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Understanding Molecular Formation and Bonding: Covalent and Ionic Bonds

In this lecture, Dr. Andrew Brandt explores the fundamentals of molecular formation and chemical bonding. He explains that molecules, the basic units of chemical compounds, consist of neutral atoms that bind together through various types of bonds. The discussion includes covalent bonds, where electron pairs are shared, and ionic bonds, formed through electrostatic attraction between charged ions. Examples such as H₂, NaCl, and their properties illustrate these concepts. A reminder for a test on Chapter 8 is also provided.

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Understanding Molecular Formation and Bonding: Covalent and Ionic Bonds

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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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