Molecular Geometry and VSEPR Theory
the properties of molecules depend not only on the bonding of atoms but also on the molecular geometry, or 3-D arrangement of a molecule’s atoms in space. molecular polarity = the uneven distribution of molecular charge (NOT electrical charge, which refers to the charges within the atoms themselves) the polarity of a molecule determines its chemical and physical properties this is especially important for molecules of liquids and solids, because they are so close together (when compared to the molecules of gases) unfortunately, the chemical formula of a compound does not tell you much about its molecular geometry, so we have a theory to help….
all diatomic (2-atom) molecules Ex: HCl = are “linear” (duh). VSEPR (Valence-Shell, Electron-Pair Repulsion) Theory = states that the repulsion between the sets of valence-level electrons in an atom causes these sets of electron pairs to be oriented as far apart as possible in all three dimensions. rules of VSEPR theory: all diatomic (2-atom) molecules Ex: HCl = are “linear” (duh). 3-atom molecules can be one of 3 different shapes, depending on the number of unshared pairs the central atom has: if it has no unshared pairs, then Ex: BeF2 = the shape is “linear.” (AB2) angle = 180 if it has one unshared pair, then Ex: SnCl2 = the shape is “angular.” (AB2E) angle = 120
if it has one unshared pair, then Ex: SnCl2 = the shape is “angular if it has one unshared pair, then Ex: SnCl2 = the shape is “angular.” (AB2E) angle = 120 if it has two unshared pairs, then Ex: H2O = the shape is “bent.” (AB2E2) angle = 105 4-atom molecules can be one of two different shapes, again, depending on the number of USPs the central atom has: if it has no USPs, then the shape Ex: BF3 = is “trigonal-planar.” (AB3) angle = 120 if it has one USP, then the shape Ex: NH3 = is “trigonal-pyramidal.” (AB3E) angle = 107
if it has one USP, then the shape Ex: NH3 = is “trigonal-pyramidal if it has one USP, then the shape Ex: NH3 = is “trigonal-pyramidal.” (AB3E) angle = 107 5-atom molecules can be only one Ex: CH4 = shape: “tetrahedral.” (AB4) angle = 109.5 6-atom molecules can be only one Ex: PCl5 = shape: “trigonal-bipyramidal.” (AB5) angles = 90 on z-axis,120 on x & y-axis 7-atom molecules can be only one Ex: SF6 = shape: “octahedral.” (AB6) angles = 90 on z-axis, 90 on x & y-axis