Dr. Fred Omega Garces Chemistry 100 Miramar College 6.04 Polarity of Molecules Predicting Molecular Polarity Dr. Fred Omega Garces Chemistry 100 Miramar College

VSEPR Table: Revisited Chemicals with Lone-pairs will always be polar.

Magnetic & Molecules Molecules and Magnets Molecules possess magnet-like properties. Magnets have poles (+)-end (-) end. Molecules too can behave similarity to that of magnets with poles (dipole). Magnets: Magnet behavior - polar (with poles) Non magnet behavior - nonpolar (without poles) Molecules

Symmetry and Leverage: Tug of War and Polarity The Charge distribution may cancel out (nonpolar) or there may be a net distortion (polar) Analogy: 1. No one wins: nonpolar 2. One team wins: polar 3. a) no one wins: nonpolar b) one team wins: polar c) two team wins polar

Not all Covalent bonds are based on Mutual Sharing Not all covalent bonds are made by bonding electrons being mutually shared between atoms 3. Because oxygen has more protons than hydrogen, it has a greater attraction for the bonded electron (it has a stronger electronegativity) this gives oxygen a  - charge and hydrogen a  + charge 4. This separation of partial charge results in the polar properties of water. 1. The oxygen atom is proceeding to form a bond with the Hydrogen atom 2. The oxygen atom and the hydrogen atom form bonds by sharing electrons

Electronegativity Compounds: When atoms combine to form compounds, electrons can either transfer from one atom to another, partially transfer from one atom to another or the electrons can be mutually shared between atoms. Transfer, partially transferred or equally shared: The behavior of the electrons is determined by the strength by which atoms attracts the bonding electrons. Electronegativity. The degree of which an atom attracts an electron. The greediness of an atom for bonding electrons. Small atoms with the highest ionization energy have the highest Electronegativity. Most Stingy Atoms near the north east corner of the periodic table are the most electronegative. Degree of polarity is based on difference of EN, or D c

Unequal sharing of bonding electrons lead to polar bonds. Electronegativity When there is an unequal sharing of electrons between two atoms; the more stingy atom draws electrons bonded to itself. Electronegativity - Ability of an atom to draw bonded e- on to itself. If an atom is “stingy” for bonded electrons, then that atom is more electronegative that the atom it is bonded to. Trend ? Unequal sharing of bonding electrons lead to polar bonds.

Determining Molecular Polarity Flowchart (Details later) CO NO N2 ICl HI H2 How many central atoms? None Dc 1 only 2 or more Are there lone pairs on central atom? Are there lone pairs on any central atom? Yes No Yes No Polar* Are all terminal atoms the same element? Polar* Are all central atoms the same element? No Yes Yes No Polar* Non-polar Are all terminal atoms the same element? Polar No Yes Polarity depends on symmetry Non-polar *This is true for everything that has up to 4 regions of electron density (i.e., what we’re studying in this class). It may not be true for other structures, such as an octahedron.

Dipoles and its mechanism Recall the criteria for Covalent - Polar Covalent - Ionic Compounds Dc Bond Dipoles: Cl-C-C-Cl H-O-H H-Cl H-F Molecules possessing net dipole moment are polar

Polar Bonds and Polar Molecules Electron cloud distortion due to electronegative atom drawing electrons on to itself. Polar Molecules If the electron cloud is distorted over the entire molecule (i.e., winner in the tug-of-war) then molecule is polar. Nonpolar molecule If bonds are not polar or if polar bonds cancel each other (no winner in the tug-of-war), then molecule is non-polar. (Polar bonds does not necessitate polar molecules)

Polar Molecules in the presence of a magnetic field Molecules will align along a magnetic field based on their polar properties. The H2 molecules shown on the left are not influenced by a magnetic field since H2 is a nonpolar chemical. On the other hand the HF molecule orientated themselves such that the the partially negative fluoride are orientated towards the positive charge in the magnetic field. This also applies to molecules with more than two element. In H2O, the molecule is polar and it orientates itself in the presence of a magnetic field. Note however that dichloroethyne is nonpolar and is not influenced by a magnetic field.

Water, a special polar molecule: More later Water is a special molecule that is unique compared to other light weight molecule. Ordinarily, light weight molecule exist as a gas at temperatures above 0°C. Example are CH4, N2, O2, SO2, NH3. The special properties of water is due to its polar properties and its ability to stick among itself. The molecular mechanism by which water sticks to themselves is due to its polar nature and something called H-bonding. H-bonding will be covered more later in this chapter. As a result of this mechanism, water is a liquid at room temp, has a lower density in its solid state, has a low vapor pressure among others. Properties: Density - low density Surface tension - bugs walking on water Capillary action - method of which trees drink Specific Heat Viscosity - high viscosity Boiling point / melting point Higher than expected: Heat of fusion/Vaporization Vapor pressure - low vapor pressure Elaborate later in next section

Determining Molecular Polarity Flowchart (Details later) How many central atoms? None Dc 1 only 2 or more HF(2.1, 4.0) Binary Molecules HBr(2.1, 2.8) Are there lone pairs on central atom? Are there lone pairs on any central atom? FF(4.0, 4.0) MgO(1.2, 3.5) Yes No Yes FeS No NaCl(0.9, 3.0) Polar* Are all terminal atoms the same element? CuS(1.9, 2.5) Polar* Are all central atoms the same element? No Yes Yes No Polar* Non-polar Are all terminal atoms the same element? Polar No Yes Polarity depends on symmetry Non-polar *This is true for everything that has up to 4 regions of electron density (i.e., what we’re studying in this class). It may not be true for other structures, such as an octahedron. CO(2.5, 3.5) HH(2.1, 2.1) NO(3.0,3.5)

Determining Molecular Polarity Flowchart (Details later) How many central atoms? None Dc Binary Molecules NaCl CuS FeS MgO HF NO HH FF HBr CO 0.0 0.5 1.0 1.9 0.6 2.1 0.7 2.3

Determining Molecular Polarity Flowchart CO H2O2 F2CCH2 H3CCH3 H3NBH3 O2 CO2 CH2F2 O3 NCCH3 NCCl How many central atoms? None Dc CO or O2 1 only 2 or more Are there lone pairs on central atom? Are there lone pairs on any central atom? Yes No Yes No Polar* Are all terminal atoms the same element? Polar* Are all central atoms the same element? O3 H2O2 No Yes Yes No Polar* Non-polar Are all terminal atoms the same element? Polar CO2 H3NBH3 CH2F2 :NΞC-Cl No Yes Polarity depends on symmetry Non-polar H3CCH3 *This is true for everything that has up to 4 regions of electron density (i.e., what we’re studying in this class). It may not be true for other structures, such as an octahedron. F2CCH2 :NΞC-CH3

Determining Molecular Polarity NP How many central atoms? Are there lone pairs on central atom? on any central atom? 1 only 2 or more Polar* Are all terminal atoms the same element? Yes No Non-polar Are all central atoms Polar Polarity depends on symmetry NºC-CH3 C O

Determining Molecular Polarity NP How many central atoms? Are there lone pairs on central atom? on any central atom? 1 only 2 or more Polar* Are all terminal atoms the same element? Yes No Non-polar Are all central atoms Polar Polarity depends on symmetry Nº C-CH3 C O

Molecular Polarity: Examples Linear BeH2 (electron deficient molecule) Trigonal BH3 (electron deficient molecule) Tetrahedral CH4 Pyramidal NH3 Bent H2O Tetra-Bent SO2 Trig-Bent O3 NO3- CS2 H2O2 C3H4 CH3Cl SiO2 COCl2 SO3 OF2 C2H2 H2CO PF3 CI4 HCN NF3 Assignment on VSEPR and Polarity Determination

Summary Key to determine Polarity of molecule. • Lewis Structure Atoms with different EN - Polar. • VSEPR Structure Symmetry (or distortion) of e- cloud; (tug-of-war). Central atom (A) has lone pair of electrons. • Polarity Net distortion of the electron Cloud.