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Occtet rule
- 1. Bonding in Molecules Part-I: Octet Rule (Remedial Teaching Lecture-1) Prepared by D. R. Shinde Associate Professor in Chemistry Department of Chemistry P.D.E.A’s. Prof. Ramkrishna More Arts, Commerce and Science College Akurdi, Pune-411044 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 1
- 2. In this lecture we will learn - • Definition and characteristics of a molecule • Early attempt to explain formation of molecule by Lewis and always referred as Octet rule • Examples with explanation in support to Lewis structure of molecule 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 2
- 3. . Definition of Molecule: Two or more atoms of same or different elements unite together to form the molecule. Example – Two H atom unite together to form H2 Molecule. + H atom H atom H –H i.e. H2 Molecule CO2 Molecules C OO HCl Molecules H Cl CH4 Molecule C H N2 Molecules N N O2 Molecules O O CO Molecules C O Likely we have formation of following molecules 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 3
- 4. ii. Thus, each molecule have definite composition: Hydrogen gas molecule – Two H atoms i.e. H2, but not 3 or 4 H atoms. Likely, Nitrogen molecule has two N atoms (N2), Methane molecule has one C and 4H atoms (CH4), etc. i. How the composition of molecules an be obtained? • From chemical reactions. • Example - Half mole of O2 gas react with one mole of H2 gas to form H2O molecule. Characteristic of Molecules i. Molecules have definite atomic composition. ii. There exists force of attraction among the atoms of molecule. iii. Molecules have definite geometry. iv. Molecules are stable than atoms. The theories are proposed to explain these properties of molecule 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 4
- 5. Scientist Lewis studied electronic structure of atoms. He found that the no of atoms in an molecule are related to the electronic structure of individual atom. The most important observation is that - inert gases do not form molecule. Scientist Lewis predicted that it must be related to electronic structure of an element. Electronic structure of inert gases He (At. No.- 2) – 1s2 Ne (At. No.- 8) – 1s2, 2s22p6 Ar (At. No.- 18) – 1s2, 2s22p6, 3s2 3p6 Kr (At. No.- 36) – 1s2, 2s22p6, 3s2 3p6, 4s2 3d10 4p6 Xe (At. No.- 54) – 1s2, 2s22p6, 3s2 3p6, 4s2 3d10 4p6 , 5s2 4d10 5p6 The important characteristic of inert gases is that outer shell of electron has 8 electrons. This electronic arrangement in inert gases must be stable hence inert gases do not form molecule easily.05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 5
- 6. Lewis observed that atoms unite to form molecule so that by sharing electrons in outer most shell, each atom complete inert gas configuration. Octet Rule: Each atoms in a molecule share No of electrons of outer most shell with other atoms to complete inert gas configuration or aquire eight electrons (Octet) in their outer shell of electron. Number of example can be given to prove Octet rule. What is the strategy that we should apply Octet rule to the molecules. The strategy: i. Write electronic configuration of each atom in a molecule. ii. Count number of electrons in outer shell of each atom iii. Draw the diagram showing outer shell electrons of atom iv. Draw the molecule. iv. Count electrons around each atom in molecule.05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 6
- 7. E. C. of C 1s2, 2s2 2p2 C has 4 electrons in its outer Shell × × × C × C has 4 electron less to complete inert gas configuration. E. C. of H 1s1 H has 1 electrons in its outer Shell • H H has 1 electron less to complete inert gas configuration. E. C. of N 1s2, 2s2 2p3 N has 5 electrons in its outer Shell N has 3 electron less to complete inert gas configuration. N × ×× × × One H atom will combine with another H atom to complete inert gas configuration i.e. H2 molecule will be formed ו H H Formation of molecules molecules In molecule Electrons of one atom will be shown by dot and that of another atom will by cross. Electrons in overlap region are associated to both atoms hence they are counted on each atom. Thus in above molecule for each atom inert gas configuration is complete. O has 2 electron less to complete inert gas configuration. E. C. of O 1s2, 2s2 2p4 O has 6 electrons in its outer Shell O ×× ×× × × 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 7
- 8. O ×× •× •× ×× HH 2 H atoms will combine with one O atom to complete inert gas configuration i.e. OH2 molecule will be formed 3 H atoms will combine with one N atom to complete inert gas configuration i.e. NH3 molecule will be formed N •× •× •× ×× H H H C × × × × • H N × × × ×× • H O ×× × × ×× • H 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 8
- 9. Limitations of Lewis concept: i) Molecules with incomplete Octet (e.g. BF3) and expanded Octet (e.g. PCl5, SF6) are formed and are stable. This is not explained why on the basis of Lewis concept. B •× •× •× F F F B in BF3 has six electrons around it. B has incomplete Octet, 2 electrons are less to complete the Octet P •× Cl Cl Cl ClCl P in PCl5 has 10 electrons around it. P has expanded Octet, 2 electrons are extra than Octet S •× F •× F F F F S in SF6 has 12 electrons around it. S has expanded Octet, 4 electrons are extra than Octet ii) Geometry of molecules are not explained. ii) Forces of attraction in molecule are not explained. 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 9
- 10. 05/08/2020 D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More College, Akurdi, Pune 10