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Unformatted text preview: Bonding and Structure 1 Copyright, Arizona State University Copyright, Arizona State University Bonding and Structure II Better Models (more...) ELECTRONS IN ORBITALS primarily determine organic structure, shape and reactivity. We need to know 1. What do ATOMIC and MOLECULAR ORBITALS REALLY LOOK LIKE? This tells us where the electrons are in atoms and molecules. 2. WHAT ARE THE (RELATIVE) ENERGIES of electrons in atomic and molecular orbitals? This tells us about the relative reactivities of electrons. 1 Simple Quantum Chemistry Description of Orbitals and Bonding (more...) Quantum Mechanics provides a more realistic description of the location and energies of electrons in atoms and molecules, you need to understand this!! An Apparent Problem: Quantum Mechanics shows that it is impossible to say exactly WHERE an electron is in an orbital, only the probability of finding the electron at a particular point in space can be obtained. HOWEVER, this is NOT REALLY A PROBLEM , since this is a natural consequence of the WAVE NATURE of electrons. Without an understanding of this wave nature we cannot properly understand orbitals and bonding in molecules . Quantum Mechanics describes atomic and molecular orbitals in terms of a WAVEFUNCTION EQUATION , which incorporates and describes this WAVE NATURE of electrons in orbitals The WAVEFUNCTION is given the Greek letter capital Psi, and for atomic orbitals has the form: = f ( n, l, m, s, r, ...) wavefunction symbol is a function of these parameters (and more) distance from nucleus quantum numbers Here, n , l , m , and s are the QUANTUM NUMBERS that you encountered in General Chemistry: n = PRINCIPLE quantum number l = ANGULAR MOMENTUM quantum number m = MAGNETIC quantum number s = SPIN quantum number Another parameter that is important for us is the distance of the electron from the nucleus , r . Wavefunctions exhibit WAVE BEHAVIOR , i.e. just like any wave they can be positive, negative or zero Solving the WAVE EQUATION for the different QUANTUM NUMBERS gives the different atomic orbitals The probability of finding an electron at a particular distance from the nucleus, P(r), is given by the value of the wavefunction squared, this is our answer to where are the electrons in an orbital ! P(r) = | (r)| 2 Quantum Mechanics is highly mathematical, HOWEVER, we do it in pictures , as a plot versus distance from the nucleus , which is much easier! OUR GOALS : 1. Learn to draw PICTURES of the WAVEFUNCTIONS of an atomic orbital, they will LOOK like waves ! 2. Learn to COMBINE THE PICTURES of atomic orbital WAVEFUNCTIONS , so that we can MAKE BONDING MOLECULAR ORBITALS 3. Learn to draw PICTURES of BONDING MOLECULAR WAVEFUNCTIONS , so that we can understand where the electrons are in molecules, and thus get information about their relative energies, in molecules At this point you should be confused since all of this is difficult to understood without examples . Bonding and Structure...
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