# modeling - Molecular Structure and Geometry Objective: The...

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Molecular Structure and Geometry Objective: The purpose of this exercise is to help you develop an understanding of the structure and geometry of species with covalent bonds. Safety Tips: Even though there are no hazards explicit with this exercise, good lab practice requires that you wear goggles whenever you are in lab. Background: Covalent bonding of atoms in molecules occurs when electron pairs are shared between two nuclei. These electrons come from the outermost shell of the atom and are called valence electrons; core electrons present in shells of lower principal quantum number are not typically involved in bonding. A Lewis dot structure is a notation system used to represent the valence electrons around an atom and includes the electrons used in bonding and those that are non-bonding (lone pairs). When drawing a Lewis structure, the electrons are grouped into four pairs, reflecting the octet rule: the tendency of molecules and polyatomic ions to have structures in which eight electrons surround each atom. An octet is regarded as a stable configuration of electrons based on the limited reactivity of the noble gases, which, except for Helium, have eight electrons in their valence shell. There are some exceptions to the octet rule, both with fewer than eight and more than eight electrons. Compounds of beryllium, boron, and aluminum commonly have fewer than eight electrons in their valence shell. Elements in the third period or higher often form compounds in which the center atom has ten or twelve electrons located around it. This is often obvious from the formula, but there are many cases where you must follow the rules for Lewis structures for the true electron count to be obvious. In these cases, d orbitals have been hybridized with the s and p valence orbitals to accommodate the additional electrons. To write a Lewis structure, one follows a set of rules to place the electrons. These rules are explained in detail in your textbook, and are summarized below. Guidelines for Writing a Lewis Structure: 1. Determine a center atom. The center atom will usually be the atom with the greatest ability to form bonds to other atoms (not a H or halogen because they can each form only one bond) and is usually the atom of lowest electron affinity. 2.

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## This note was uploaded on 07/29/2010 for the course SCI none taught by Professor None during the Spring '10 term at Armstrong State University.

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modeling - Molecular Structure and Geometry Objective: The...

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