NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 IONIC OR ELECTROVALENT BOND This bond comes into existence by the transference of electrons between the participating atoms. The atom losing electron gets converted into cation while the other atom gaining electron changes into anion. The electrostatic force of attraction which holds the oppositely charged ions is termed as electrovalent bond. The number of electrons lost or gained by the atom is known as its electrovalency. The necessary conditions for forming stable ionic bond are (i) Low ionisation energy of element forming cation. (ii) High electron affinity of element forming anion. (iii) High lattice energy resulting from the formation of crystal by close packing of gaseous ions of opposite charges. Electrovalency : It is defined as number of electron lost or gained by an atom e.g. Na has electrovalency 1, Mg has 2. It is equal to their valence electrons respectively. Characteristics of Ionic Compounds These are hard and brittle crystalline solids with high melting and boiling points. They consist of ions held by strong electrostatic forces of attraction in the solid state and hence do not conduct electricity. When fused (in molten state) or in aqueous solution they behave as good conductors of electricity because ions become free to move and carry current. They are soluble in polar solvents like water but are insoluble in nonpolar solvents like CCl4, CS2, benzene etc. FAZAN’S RULE In ionic bond, some covalent character is introduced because of the tendency of the cation to polarise the anion. In fact cation attracts the electron cloud of the anion and pulls electron density between the two nuclei. + + Cation Anion Polarised Electron cloud of anion According to Fazan’s rule, the magnitude of covalent character in the ionic bond depends upon the extent of polarisation caused by cation. In general, Smaller the size of cation, larger is its polarizing power. Larger the anion, more will be its polarizability. The extent of polarisation will also be favoured by More charge on cation and anion. Presence of a non polar solvent Cation with an electronic configuration other than noble gas. LATTICE ENTHALPY It is the amount of energy released when gaseous cation and anion are brought closer from infinity to form 1 mole of ionic compound. Factors affecting lattice enthalpy The charges on the ions : Larger the magnitude of charge on the ions higher is the value of lattice enthalpy. The radius of the ions : Smaller the size of ions, more will be the value of lattice enthalpy.
NEET Chemical Bonding and Molecular Structure 77 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 COVALENT BOND This bond is formed by the mutual sharing of electrons between the participating atoms of same or different elements. Depending upon the number of electrons shared by each atom in bond formation, a single, double or triple bond may result. The number of electrons contributed by an atom for sharing is known as its covalency. Covalency : It is defined as number of electron shared e.g. hydrogen has covalency 1, oxygen has 2, nitrogen has 3. Formation of covalent bonds between atoms of different elements e.g., H O2 , CCl4, etc. (a) H O H 2e– 8e– 2e– O — — H H  H atoms attain a duplet of electrons and O the octet, here O has 2 lone pairs (b) Cl Cl Cl Cl C Cl—C—Cl Cl Cl 8e– 8e– 8e– 8e– 8e  Each of the four Cl atoms along with the C atom attains octet of electrons, here each Cl atom has 3 Ione pairs. Multiple covalent bonds (a) 8e– 8e– O O  O O —— (Oxygen molecule) (b) 8e– C H 8e– C  C C H H H H —— H H H (ethene molecule) (c) N N N  ——— N (Nitrogen molecule) 8e– 8e– Characteristics of Covalent Compounds Under normal conditions they are generally liquids and gases. They have low melting and boiling points and are soluble in nonpolar solvents, insoluble in polar solvents and do not conduct electricity even in fused or dissolved state. They exhibit isomerism because of the rigid and directional nature of the covalent bond.
78 Chemical Bonding and Molecular Structure NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Polar and non-polar covalent bonds. If a covalent bond is formed between similar atoms, the shared pair of electrons lies almost in the centre and electron cloud is uniformly distributed around the two atoms. Such a covalent bond is called non-polar covalent bond. On the other hand, if a covalent bond is formed between the different atoms then the shared pair is displaced towards the more electronegative atom causing greater concentration of electron density around the more electronegative atom. Such a covalent bond develops some ionic character and is called polar covalent bond. e.g., + [H : Cl] H—Cl– A polar covalent bond The bonding electrons are attracted more strongly by Cl than by H.  e.g., Cl : Cl Cl – Cl  A nonpolar covalent bond Octet Rule Atoms of various elements tend to gain, lose or share valance electrons during the formation of molecules such that there are eight electrons or octet in their valence shells. Limitations of octet rule Formation of compounds with electron deficient atoms Li Cl F Be F Cl B Cl Cl In LiCl, Li atom has only 2 electrons; in BeF2, Be atom has four electrons and in BCl3 the boron atom has only six electrons in its outer shell. Other such compounds are AlCl3, BF3, BeH2 etc. Formation of super octet molecules like PCl5, SF6 and IF7 In a number of compounds, there are more than eight valence electrons around the central atom. This is termed as super octet or the expanded octet. P Cl Cl Cl Cl Cl S F F F F F F I F F F F F F F 10 electrons around the P atom 12 electrons around the S atom 14 electrons around I atom Formation of compounds of xenon Octet rule says that noble gases are inert, but Xe and Kr take part in bonding and form compounds with elements like fluorine and oxygen such as XeF2, XeF4, XeOF2, XeOF4, KrF2 etc.