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Physics 3313 - Lecture 19

4/15/2009

Wednesday April 15, 2009Dr. Andrew Brandt

1. Molecules2. Bonds3. Reminder test will be through Ch 8 on April 27

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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?

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Covalent Bond

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• 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 counterbalancesrepulsive force of nuclei, leads to stable equilibriumfor 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

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Consider Carbon

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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)

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Simplest Molecule is H2+

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•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 principalelectron 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

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Symmetric Wave Functions for H2+ Molecule

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Solving Schrodinger Equation for this system is possible, but complicated. Start with conceptual approach considering symmetric wave function:

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Anti-Symmetric Wave Functions for H2+ Molecule

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Energy Approach for H2+

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