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Understanding Sigma and Pi Bonds: Formation and Characteristics of Molecular Orbitals

This text delves into the fundamental concepts of sigma (σ) and pi (π) bonds, extending beyond traditional Lewis structures. It discusses the combination of atomic orbitals, such as s and p, to form molecular orbitals, highlighting the shapes of these orbitals—spherical for s, peanut-shaped for p, and doughnut-shaped for some d orbitals. The text explains how σ bonds are formed through the direct overlap of orbitals while π bonds arise from sideways overlap. It also clarifies the hierarchy of bond types in molecules, showcasing examples like ethene and acetylene.

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Understanding Sigma and Pi Bonds: Formation and Characteristics of Molecular Orbitals

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  1. HL Bonding

  2. HL Bonding Sigma & pi bonds

  3.  and  bonds • This goes beyond the Lewis representation

  4.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals

  5.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape?

  6.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical

  7.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical p orbitals are what shape?

  8.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical p orbitals are what shape? Peanut-shaped

  9.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical p orbitals are what shape? Peanut-shaped d orbitals are what shape?

  10.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical p orbitals are what shape? Peanut-shaped d orbitals are what shape? Doughnut-shaped

  11.  and  bonds • This goes beyond the Lewis representation • Considers the combining of atomic orbitals to form molecular orbitals • Remember - s orbitals are what shape? Spherical p orbitals are what shape? Peanut-shaped d orbitals are what shape? Doughnut-shaped We will only consider s & p orbitals here

  12. s orbitals are spherical p orbitals are dumbbell (or peanut) shaped From: http://www.grenvillescience.com/chemistry/y6l/asdef.html

  13. Sigma () bonds • A bond situated symmetrically between 2 atoms

  14. Sigma () bonds • A bond situated symmetrically between 2 atoms • Formed when 2 orbitals overlap along a line drawn between the 2 nuclei

  15. Sigma () bonds • A bond situated symmetrically between 2 atoms • Formed when 2 orbitals overlap along a line drawn between the 2 nuclei • Occurs when 2 s orbitals overlap

  16. Sigma () bonds • A bond situated symmetrically between 2 atoms • Formed when 2 orbitals overlap along a line drawn between the 2 nuclei • Occurs when 2 s orbitals overlap • e.g. H2 From: http://www.grenvillescience.com/chemistry/y6l/asdef.html

  17. Occurs when an s orbital overlaps with a p orbital • E.g. HCl From: http://www.webchem.net/notes/chemical_bonding/orbitals_and_covalent_bonds.htm

  18. Occurs when an s orbital overlaps with a p orbital • E.g. HCl • Occurs when two p orbitals overlap “head on” • E.g. Cl2, methane - CH4 From: http://www.webchem.net/notes/chemical_bonding/orbitals_and_covalent_bonds.htm

  19. Pi () bond • Formed by the “sideways” overlap of two p orbitals • The overlap occurs both above and below the line drawn through the nuclei • E.g. ethene C2H4 From: http://www.everyscience.com/Chemistry/Physical/Theories_of_Bonding/.images/MolStcPiBond.gif

  20. From: http://www.cofc.edu/~deavorj/102/images/organic/pi-bonds.jpg

  21. When & What Type? • A single bond is always a sigma bond as a pi bond cannot form until after a sigma bond has formed • A double bond consists of one sigma bond and one pi bond • A triple bond consists of one sigma bond and two pi bonds. • As the electrons in the pi bonds are further from the nucleus they are weaker than sigma bonds

  22. e.g. ethene • From: http://ibchem.com/IB/ibnotes/full/bon_htm/14.2.htm • The carbon-carbon double bond contains 1  and 1  bond. The carbon-hydrogen single bonds are all  bonds. • The ethene molecule contains a total of one  bond and five  bonds

  23. Identify the number of  and  bonds in the following molecules.Explain your answer. • Nitrogen Gas • From: “http://www.windows.ucar.edu/physical_science/chemistry/n2_molecule_sm.gif” cannot be displayed, because it contains errors. • A triple bond so one sigma and one pi bond

  24. Identify the number of  and  bonds in the following molecules.Explain your answer. • Ethyne • From: http://www2.eou.edu/chemweb/molmodel/mmp9f.html

  25. Identify the number of  and  bonds in the following molecules.Explain your answer. • Ethyne • From: http://www2.eou.edu/chemweb/molmodel/mmp9f.html • 2 pi bonds in triple bond • 3 sigma bonds - 1 in triple bond and 1 in each of the carbon-hydrogen single bonds

  26. Identify the number of  and  bonds in the following molecules.Explain your answer. • Buta-1,3-diene • What is its formula? • CH2CHCHCH2 • From: http://www.scibio.unifi.it/chimorg/cap04.html

  27. Identify the number of  and  bonds in the following molecules.Explain your answer. • Buta-1,3-diene • What is its formula? • CH2CHCHCH2 • From: http://www.scibio.unifi.it/chimorg/cap04.html • Two pi bonds - one in each double bond • Eight sigma bonds - one in each of the double bonds (2) and one in each of the carbon-hydrogen single bonds (6)

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