Molecular Shape and Hybridized Orbitals CH2000: Introduction to General Chemistry, Plymouth State University Introduction: In chemistry, the three dimensional shape of a molecule is as important as the distribution of electrons within it. The two shape and distribution of electrons are interrelated, and the one rule to remember is determining shape is this: pairs of electrons repel one another to the extent possible. In other words, a given pair of electrons will take up as much space as it can, pushing other electron pairs as far away as possible. The steps for determining molecular shape are: 1. Determine the Lewis dot structure (see previous worksheet) 2. Determine the number of regions of electron density around the central atom a) Note: double and triple bonds count as one region of electron density 3. The number of regions of electron density determines the electron geometry as follows: Regions of Electron Density Electron Geometry Hybrid Orbitals 2 linear sp 3 trigonal planar sp 2 4 tetrahedral sp 3 5 trigonal bipyramidal sp 3 d or sp 2 d 2 6 octahedral sp 3 d 2 or sp 2 d 3 4. The overall geometry has now been set--this cannot change! However, not all of the regions of electron density correspond to bonds between atoms. THE MOLECULAR GEOMETRY IS GIVEN ONLY BY THE ARRANGEMENT OF ACTUAL ATOMS. BUT the arrangement of atoms is influenced by non-bonding electrons! After you have determined the electron geometry, look at the number of regions of electron density that contain bonding electron pairs, and determine the molecular geometry based on these. Once the shape of the molecule is known, its polarity a combination of the shape and the electron distribution can be determined by the following steps: 1. Draw the three-dimensional representation of the molecule. 2. Determine the polarity of each bond by comparing the electronegativities of the two atoms in the bond (see Table of Electronegativities on page 3). A bond is considered polar if the difference between the electronegativities of the atoms is greater than or equal to 0.5. 3. Indicate the dipole moments of the bonds on the 3-D drawing. 4. Sum up the individual bond dipoles to determine the overall dipole of the molecule. Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 1
Use the table below to take notes during the introduction to class. Geometry 3-D Drawing with bond angles # of atoms around central atom Linear # of lone pairs around central atom Hybrid Orbitals Trigonal planar Bent TP Tetrahedral Trigonal pyramidal Bent Td Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 2
Geometry 3-D Drawing with bond angles # of atoms around central atom Trigonal bipyramidal # of lone pairs around central atom Hybrid Orbitals See-saw T-shaped Linear TB Octahedral Square pyramidal Square planar Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 3
Table of Pauling Electronegativities Exercises For each of the following compounds: 1) Draw the Lewis dot structure (note: you already did 1-8 last week), 2) determine the molecular geometry, 3) state the type of hybridized orbitals that will occur on the central atom, 4) if the molecule is three-dimensional, draw it using dashes and wedges, 5) indicate the bond angles on your drawing, and 6) determine whether the molecule will be polar or non-polar. Geometry Hybrid OrbitalPolar? 1. PCl 4 + 2. XeF 6 2+ 3. BBr 3 4. CN - 5. COF 2 Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 4
6. N 2 O Geometry Bond Angles Polar? 7. NO 3-8. SO 4 2-9. PCl 5 10. SCl 6 11. SO 2 12. I 3 - Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 5