Compounds and Their Bonds Shapes and Polarity of Molecules

VSEPR In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible have the least amount of repulsion of the negatively charged electrons have a geometry around the central atom that determines molecular shape

Shapes of Molecules The three-dimensional shape of a molecule is the result of bonded groups and lone pairs of electrons around the central atom is predicted using the VSEPR theory (valence-shell-electron-pair repulsion)

Guide to Predicting Molecular Shape (VSEPR Theory)

Polar Molecules A polar molecule contains polar bonds (determined using electronegativity of the atoms in the bond) 0.4 <EN< 1.7 has a separation of positive and negative charge called a dipole, indicated with + and – has dipoles that do not cancel + – • • H–Cl H—N—H dipole H dipoles do not cancel

Nonpolar Molecules A nonpolar molecule contains nonpolar bonds Cl–Cl H–H or has a symmetrical arrangement of polar bonds

Determining Molecular Polarity Determine the polarity of the H2O molecule. Process: Determine shape of molecule using VSEPR (focus on symmetry) Determine polarity of the bonds (EN) Calculate net dipole

Learning Check Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. A. PBr3 B. HBr C. Br2 D. SiBr4

Solution A) pyramidal; dipoles don’t cancel; polar B) linear; one polar bond (dipole); polar C) linear; nonpolar bond; nonpolar D) tetrahedral; dipoles cancel; nonpolar

Summary A molecule will be polar if it is asymmetrical AND contains polar bonds. A molecule will be non-polar if it is asymmetrical BUT contains non-polar bonds. A molecule will be non-polar if it is symmetrical, regardless of bond type.

What Good is This?? The polarity of a molecule will tell you a lot about properties such as solubility, boiling/melting points, etc. when you compare it to other similar molecules.

VSEPR Theory and Polarity Complete the worksheet from yesterday by determining if the molecules/ions are polar or non-polar. Read section 4.5 Attempt some problems from the electronic problem set. (see unit page) Complete Page 227 #1-3