Unit 6

Read Sections 4.7, 4.11 and 4.12 before viewing the slide show.

Unit 16Shapes and Electrical Properties of Molecular CompoundsVSEPR Theory (4.11)Electronegativity (4.7)Identification of Compounds as Polar or Nonpolar (4.12)

VSEPR Theory (4.11)The geometry of molecules is important in determining some of their propertiesElectrons tend to repel each other because of their negative chargesIn considering a Lewis structure for a molecule (or ion) the geometry becomes apparent when one considers that the electrons tend to get as far away from each other as possibleThese observations are summarized in the Valence Shell Electron Pair Repulsion theory (VSEPR). Simply stated, electron pairs around a central atom arrange themselves to minimize repulsion.

Molecular Shape from VSEPR (4.11)

Application of the Previous Table (4.11)Application of the table on the previous slide involves drawing the Lewis structure and determining the number of bonded atoms and nonbonded pairs of electronsNotice the counting of atoms and nonbonded pairs only involves those around the atom of interest, circled in the table below. Molecule# of Bonded Atoms# of Nonbonded Pairs around Atom of InterestMolecular ShapeHCN|20Linear .. HN H | H31Pyramidal

More Examples (4.11) Molecule# of Bonded Atoms# of Nonbonded Pairs around Atom of InterestMolecular Shape20Linear 22Bent40Tetrahedral

Electronegativity (4.7)Recall that in ionic compounds electrons are essentially transferred from a metal to a nonmetalElectrons are not necessarily shared equally between two atoms in a covalent bondThe determining factor as to which atom will attract electrons more in a covalent bond is called the electronegativityA scale has been set up for electronegativity with fluorine having the highest electronegativity the strongest ability to draw electrons towards itself in a chemical bondA section of the electronegativity table is on the next slide

Electronegativity and the Periodic Table (4.7)

The chart below gives the electronegativity values of some of the key elements.A high value (F, for example) means a strong tendency to draw electrons toward that element in a chemical bondHydrogen is similar to carbon in terms of electronegativityThe differences in electronegativity between metals and nonmetals are typically large enough to result in a significant transfer of electrons to form ionic compoundsThe nonmetal-nonmetal bondsare covalent they involveelectron sharing, but not equally

LargestSmaller

Polar and Nonpolar Covalent Bonds (4.7)

If the difference in electronegativity between the two atoms is less than 0.5, the electrons are about equally shared and the bond is called a nonpolar covalent bondIf the difference in electronegativity between the two atoms is between 0.5 and 2.0, the electrons are drawn toward the more electronegative element and the bond is called a polar covalent bond. Basically the negative and positive charges are separated from each other.If the difference in electronegativity between the two atoms is greater than 2.0, the bond is considered to be ionic because of a significant transfer of electrons.

BondDiff. in ENBond TypeN-O3.5 - 3.0 = 0.5polar covalentAl-Cl3.0 1.5 = 1.5polar covalentC-H2.5 - 2.1 = 0.4nonpolar covalentK-O3.5 - 0.8 = 2.7ionic

Polar and Nonpolar Covalent Bonds (4.7)There are a couple of ways of representing in a polar covalent bond which way the electrons are pulledUse the greek letter with a superscript to represent whether it is at the positive or negative end of the bondUse an arrow such as that points toward the more electronegative atom. The length of the arrow may be used to indicate on a relative basis how big the difference in electronegativity is. The arrow has a size and direction which together can be referred to as the dipole moment.

BondDiff. in ENGreek Rep.Arrow Rep.N-O3.5 3.0 = 0.5Al-Cl3.0 1.5 = 1.5P-F4.0 2.1 = 1.9

Polar and Nonpolar Molecules (4.12)A molecule itself may be polar or nonpolar depending upon the geometric arrangement of its bonds. A polar molecule has separate centers of positive and negative charges, similar to north and south poles of a magnet.A nonpolar molecule has the centers of positive and negative charges at the same place, similar to a magnet in which the poles have collapsed on top of each other.The polarity of the bonds and the geometry of the molecule must both be considered to determine whether or not a molecule is polar or nonpolar.

CH4 NH3

The red represents areas of positive charge, the blue the negative chargeIn CH4, the red positive charge is evenly distributed throughout the molecule indicating a nonpolar molecule.In NH3, the negative and positive charges are separated from each other indicating a polar molecule.

Methods of Telling Polarity (4.12)A quick way to determine something about polarity of a molecule is to:Draw the Lewis structureIf the central atom has any nonbonded pairs around it, it will be a polar molecule.If the central atom has only atoms around it and they are all the same, it is a nonpolar molecule.If the central atom has only atoms around it and any of them are different, it is a polar molecule.On the previous slide, CH4 has four atoms all the same around it it is nonpolar.NH3 has a nonbonded pair of electrons and will be polarThese guidelines are general. Sometimes electronegativity differences will be small enough that a molecule, even with different atoms around the central atom, will be virtually nonpolar. An example of this is compounds containing only carbon and hydrogen these are considered nonpolar regardless of geometry.There are also molecules with 5, 6, or 7 atoms and/or nonbonded electrons bonded to the central atom. Those dont strictly follow the rules above. Those we wont worry about in this course.

A SimulationIf you would like to get some practice on molecule polarity using the PHET simulations, go to:

http://phet.colorado.eduClick on the chemistry icons in the upper left.Choose the Molecule Polarity simulationYou can Save It and then run it or Run it Now

A reasonable way of using this simulation is to focus simply on changing the electronegativity of the atoms and watching what happens to the arrow. The arrow represents what we call the dipole moment and points toward the negative charge in the molecule.

Take a look at the 2-atom, 3-atom, and molecule tabs to get a feel for how the polarity idea works.

Importance of Polarity (4.12)Properties of compounds depend to a large extent on the polarity or nonpolarity of the compound. Properties affected include:boiling pointmelting pointvapor pressuresolubilitySome of these properties and their relationship to polarity will be explored later.

Number of Bonded AtomsNumber of Nonbonded Pairs of ElectronsNumber of Sets of Electrons (sum of bonded atoms and nonbonded pairs)Molecular ShapeExamplesPicture of Shape

202LinearHCN, CO2O=C=O

303Trigonal PlanarSO3, BF2Cl

213BentSO2

404TetrahedralCH4, CHCl3, CH2Cl2

314PyramidalNH3

224BentH2O

Sheet112131415161718H 2.1Li 1.0Be 1.5B 2.0C 2.5N 3.0O 3.5F 4.0Na 0.9Mg 1.2Al 1.5Si 1.8P 2.1S 2.5Cl 3.0K 0.8Ca 1.0As 2.0Se 2.4Br 2.8I 2.5

Sheet2

Sheet3

Sheet112131415161718H 2.1Li 1.0Be 1.5B 2.0C 2.5N 3.0O 3.5F 4.0Na 0.9Mg 1.2Al 1.5Si 1.8P 2.1S 2.5Cl 3.0K 0.8Ca 1.0As 2.0Se 2.4Br 2.8I 2.5

Sheet2

Sheet3