Molecular Geometry And Polarity Laboratory Assignment Satisfaction

CHM 11500 Fall 2014 1 Objectives At the end of this activity you should be able to: oWrite Lewis structures for molecules. oClassify bonds as nonpolar covalent, polar covalent, or ionic based on electronegativity differences. oRecognize exceptions to the octet rule; draw accurate representations. oDescribe 3-dimensional shapes of simple molecules based on VSEPR theory. oPredict polarity based on geometry and individual dipole moments. IntroductionThe substances in our world exhibit remarkably different properties. At room temperature some substances are solids, others liquids, and others gases. Some participate in sudden chemical reactions, whereas others are quite inert and unreactive. Perhaps most remarkably, this wonderful diversity occurs even though the substances are comprised of a limited number of elements. Indeed, only a very small number of different elements are present in almost any pure substance we encounter in the environment or the laboratory. How can this wide diversity of properties be explained? A key to understanding the wide range of physical and chemical properties of substances is recognizing that atoms combine with other atoms to form molecules or compounds and that the shape or geometry of a collection of atoms strongly affects the properties of that substance. One reason this occurs is because the distribution of charge in a molecule affects many properties of the substance. For example, if the negative charge is concentrated in one region of a molecule its’ properties will be widely different than if the charge is distributed evenly throughout the entire molecule. In this investigation you will examine a theory that chemists use to explain different aspects of chemical bonding: Valence-shell electron-pair repulsion (VSEPR) theory. Attention will be given to how molecules are arranged in different shapes and how chemists can predict the geometry of a given molecule. It will then be shown how a molecule’s shape, along with electronegativity differences of its atoms, determines the molecule’s polarity. As suggested above, the best way to understand and predict the physical and chemical properties of substances in our world is by understanding their structure at the molecular level. 1Prepared by Dr. Ted M. Clark and Dr. Patrick Woodward, the Ohio State University, Department of Chemistry and Biochemistry. Modified by Dr. Christine Hrycyna, and Ms. Marybeth Miller, Purdue University, Department of Chemistry. Molecular Geometry and Polarity1

General Chemistry 1 Laboratory 10: Lewis Dot Structures and Molecular Geometry 10.1 Introduction 10.1.1 General G. N. Lewis (in about 1916) observed that many elements are most stable when they contained eight electrons in their valence shell. He suggested that atoms with fewer than eight valence electrons bond together to share electrons and complete their valence shells. Bonds form tending to stabilize a chemical system by releasing energy. The larger the amount of energy released during the formation of a bond, the more stable the bond will be. If two atoms release energy (Exothermic reaction) by forming a bond, then the atoms will be more stable by staying together than they would be as individual atoms. 10.1.2 Valence Electrons The electronic configuration of an atom is given by listing its subshells with the number of electrons in each subshell, as shown in Table 1 . According to the Aufbau principle , the electrons of an atom occupy quantum levels or orbital’s starting from the lowest energy level, and proceeding to the highest, with each orbital holding a maximum of two paired electrons (opposite spins). Study the third column of complete electronic configurations carefully so you understand how electrons are added to the subshell of lowest energy until it reaches its capacity; then the subshell of the next energy level begins to be filled. The electrons on the highest numbered subshells are the valence electrons , which comprise the valence shell of the atom and participative in chemical bonding. Table 1: Electron Configurations and Oxidation Numbers Element Name Atomic Number Electron Configuration Valence Shell Hydrogen 1 1 s 1 1 s 1 Helium 2 1 s 2 1 s 2 Lithium 3 1 s 2 2 s 1 2 s 1 Beryllium 4 1 s 2 2 s 2 2 s 2 Boron 5 1 s 2 2 s 2 2 p 1 2 s 2 2 p 1 Carbon 6 1 s 2 2 s 2 2 p 2 2 s 2 2 p 2 Nitrogen 7 1 s 2 2 s 2 2 p 3 2 s 2 2p 3 Oxygen 8 1 s 2 2 s 2 2 p 4 2 s 2 2 p 4 Fluorine 9 1 s 2 2 s 2 2 p 5 2 s 2 2 p 5 Many chemists record the electron configuration of an atom by listing the noble gas symbol in brackets followed by the remaining electron configuration of an element. This process easily

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