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Unformatted text preview: Bonding and Structure 1 Copyright, Arizona State University Copyright, Arizona State University Bonding and Structure I The Basics (more...) 1 Energies of Electrons in Atoms and Molecules (more...) Bonding , structure , shape and reactions of organic molecules are determined PRIMARILY by the Energies of the electrons in ATOMIC and MOLECULAR ORBITALS In organic chemistry we use MODELS, generally the most useful model is the simplest one that explains what we are trying to understand. Here we introduce a simple model of bonding and some basic factors that determine electron energies. Later we will develop more sophisticated models for orbitals and bonding to understand electron energies 1.1 Energies of Electrons in Atomic Orbitals (more) Simple picture of factors controlling electron energies (a better description is coming in the next section) electron stabilized by nucleus, held "tightly" close to nucleus, relatively low energy electron 3+ H Li 1s 1 1s 2 2s electron in larger 2s A.O., further from nucleus and shielded by two 1s electrons, not held tightly, relatively high energy electron C 1s 2 2s 2 2p 2 electron in even larger 2p A.O., but, higher positive charge on the nucleus (6+), thus outer electrons held reasonably tightly by nucleus, moderate energy 6+ O 1s 2 2s 2 2p 4 even higher positive charge on the nucleus (8+), but now atomic orbital has another negatively charged electron, outer electrons held reasonably tightly by nucleus, moderate energy 8+ 1.2 Quantitative Energies of Electrons in Atoms (more) Quantitative information about the relative energies of electrons is obtained from measurements of Ionization Energies, or Ionization Potentials (IPs) The "first" IP is the energy required to completely remove the highest energy electron from an atom or molecule Example from General Chemistry (energies in eV, i.e. electron Volts) energy of an electron that is infinitely far from any nucleus H 1s Li 1s 2 2s N 1s 2 2s 2 2p 3 O 1s 2 2s 2 2p 4 C 1s 2 2s 2 2p 2 1s IP ~ 13.6 1s 2s 1s 2p y 2s "core" electrons, not involved in reactions/bonding "valence" electrons, are involved in reactions/bonding 1s 2p y 2s 1s 2p y 2s 2p x 2p x 2p x 2p z 2p z IP ~ 5.4 IP ~ 11.3 IP ~ 14.5 IP ~ 13.6 Energy Electrons in atoms are low in energy because they are negatively charged and are stabilized by the positively charged nucleus Bonding and Structure 2 Copyright, Arizona State University Electrons that are held "less tightly" by the nucleus are HIGH in energy, and thus require less energy to remove from an atom, and thus have a l ow IP The IP is not equal to the electron energy , but provides the relative energy information we seek many factors influence atomic I.P.s , (orbital size, nuclear charge, orbital occupancy, etc.), however, a detailed understanding is not necessary at this point , however, should know that in general .......
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