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Physics 3313 - Lecture 19

Physics 3313 - Lecture 19. Wednesday April 15, 2009 Dr. Andrew Brandt. Molecules Bonds Reminder test will be through Ch 8 on April 27. Molecules. Except for inert gasses elements generally combine to form chemical compounds

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Physics 3313 - Lecture 19

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  1. Physics 3313 - Lecture 19 Wednesday April 15, 2009 Dr. Andrew Brandt • Molecules • Bonds • Reminder test will be through Ch 8 on April 27 3313 Andrew Brandt

  2. Molecules • Except for inert gasses elements generally combine to form chemical compounds • Basic unit is the molecule—group of neutral atoms held together strongly enough to act as a single particle • Molecules can be studied by examining their absorption or emission spectra and through chemical properties • Molecules of a certain type have definite composition and substructure Ex. Hydrogen always has 2 H atoms; water H2O; salt Na Cl • If one atom is removed or another becomes attached then you have a different molecule with different properties • If the energy of the molecule is > < or = to the sum of the atoms energy a molecule can be formed? • < • How are molecules formed? 3313 Andrew Brandt

  3. Covalent Bond • Covalent bond-one or more pairs of electrons shared between two atoms. Electrons spend more time between the atoms leading to a net attractive force • The attractive force of electrons counterbalances • repulsive force of nuclei, leads to stable equilibrium • for H2 atom with a separation of 0.0742 nm and a total molecule energy of -4.5 eV. • So if you add 4.5 eV the molecule will split into H+H 3313 Andrew Brandt

  4. Consider Carbon 3313 Andrew Brandt

  5. Ionic Bond • An ionic bond is formed between oppositely charged ions • The electron is transferred from one atom to another, and the oppositely charged ions attract each other • Example NaCl: Cl has 17 electrons so one more closes sub shell, while Na has 11 electrons so losing one also closes a shell, Na+ and Cl- ions attract each other. Note the molecular structure of NaCl is an aggregate of ions, not as regular a molecular structure as covalent bond atoms. • Some molecules not purely covalent or ionic: HCl , CL atom attracts shared electron more strongly than H (like kids some share better than others) 3313 Andrew Brandt

  6. Simplest Molecule is H2+ • This molecule is one electron shared by two protons • If barrier is not too wide electron can tunnel between the two potentials • If R=0.1 nm electron transits 1 quadrillion times a second. • If R=1nm transits only 1/second • According to uncertainty principal • electron confined to a larger distance has less momentum and KE, so total energy of H2+ electron is less than that of electron in H with an H+ (proton) nearby, so if repulsion between protons not too great H2+ could be stable 3313 Andrew Brandt

  7. Symmetric Wave Functions for H2+ Molecule Solving Schrodinger Equation for this system is possible, but complicated. Start with conceptual approach considering symmetric wave function: 3313 Andrew Brandt

  8. Anti-Symmetric Wave Functions for H2+ Molecule 3313 Andrew Brandt

  9. Energy Approach for H2+ 3313 Andrew Brandt

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