Ch13 notes part 1 - CH301 Chapter 13 part 1 CHEMICAL...

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CH301 Chapter 13 part 1 CHEMICAL BONDING All of our ideas of bonding are just MODELS!! A bond is an interaction that holds atoms together Different interactions cause varied properties. Main two models: IONIC bonding : electrons are transferred --> IONS. Ions held together by electrostatic force. METALS lose electrons to form CATIONS NON-METALS gain electrons to form ANIONS. COVALENT bonding: electrons are shared somehow. Why bonding happens: as a result, the species is more stable than the isolated atoms. IONIC BONDING Ionic materials conduct electricity when molten: so…they must consist of IONS: CATION: from the element with a low ionization energy ANION: from the element with a large (more negative, more favorable) electron affinity. INTERACTIONS BETWEEN IONS Coulomb potential energy between one positive ion and one negative one: (1/4 π ε 0 ) = 2.31 x 10 -19 J.nm ε 0 = vaccum permittivity (another constant) Q 1 = charge number on ion 1 (e.g., a +2 ion has Q = +2) r = distance between the two ions FORMING IONIC BONDS Sodium chloride, for example: r = 276pm, Na + , Cl - This gives V = (+1)(-1) x 2.31 x 10 -19 J.nm / 0.276nm = - 8.37 x 10 -19 J For one mole this would be - 504kJ/mol Note two positive ions would give a positive answer. . less stable. . the ions repel! COVALENT BONDING First consider between identical atoms. No incentive for either to gain or lose electrons The two nuclei REPEL, The electrons REPEL, but-the electrons are ATTRACTED to the nuclei! See Figure 13.1: (a) Interaction of two hydrogen atoms: A simple example of COVALENT BONDING. Note how the electrons are SHARED between the nuclei! Also see Fig 13.1 (b) Energy diagram for the H 2 molecule - note where we see repulsion, the actual bond length, and the bond energy. IONIC, COVALENT…?? Its not that easy!!! Consider a COVALENT bond between two DIFFERENT atoms: The sharing of electrons is NOT equal!!!!! See Figure 13.2: The effect of an electric field on hydrogen fluoride molecules. They line up and orient themselves! HF has a POLAR COVALENT BOND ELECTRONEGATIVITY Ability* of an atom in a molecule to attract shared electrons to itself MOST electronegative element is FLUORINE SECOND most electronegative element is OXYGEN *measured using the Pauling scale. See Figure 13.3:The Pauling electronegativity values as updated by A.L. Allred in 1961. (In 3-D)
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Bond Type and Electronegativity (Fig 13.11) Consider two different atoms, X and Y. They have a bond between them, which we represent as a line. A BOND is a PAIR of electrons, one from each of the atoms. Depending on the relative electronegativites of X and Y there are three possibilities for the electrons in that bond: X and Y have identical electronegativities. This is most common if X and Y are the same element. Neither dominates and so the electrons
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Ch13 notes part 1 - CH301 Chapter 13 part 1 CHEMICAL...

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