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Valence Shell Electron Pair Repulsion (VSEPR) Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory. Chemistry 11 November 7, 2008. Vsepr Theory. What is it? Model developed to predict the shapes of molecules Remember… Atoms are bound together by electron pairs called bonding pairs

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Valence Shell Electron Pair Repulsion (VSEPR) Theory

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  1. Valence Shell Electron Pair Repulsion (VSEPR) Theory Chemistry 11 November 7, 2008

  2. Vsepr Theory • What is it? • Model developed to predict the shapes of molecules • Remember… • Atoms are bound together by electron pairs called bonding pairs • These can be single (one pair e- = single bond) or multiple (2 pair e- = double bond; 3 pair e- = triple bond) • Some atoms in a molecule can also have pairs of electrons not involved in bonding called lone pairsor non-bonded pairs

  3. VSEPR Theory • Why are we concerned with the shape of molecules? • Will help us later when we discuss polarity • What causes molecules to take on a different shape? • Electron pairs (bonding pairs and lone pairs) are negatively charged and will repel each other • These pairs of electrons tend to occupy positions around the atom that minimize repulsions and maximize the distance of separation between them

  4. Three types of repulsions take place in an atom • Lone Pair – Lone Pair (LP-LP) • Lone Pair – Bonding Pair (LP-BP) • Bonding Pair – Bonding Pair (BP-BP) • Lone pairs occupy more space than bonding electron pairs • Double bonds occupy more space than a single bond

  5. A molecule must avoid these repulsions to remain stable. • When repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred. • Lone pair-lone pair (LP-LP) repulsion is considered to be stronger than the lone pair-bonding pair (LP-BP) repulsion, which in turn is stronger than the bonding pair-bonding pair (BP-BP) repulsion. LP-LP > LP-BP > BP-BP

  6. Things to Remember • When assigning a VSEPR Shape to a molecule, we focus on the central atom and the bonding pairs or lone pairs associated with it • Ex. CO2 • Electron pairs are considered to exist in a domain • Domains can be made up of: • A lone pair • A single bond • A double bond • A triple bond • Thus, all are considered to be one electron pair Domain

  7. Steps to Determine VSEPR Shape • Draw the Lewis structure of the molecule • Determine the central atom (the least electronegative) • Determine the number of bonding pairs • Determine the number of lone pairs • Consult the VSEPR chart to find the shape

  8. VSEPR Chart

  9. Carbon Dioxide CO2 • As a Lewis Dot diagram: • Has two electron pairs - two sets of double bonds – two bonding pairs • Creates a LINEAR shape according to VESPR

  10. Methanal CH2O • As a Lewis Dot diagram: • Has three electron pairs - three bonding pairs and no lone pairs • Note: Only has three bonding pairs because the double bond is considered to be one bonding pair! • Creates a TRIGONAL PLANARshape according to VESPR

  11. Methane CH4 • As a Lewis Dot diagram: • Has four electron pairs - four bonding pairs and no lone pairs • Creates a TETRAHEDRALshape according to VESPR

  12. Ammonia NH3 • As a Lewis Dot diagram: • Has four electron pairs - three bonding pairs and one lone pair • Creates a PYRAMIDALshape according to VESPR

  13. Water H2O • As a Lewis Dot diagram: • Has four electron pairs - two bonding pairs and two lone pairs • Creates a BENT shape according to VESPR

  14. Practice • NF3 • CCl2F2 • NCl3 • SO42- • COCl2 • NO2- • NH4+ • Draw the VSEPR shape for each of these molecules • HCN • CCl4 • SI2 • H3O+ • CH2F2 • AsCl3 • NO2+ • CH3F Finished? Assign VSEPR shapes to the homework sheet (questions 1-5) from last class. 

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