Unit 2 Module 2-1 - Unit 2 Module 2 Looking for Patterns John Pollard University of Arizona Knowing both the atomic composition and connectivity of

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Unit 2 Module 2 Looking for Patterns John Pollard University of Arizona Knowing both the atomic composition and connectivity of the atoms in a molecule is fundamental to determining the identity and properties of a substance. The idea that molecules are comprised of atoms connected to each other in some way and arranged in particular geometries in space is very useful in explaining and predicting properties. To fully describe molecular structure, we must first understand the bonding model used to explain how atoms connect to each other A Bonding Model In molecules, atoms are thought to be bonded together by the electrostatic interactions between the electrons and protons. Electrons, being negatively charged are attracted to the protons residing in the nuclei of the atoms bonded. There is a balance between attraction and repulsion (between like charges) that establishes a lowest energy distance between the nuclei called the bond length. In addition to the electrostatic forces involved in bonding, electrons (and protons) have an intrinsic property called “spin”. Spin generates an additional force between electrons that is magnetic in nature and weaker than the electrostatic forces involved in bonding. When electrons are confined to the same region of space, like in between two nuclei, “spin pairing” reduces the energy of the system. The spin pairing of 2 electrons in a bond is often symbolically represented by an upward and downward pairing of arrows. Electrons do not behave as “classical” particles in that we cannot know their position and velocity simultaneously. Electrons are more accurately described as particles that occupy regions of probability. The electron density is a measure of this probability. When a simple covalent bond is formed between atoms, there is an increase in electron density between the atoms involved. In addition, the electrons are delocalized over the bonding region and there is a spin pairing between the two electrons involved. Therefore, two electrons are involved in each bond. Atoms can have more than one bond between them but in general, every pair of electrons will be spin paired. Chemical bonds are not static. The nuclei and electrons involved are constantly in motion. The atoms involved in a bond vibrate around their equilibrium position with a frequency that depends on the bond strength and atomic masses. The vibrational modes that occur in molecules are also quantized, meaning there are only specific vibrational modes with specific energies accessible to a molecule. As with the energy level diagrams used to represent the absorption or emission of electromagnetic radiation by atoms, one can represent the various vibrational modes of a molecule in the same fashion.
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The absorption of infrared (IR) radiation at specific frequencies can be used to detect the presence of specific bonding motifs. The IR spectrum is a sort of fingerprint for a molecule and can reveal important information about the presence of certain types of bonding.
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This note was uploaded on 12/08/2009 for the course CHEM 151 taught by Professor Staff during the Fall '08 term at University of Arizona- Tucson.

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Unit 2 Module 2-1 - Unit 2 Module 2 Looking for Patterns John Pollard University of Arizona Knowing both the atomic composition and connectivity of

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