2TopicsVSEPRThe Different GeometriesElectronic and MolecularBond AnglesIsomers
3VSEPRThe Valence Shell Electron Pair Repulsion Theory states that electron pairs will take up postions in space, as far apart from one another as possible.This is because like charges repel each other.
4Nonbonded vs. Bonded Pairs Nonbonded pairs will take up more space around the central atom than bonded pairs.This will affect the bond angles and the shape of the molecule.
5Ex. In a linear molecule such as CO2, the bond angles are 180o Bond Angles are the angles formed between the atoms bonded around the central atom.Ex. In a linear molecule such as CO2, the bond angles are 180oO = C = O180o
6Names of Molecular Geometries Linear has 180o bond anglesAngular (Bent)~ 105o or ~120oTriangular Planar ~ 120oPyramidal ~ 107oTetrahedral ~ 109o
7There are exceptions to the octet rule There are exceptions to the octet rule. The names of these Geometries are...Square PlanarTriangular BipyramidalOctahedralIrregular Tetrahedral or see-sawSquare PyramidalT - Shaped
8Don’t be disappointed but... We will not be covering the exceptions to the octet rule.
9Two Types of Geometries 1. The electronic geometry is the shape of the particle, due to electron pairs around the central atom.
10Types of Geometries...2. The Molecular geometry is the shape of the particle, made by the atoms bonded to the central atom.
11How To Determine The Shape Of a Particle. 1. Draw the Lewis Dot Structure.2. Count up how many positions are occupied around the central atom, (Atoms and / or electron pairs).
12Use the following guide lines to draw a starting shape: Two positions occupied = Linear.Three positions occupied = Triangular Planar.Four Positions occupied = Tetrahedral.
13This is only how to start the drawing. There is more to come ! Ex. Consider H2OThe dot structure of water will show that there are 4 positions occupied around the O atom. Two positions are Hydrogens and two positions are nonbonding pairs of electrons.
14The dot structure of water... 41:H O H:..23So we start by drawing a tetrahedral shape.
15Tetrahedral ShapeTwo lines are on the same plane, the third is pointing back and the fourth points forward.O
16H H Now put the bonded atoms on the shape. O 105o Bond angles between bonding atoms are ~105oOHH105o
17What this diagram shows is... A structure that has 4 positions around it has an electronic geometry of TETRAHEDRAL.If only Two of the Four positions are bonded, then the molecular geometry is ANGULAR.
18Ex. 2 Four Positions Occupied Consider NH3The dot structure of NH3 has 4 positions around the Nitrogen.Notice that there are only 3 bonds.
19107o Draw the structure for 4 positions. add the bonded atoms N H H H Bond angles between bonding atoms are ~ 107o
20What this diagram shows is... Since there are 4 positions occupied, the electronic geometry is still tetrahedral.Only 3 of the 4 positions are bonded atoms, so the molecular geometry is PYRAMIDAL.
21Ex. 3 Four positions occupied and all four are bonded. HH C HCH4..::..Four positions occupied and all four bonded make a tetrahedral electronic and molecular geometry.
22Some models of molecules with 4 occupied positions... Tetrahedral electronic and molecular geometries~109o angles
30Two Occupied Positions Are Always Linear Electronic and Molecular Ex. CO2The dot structure shows that there are two positions occupied and both positions are bonded.
31.. : :: :: : .. O C O Two Positions ... Note that the CENTRAL ATOM has no nonbonded electrons. If it did, they would cause the molecule to bend...
32Two positions occupied with one postion bonded... Ex. CN-C N:::::When ever there are only TWO Atoms, both the electronic and molecular geometries are Linear.
33Molecular Diagrams With Two Occupied Positions... Ex. CN-
34Two Positions... 180o bond angles Two resonance structures of CO2 - LINEAR180o bond angles
35IsomersIsomers are two or more compounds with the same molecular formula but different bonding arrangements. Some of them have different physical and chemical properties while others only have reactive differences.
36Constitutional or Structural Isomers These isomers have their atoms bonded in a different order.The order of bonding may change the functional group.It may involve branching in the carbon chain.The double bonds may be in different locations.
37Structural Isomers .. .. .. .. : : : : : .. .. .. .. .. .. Ex. C3H8O H H HH C C C O H........:::::Propanol-->..........H H HH C O C C H<--Methoxy ethane..
38Example; BranchingC 4 H10H H H HH C C C C H........:::::........continued...