CHM 101 General Chemistry I - Hybridization & Molecular Shapes PDF

CHM 101 GENERAL CHEMISTRY 1 HYBRIDIZATION AND SHAPES OF MOLECULES By DR. OLUBUNMI A. ADEWUSI Department of Chemistry, Lagos State University, Ojo, Lagos  LEARNING OBJECTIVES Know what sigma and pi bonds are, and examples o...

CHM 101 GENERAL CHEMISTRY 1 HYBRIDIZATION AND SHAPES OF MOLECULES By DR. OLUBUNMI A. ADEWUSI Department of Chemistry, Lagos State University, Ojo, Lagos  LEARNING OBJECTIVES Know what sigma and pi bonds are, and examples of simple molecules having them. Understand sp2, and sp3 orbital hybridization with specific examples. Understand the principles of the valence shell electron pair repulsion (VSEPR) theory used in the prediction and the interpretation of shapes/geometry of simple molecules and ions. Understanding of the term bond angle. Know the shapes/geometry and bond angles of BeCl2, BCl3, CH4, NH3, H2O, CO2, PCl5, SF6 and C2H4. Be able to use VSEPR to predict shapes/geometry, bond angles of similar molecules to the ones given above.  SIGMA (σ) BOND Sigma bonds are the strongest type of covalent bonds due to the direct overlap of valence orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons. A single bond is usually a sigma bond. ONE HYDROGEN (H2) 1s1 1s1 The simplest molecule to use to explain is the hydrogen molecule. Each atoms provides a 1s electron which is used in covalent bonding through the overlap of the two 1s orbitals, hence forming a pair within the two overlapping orbitals The two electrons forms a strong bond like a glue because of the strong attraction they experience to the positive nuclei of the atoms  TWO METHANE (CH4) ✓ The simplest organic molecule ✓ Carbon 1s2, 2s2, 2px1, 2py1, 2pz ✓ A concept called orbital hybridization takes place for the carbon atom to be able to have a single bond each with the four hydrogen atoms. This is explained by valence bond theory (VBT) ✓ The four valence orbitals of the carbon (one 2s and three 2p orbitals) combines to form four equivalent hybrid sp3 orbitals because they are formed from mixing one s and three p orbitals. In the new electron configuration, each of the four valence electrons on the carbon occupies a single sp3 orbital. ✓ The four C-H bonds in methane are arranged with tetrahedral geometry about the central carbon, with each bond hav the same length and strength. ✓ The sp3 hybrid orbitals are oblong in shape similar to the p orbitals of which they are partly made up of with two lobes of opposite sign but the two lobes have different sizes unlike the regular p orbital. The bigger lobes are directed towards the four corners of a tetrahedron, making the angle between any two orbital in methane to be 109.5o.  sp3 hybridized carbon sp3 hybridized carbon bonded to four hydrogen atoms ✓ The four C-H bonds in methane are of equal length of 1.09 Å (1.09 x 10-10m). Each is formed from the overlap between a half-filled 1s orbital in a hydrogen atom and a sp3 hybrid orbital in the central carbon. THE SOLID DASH/WEDGE SYSTEM It is a conventional way of drawing 2- dimentional structures of molecules. A solid wedge simply represents a bond that is meant to be pictured emerging from the plane of the page. A dashed wedge represents a bond that is meant to be pictured pointing into, or behind, the plane of the page. Normal lines imply bonds that lie in the plane of the page.  THREE AMMONIA (NH3) ✓ Nitrogen is sp3 hybridized. ✓ The bonding arrangement is also tetrahedral where the three N-H bonds of ammonia forms the base of a trigonal pyramid. The fourth orbital which has the lone pair forms the top of the pyramid. ✓ The lone pair due to its slightly greater repulsive effect pushes the three N-H bonds away from the top of the pyramid, making the H-N-H bond angles not to tetrahedral at 107.3˚.  FOUR WATER (H2O) ✓ The overlap of sp3 hybrid orbitals on oxygen with 1s orbitals on the two hydrogen atoms makes the bonding in water to occur. ✓ The two nonbonding lone pairs on oxygen would be located in sp3 orbitals. A molecule of water molecule is ‘bent’ at an angle of approximately 104.5˚(explained by VSERP).  pi (π) BONDS sp2 hybridization FIVE ETHENE C2H2 Valence bond theory, and the hybrid orbital concept, describes bonding in double-bonded compounds like ethene. ✓ The 2s, 2px and 2py orbitals combine to form three sp2 hybrid orbitals, leaving the 2pz orbital unhybridized. Characteristics of the ethene molecule relating to its bonding. ✓ It is a planar (flat) molecule. ✓ Bond angles are approximately 120 o, and the C-C bond length is 1.34 Å, significantly shorter than the 1.54 Å single carbon-carbon bond in ethane. ✓ There is no rotation about the carbon-carbon double bond. The valence shell electron pair repulsion (VSEPR) theory More Examples  REFERENCES 1. Pearson-International. A-Level-Chemistry 2. Education-career. Online image from https://www.apa.org/ on June 6, 2023. 3. Chem.libretexts.org. Structure and Properties of Organic Molecules. Hybridization and Molecular Shapes (Review). Online excerpt on June 6, 2023.