Wade Chapter 2 - 2 Structure and Properties of Organic Molecules H C C H I n Chapter 1 we considered how atoms bond together to gain noble-gas

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upward displacement downward displacement rest position + rest position d d Á Figure 2-1 A standing wave. The fundamental frequency of a guitar string is a standing wave with the string alternately displaced upward and downward. Structure and Properties of Organic Molecules 2 CC H H I n Chapter 1, we considered how atoms bond together to gain noble-gas configura- tions, forming molecules in the process. Using the octet rule, we drew Lewis struc- tures for organic molecules and used these diagrams to determine which bonds are single bonds, double bonds, and triple bonds. We discussed various ways of drawing organic structures, and we saw how resonance structures represent molecules whose actual bonding cannot be described by a single Lewis structure. Chapter 1 does not explain the actual shapes and properties of organic mole- cules. To understand these aspects of molecular structure we need to consider how the atomic orbitals on an atom mix to form hybrid atomic orbitals and how orbitals on dif- ferent atoms combine to form molecular orbitals . In this chapter, we look more close- ly at how combinations of orbitals account for the shapes and properties we observe in organic molecules. 2-1 Wave Properties of Electrons in Orbitals We like to picture the atom as a miniature solar system, with the electrons orbiting around the nucleus. This solar system picture satisfies our intuition, but it does not accurately reflect today’s understanding of the atom. About 1923, Louis de Broglie suggested that the properties of electrons in atoms are better explained by treating the electrons as waves rather than as particles. There are two general kinds of waves, traveling waves and standing waves . Examples of traveling waves are the sound waves that carry a thunderclap and the water waves that form the wake of a boat. Standing waves vibrate in a fixed location. Stand- ing waves are found inside an organ pipe, where the rush of air creates a vibrating air column, and in the wave pattern of a guitar string when it is plucked. An electron in an atomic orbital is like a stationary, bound vibration: a standing wave. To understand the features of an orbital (a three-dimensional standing wave) more easily, let’s consider the vibration of a guitar string as a one-dimensional analogy (see Figure 2-1). If you pluck a guitar string at its middle, a standing wave results. In this mode of vibration, all of the string is displaced upward for a fraction of a second, then downward for an equal time. An instantaneous picture of the waveform shows the string displaced in a smooth curve either upward or down- ward, depending on the exact instant of the picture. The waveform of a 1 s orbital is like this guitar string, except that it is three- dimensional. The orbital can be described by its wave function , which is the mathematical description of the shape of the wave as it vibrates. All of the wave is positive in sign for a brief instant; then it is negative in sign. The electron density c , 39
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This note was uploaded on 04/29/2008 for the course CHM 2210 taught by Professor Reynolds during the Spring '01 term at University of Florida.

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Wade Chapter 2 - 2 Structure and Properties of Organic Molecules H C C H I n Chapter 1 we considered how atoms bond together to gain noble-gas

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