STOICHIOMETRY:

MOLECULAR GEOMETRYCHM124

MOLECULAR GEOMETRY refers to the three dimensional arrangement of atoms in a molecule. can be reliably predicted by the valence-shell electron-pair repulsion (VSEPR) model.

VSEPR it accounts for the geometric arrangements of electron pairs around a central atom in terms of the electrostatic repulsion between electron pairs. this model is based on the assumption that chemical bonds and lone pairs tend to remain as far apart as possible to minimize repulsion.

Rules Governing the Use of VSEPR1. As far as electron pair repulsion is concerned, double bonds and triple bonds can be treated like single bonds. (In reality, multiple bonds are larger than a single bond; that is, because there are two or three bonds between two atoms, the electron density occupies more space).2. If a molecule has two or more resonance structures, we can apply the VSEPR model to any one of them. Formal charges are usually not shown.

MOLECULES IN WHICH THE CENTRAL ATOM HAS NO LONE PAIRS:

Species TypeOrientation of Electron PairsPredicted Bond AnglesExampleImage

AX2Linear180oBeCl2

AX3Trigonal Planar120oBF3

AX4Tetrahedral109.5oCH4

AX5Trigonal Bipyramid90o1200180oPF5

AX6Octahedral90o180oSF6

AX7Pentagonal Bipyramidal72o90oIF7

MOLECULES IN WHICH THE CENTRAL ATOM HAS ONE OR MORE LONE PAIRS

Species TypeOrientation of Electron PairsPredicted Bond AnglesExampleImage

AX2EBent or Angular