CHM 2103 Inorganic Chemistry Lecture 8 PDF
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University of Guyana
Brij B. Tewari
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This document provides lecture notes on inorganic chemistry, specifically focusing on Lewis structures, homonuclear and heteronuclear interactions, and aggregation of atoms. It details the formation of molecules through various bonding interactions.
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# CHM-2103 ## Inorganic Chemistry-I ### Lecture No. 8 - Lewis Structure - Homonuclear Interaction - Heteronuclear Interaction - Aggregation of Atoms - Formation of Molecules **Prof BRIJ B.TEWARI** -1- ## Lewis Structure - Lewis structures contain definite electron pair bonds, that are transfe...
# CHM-2103 ## Inorganic Chemistry-I ### Lecture No. 8 - Lewis Structure - Homonuclear Interaction - Heteronuclear Interaction - Aggregation of Atoms - Formation of Molecules **Prof BRIJ B.TEWARI** -1- ## Lewis Structure - Lewis structures contain definite electron pair bonds, that are transferable from one species of molecule to another. - Although lone pair do not contribute directly to the bonding, they do influence the shape of molecule and its chemical properties. - The octet rule provide a simple way of constructing Lewis structures, which are diagrams that show the pattern of bond in a molecule - Lewis structure can be constructed in following steps: 1. Determine the total number of valency electrons that are to be included in the structure Example: H1 = 1 e- N7 = 1s², 2s², 2p³ = 5e- O8 = 1s², 2s², 2p⁴ = 6e- Each negative charge on an ion corresponds to an additional electron, each positive charge corresponds to one electron loss 2. Write the chemical symbols of the atoms in the arrangement. Usually less electronegative element is the central atom 3. Distribute the electrons in pairs so that there is one pair of electrons between each pair of atoms, bonded together. 4. Supply electron pair until each atom has an octet. **Examples** 1. Write Lewis structure of BF<sub>4</sub><sup>-</sup> ion Answer: Total number of Valency electrons: = 3 + (4 x 7) = 31 e- Single negative charge shows the presence of an additional electron = 31 + 1 = 32 e- So 32 e- in 16 pairs accommodate around 5 atoms [ : F : B : F : : F : ] : F : 2. Write Lewis Structure of PCl<sub>3</sub> Answer: Total number of valency e- = 5 + 7x3 = 26e- So 26 e- in 13 pairs accommodate around 4 atoms : Cl : C : : Cl : : Cl : - Lewis Structure for some common molecules. ![[image of chemical Lewis structures]] -6- ## Homonuclear Interactions For the explanation of homonuclear interactions, take simple example of H<sub>2</sub> molecule which is formed by two atomic hydrogen A and B. ![[image of H2 molecule, bonding and anti-bounding mo]] Figure 1. Two atomic hydrogen atoms give bonding and anti-bonding molecular orbitals -7- ![[image of energy level diagram for H2 molecule]] Figure 2. Relative energy levels of molecular orbitals and their constituents atomic orbitals of H<sub>2</sub> Since A and B are identical atoms their atomic orbital obviously contribute equally to molecular orbitals. Two molecular orbitals derived by combining the 1s atomic orbitals are represented as: Ψ<sub>m</sub> = N [Ψ<sub>A</sub> + Ψ<sub>B</sub>] Ψ<sub>m</sub>* = N*[Ψ<sub>A</sub>-Ψ<sub>B</sub>] In these expression N and N* normalize the molecular wave function, so that for each orbital the probability of finding an electron somewhere in it is unity. They are given by: N = 1/√2(1+S); N* = 1/√2(1-S) Where S = overlap integral which is a measure of extent to which two wave function coincide. The molecular wave function Ψ<sub>m</sub> * and Ψ<sub>m</sub> are bonding and antibonding molecular orbitals are shown in Figures 1, 2 and 3. -9- The pictorial representation of combination of s orbitals given as follows: ![[image of pictorial representation of s orbitals]] Figure 3. Formation of molecular orbitals for H<sub>2</sub> (a) Ψ<sub>A</sub> and Ψ<sub>B</sub> for individual atoms (b) Ψ<sub>m</sub> (wave function for bonding M.O.) (c) Probability function for the bonding orbital (Ψ<sub>m</sub>)² (d) Ψ<sub>m</sub>* (wave function for antibonding Molecular orbital) (e) Probability function for the anti bonding orbital (Ψ<sub>m</sub>*)² A pictorial representation of the combination of the p orbitals are given in Figure-4. -11- ![[image of pictorial representation of combination of p orbitals]] Figure 4: The combination of 2 p orbitals showing formation of (a) σ2p bonding mo (b) σ2p antibanding mo (c) π2p bonding Mo (d) π2p antibonding Mo -12- ## Heteronuclear Interaction The molecular orbitals of heteronuclear diatomic molecule differ from those homonuclear diatomic molecule by having unequal contribution from each atomic orbital HF, HCl, LiCl, NaCl, CN, CO, NO etc. are examples of heteronuclear diatomic molecules. In heteronuclear molecule like AB two orbitals of different energy are combined to give molecular orbitals as shown in Figure below: ![[image of energy level diagram for heteronuclear molecule AB]] Figure: The relative energy levels of molecular orbitals and their constituents atomic orbitals for heteronuclear molecule AB. -14- The molecular wave function for heteronuclear diatomic molecule is given as: Ψ<sub>m</sub> = N[Ψ<sub>A</sub> +λΨ<sub>B</sub>] and Ψ<sub>m</sub>* = N[Ψ<sub>B</sub>-λΨ<sub>A</sub>] where B is the element contributing the lower energy orbital; λ is a constant reflecting extent to which the Ψ<sub>B</sub> orbitals contributes to molecular wave function. - In Case of HF, λ >> 1 the 2p<sub>z</sub> wave function for fluorine contributes more than hydrogen, 1s wave function. The electron density is greater around the fluorine nucleus than around the proton. The value of λ is indication of polarity of the H-F bond. -16- ## Aggregation of Atoms ## Formation of Molecules Solids are composed of atoms stuck together in various ways. This observation explained by two assumptions (a) There are force of attraction between atoms tending to pull them together (b) A collection of atoms will tend to arrange itself so that its potential energy is minimum The tendency of an atom to arrange themself in a configuration of minimum potential energy is opposed by the disruptive effective of any kinetic energy possessed by the atoms. The Actual state of aggregation of any group of atoms at any given temperature is determined by the interplay of two equally important factors. (a) The tendency to a potential energy minimum (b) The disruptive effect of kinetic energy. -18- The atomic aggregates can be divided into two classes 1. Strong bonding of atoms into discrete molecules 2. Strong banding of atoms into infinite arrays. 1. All substances in the gas phase exist as discrete molecules or as atoms. H<sub>2</sub>, O<sub>2</sub>, H<sub>2</sub>O, CO<sub>2</sub>, CH<sub>4</sub> are all examples of substances which form discrete molecules ![[image of molecular shapes for a few molecules]] Fig. 1. Shapes of some simple molecules 2. Strong bonding of atoms into infinite arrays. No discrete molecule can be detected in substances exhibiting this structural form. It is divided into three subclasses. (a) Strong bonding in three dimensions. Examples: Diamond and NaCl These both solids have all units of macroscopic crystal ![[image of diamond and sodium chloride crystal structure]] Fig. 2. Network lattices (b) Strong bonding in two dimensions; weak bonding in one dimensions; Chain lattices. Examples: Copper(II) chloride In which infinite chain lie parallel adjacent chain being held together comparatively weak forces. ![[image of Copper(II) chloride chain structure]] Figure-4. (c) Strong bonding in one dimensions; weak bonding in two dimensions; Chain lattices. Examples: CuCl<sub>2</sub> ![[image of CuCl2 chain structure]] Fig. 4 Chain lattice; copper(II) chloride (CuCl<sub>2</sub>).
