Unit 3: Types of Bonding & Geometry Chem. II Name Unit Three March 7, 2017 Lab #3: Three-Dimensional Models of Covalent Molecules A single covalent bond is formed when two atoms share a pair of electrons. Each atom provides one of the electrons of the pair. If the two atoms are alike, the bond is said to be non-polar covalent. If the two atoms are not alike then the atom with the stronger electro negativity may attract the shared electron. The rules for electro negativity differences are as follows: 1. If the electro negativity difference (usually called EN) is less than 0.5, then the bond is non-polar covalent. 2. If the EN is between 0.5 and 1.6, the bond is considered polar covalent 3. If the EN is greater than 2.0, then the bond is ionic. 4. If the EN is between 1.6 and 2.0 and if a metal is involved, then the bond is considered ionic. If only nonmetals are involved, the bond is considered polar covalent. When atoms share more then one pair of electrons then a double or triple bond is possible. A group of atoms held together by covalent bonds is called a molecule. Molecules can be either polar or non-polar. If bonds are non-polar, the molecule is non-polar. If bonds are polar,molecules can still be non-polar if the charge distribution throughout the molecule is symmetrical. A molecule s symmetry depends on its shape, that is, the positions in space of the atoms making up the molecule. Some possible shapes are linear, bent, pyramidal, and tetrahedral. Although we represent molecules on paper as being one-dimensional for convenience, they are actually three-dimensional. By building molecular models, chemists come to understand the bonding, shapes, and polarity of even the most complex molecules. Purpose: Build three-dimensional models of some simple covalent molecules. Predict their shapes and polarities from knowledge of bonds and molecule polarity rules. Equipment: molecular model building set 1
Unit 3: Types of Bonding & Geometry Procedure: 1. Obtain a molecular model building set. Study the color code identifying the different kinds of atoms. 2. Observe that the following atoms have one hole (bonding site): hydrogen, flouring, chlorine, bromine, and iodine. The atoms with two holes are oxygen and sulfur. A nitrogen atom has three holes, and a carbon atoms has four holes. 3. Construct models of the following molecules labeled on the observation/data table below. Fill in the table. Geometry formula Valence e- s & Electron pairs Electrongroup Lewis structure (Drawn with VSEPR in mind) Bond Dipole E.N.=? Molecule polar or nonpolar? CH4 8v.e- 4e.p. tetrahedral tetrahedral H C H H H E.N.= 0.4 non-polar CO2 BF3 2
Unit 3: Types of Bonding & Geometry formula Valence e- s & Electron pairs Electron-group Lewis structure (Drawn with VSEPR in mind) Bond Dipole E.N.=? Molecule polar or nonpolar? CCl4 NH3 H2O SCl2 3
Unit 3: Types of Bonding & Geometry formula Valence e- s & Electron pairs Electron-group Lewis structure (Drawn with VSEPR in mind) Bond Dipole E.N.=? Molecule polar or nonpolar? IF4 + I3 - SO2 ICl4-4
Unit 3: Types of Bonding & Geometry formula Valence e- s & Electron pairs Electron-group Lewis structure (Drawn with VSEPR in mind) Bond Dipole E.N=.? Molecule polar or nonpolar? AsF5 CO3 2- Discussion Questions: 1. Which molecules were nonpolar because all bonds were nonpolar (in other words there were no dipoles present)? 5
Unit 3: Types of Bonding & Geometry 2. Which molecules had polar covalent bonds but were nonpolar because of symmetry (in other words the dipole moment equals zero)? 3. Explain what was present in the electron that produced polar molecules? What shapes were these molecules? 4. Explain the two factors that determine a molecules overall shape according to VSEPR. 6
Unit 3: Types of Bonding & Geometry Conclusion: Explain the factors that determine whether or not a molecule is polar. Start at the beginning, be complete and clear!! Identify the molecules in this lab that were polar because the factors you have discussed were present in the molecule. 7
Unit 3: Types of Bonding & Geometry 8