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Molecular Geometry and Bonding Theories

Molecular Geometry and Bonding Theories. Notes 1. Molecular Shapes. Lewis Structures give atomic connectivity they tell what is physically connected to which atom but it tells little about geometry. The shape of a molecule is determined by the bond angle

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Molecular Geometry and Bonding Theories

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  1. Molecular Geometry and Bonding Theories Notes 1

  2. Molecular Shapes • Lewis Structures give atomic connectivity they tell what is physically connected to which atom but it tells little about geometry. • The shape of a molecule is determined by the bond angle • The angle made by the lines joining the nuclei of the atoms in a molecule • Consider CCl4 • Experimentally the bond angles are all 90o so the molecule is not planar all are located on vertices of a tetrahedron with C at the center • Geometries are such that they minimize electron repulsion

  3. VSEPR • A covalent bond forms when two atoms electrons occupy the space between the nuclei • The region of sharing is an electron domain • A non bonding pair or lone pair defines an electron domain located principally on one atom • Example NH3 has three bonding pairs and one lone pair • VSEPR predicts geometries to minimize electron repulsions • The arrangement of electron domains about the central atom of an Abn molecule is its electron- domain geometry.

  4. Electron domain geometries • Linear- two electron domains • Trigonal planar- three domains • Tetrahedral- four domains • Trigonalbipyramidal- five domains • Octahedral- six domains

  5. Steps to geometry • 1. Draw the lewis structure • 2. count the total number of electron pairs around the central atom • 3. Arrange the electron pairs in one of the previous geometries to minimize the electron repulstions • 4. Determine the 3D structure of a molecules • 5. Ignore lone pairs in the molecular geometry • Describe the molecular geometry in terms of the angular arrangement of the bonded atoms • Multiple bonds count as one electron domain

  6. The effect of Nonbonding electrons and multiple bonds on bond angle • We refine VSEPR to predict and explain slight distortions from ideal geometries • Consider three molecules with tetrahedral electron domain geometries CH4, NH3 and H2O. • By experiment HXH bond angle decreases from C (109.5o in CH4) to N (107 in NH3 to O(104.5 in H2O) • A bonding pair of electrons is attracted by two nuclei. They do not repel as much as the nonbonding pairs, which are primarily attracted to only one nucleus • Electron domains for nonbonding electron pairs thus exert greater repulsive forces on adjacent electron domains • They tend to compress bond angles • The bond angle decreases as the number of nonbonding electron pair increases.

  7. Similarly,electrons in multiple bonds repel more than electrons in single bonds

  8. Molecular shapes • Three atoms • Linear • Bent • Four atoms • Trigonal planar • Trigonalbipyramidal • T- shaped • Five atoms • Tetrahedral • Square planar • See Saw

  9. Six atoms • Trigonalbipyramidal • Square pyramidal • Seven atoms • Octahedral

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