Building Molecular Models

Name:______Pd.:______

VSEPR Lab

Introduction

Most molecules in this activity obey the octet rule. Their Lewis-Dot structures are straightforward. Recall that to draw the Lewis-Dot structure of a molecule obeying the octet rule, eight electrons on each atom (except Hydrogen and a few others) are arranged in four pairs. Each pair either participates in a covalent bond or is present as a lone pair. Some molecules have an odd number of valence electrons or may have an expanded octet. Remember the list of atoms that break the octet rule can be found in your notes.

According to the Valence Shell Electron Pair Repulsion (VSEPR) Theory, the areas of high electron density move as far apart from each other as possible, without leaving the atom. It is the repulsion of lone pairs of electrons that causes the geometry or molecular structure of the molecule. The molecular shape is determined by the positions of the atoms in a molecule, not the positions of the electrons. The possible shapes for the molecules in this lab are linear, bent, trigonal pyramid, tetrahedral and trigonal planar.

Shape Number of pairs of Number of Bond angle Hybridization electrons shared pairs Linear 1 (2, 3 or 4) 1, 2, 3 or 4 180 sp Trigonal Planar 3 3 120 sp2 Bent 3 2 120 sp2 Bent 4 2 104.5 sp3 Trigonal Pyramid 4 3 107 sp3 Tetrahedral 4 4 109.5 sp3

Knowing the geometry of a molecule allows one to predict whether it is polar or nonpolar. Polarity of a molecule can also be determined by subtracting the electronegativity values of the individual atoms in a bond. A bond that is polar has a positive and negative end. A bond between two identical atoms or two atoms with similar electronegativities is nonpolar. The symmetry of a molecule with polar bonds determines the molecular polarity.

Ball-and-stick models will be used to represent the shapes of molecules in this activity. Several molecular models will be built. The molecular formulas, the Lewis–dot structures, sketches of the molecules, the names of the molecular shapes, the approximate bond angles, and polarities will be recorded in a data table.

Purpose

To predict shapes, bond angles, polarities and hybridization of some molecules and to build models of these molecules confirming the predictions. Procedure

1. Write the molecular formula, valence electron total, and draw the Lewis Structure for the following sixteen molecules:

1. NH3 9. H2 2. N2 10. C2H2 3. CH2Cl2 11. HF 4. Cl2 12. SO2 -2 5. SO4 13. C2H4 6. H2S 14. CH4O - 7. NO3 15. H2O2 8. CO2 16. CH2O

2. Using ball-and-stick models, build a model of each of the assigned molecules. After building each model, HAVE IT APPROVED BY YOUR TEACHER. Draw a sketch of each model as soon as it is checked; then complete the remaining entries in your data table.

3. Please put all of the pieces of the model kit in the appropriate places in the box and return the box to the teacher.