Review Draw the Lewis structure for CH 3 OH. Include polarity of the bonds. How many unpaired electrons in the center atom of oxygen?
Chapter 8 “Molecular Shape” Are we living in a two dimensional world? Lewis structures show 2-dimensional pictures of molecules. In real life molecules are 3-dimensional.
We are learning to… Identify the shapes of small molecules; Describe and apply the VSEPR theory;
8-1 The Shape of Small Molecules The shape of a molecule (molecular geometry) is described by the geometric figure formed when the atomic nuclei are imagined to be joined in straight lines. The bond angle is the geometric angle between two adjacent bonds.
8-1 The Shape of Small Molecules Why do we use models? Just as architects use models to see how a building will look when finished, chemists use models to help visualize how atoms of molecules are arranged in space. Ball & stickSpace-filling Wire EXAMPLES
VSEPR Theory The VSEPR theory helps explain the shapes of simple molecules. Valence Shell Electron Pair Repulsion (VSEPR) theory states that, in small molecules, the pairs of valence electrons are arranged as far apart from each other as possible (due to repulsion of electrons). This results in geometries that minimize the energy of molecules.
VSEPR Theory: Electron Group Geometry & Molecular Geometry This leads to the Molecular Geometry, which shows how bonded atoms are arranged around that same central atom. -This is due to charge repulsion of the electrons that surround the atoms.
Common Molecular Shapes LINEAR TRIGONAL PLANAR TETRAHEDRAL TRIGONAL PYRAMIDAL BENT
LINEAR 180º bond angle CO 2 or O 2 2 lone pairs of electrons
TRIGONAL PLANAR 120º bond angle 1 central atom surrounded by 3 other atoms. No loan pair of electrons in the central atom. BCl 3 3 electron domains
TRIGONAL PYRAMIDAL 107º bond angle 1 lone pair of electrons in the central atom. NH 3 4 electron domains
TETRAHEDRAL 109.5º bond angle 1 central atom surrounded by 4 other atoms. No unshared electrons CH 4 4 electron domains
BENT 105º bond angle 2 lone pairs of electrons in the central atom. OF 2 4 electron domains
Exit Task Draw the structure of NBr 3. How many lone pairs of electrons in the center atom? What is the geometry of NBr 3 ? What is one factor that determines the geometric shape of a molecule?
PROPERTIES OF BONDS In our models the bonds are all the same length, but this is NOT true in reality. As one moves down a Group of the Periodic Table, the atoms form longer bonds. Atoms get larger moving down a Group. Multiple bonds are shorter and stronger than single bonds. The more electrons in a bond, the greater the attraction to the positive nuclei of a bond. Electrons act as the ‘electrical glue’ between the two nuclei.
Characteristics of Various Bonds) BOND BOND TYPEDISTANCE (nm)STRENGTH (kJ/mol) H-HSingle0.075436 C-HSingle0.109413 F-FSingle0.128155 Cl-ClSingle0.198242 Br-BrSingle0.228193 I-ISingle0.266151 O-OSingle0.132146 O=ODouble0.121498 S=SDouble0.189 C-OSingle0.143358 C=ODouble0.121745 C Ξ OTriple0.1131046 C-NSingle0.147305 C=NDouble0.138615 C Ξ NTriple0.116887 C-CSingle0.154347 C=CDouble0.134614 C Ξ CTriple0.120839 (Source: Textbook and Teaching resources; also Kotz & Treichel, “Chemistry & Chemical Reactivity,” 5 th Ed., Thomson, Brooks/Cole, 2003, pp 354-356.)
8-2 POLARITY Recall what we learned about polar & non-polar bonds. Are electrons shared equally in all bonds? WWhat happens if they are not shared equally? Because of the polarity of bonds and how they are arranged around a central atom, molecules may also be polar or non-polar. Dipole: a polar molecule, one that has a positive end and a negative end. What determines the polarity of a molecule? TTogether the shape of a molecule and the polarity of its bonds determine if a molecule is polar or non-polar.
Properties of Polar Molecules Formaldehyde A dipole because of the imbalance of polar bonds. A gas, but very soluble in water. Carbon Dioxide Not polar because the effect of the two polar bonds cancel. Gas at room temperature due to lack of attraction between molecules; also soluble in water. Water A dipole because the bent shape of the molecule does not cancel the polar bonds. The shape of the water molecule has a major impact on it’s properties, such as melting, boiling points, solubility and physical states at room temperature. Large Molecules Dipoles are critical to the functions of life. Proteins, DNA/RNA, carbohydrates, lipids, etc.