MSE306Chap3Bonding-2

MSE306Chap3Bonding-2 - Bonds Between Neutral Atoms Ionic...

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Bonds Between Neutral Atoms Ionic bonding is simple to understand due the attraction of unlike charges and repulsion of like charges. Now we will consider 3 types of bonds between neutral atoms van der Waals Covalent Metallic
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Dipoles A pair of oppositely charged particles Even though the total charge is neutral, they produce an electrostatic field capable of exerting a force on other electrical charges. The corresponding field (and force) are… ( 29 ( 29 ( 29 ( 29 [ ] 2 2 1 2 1 1 1 2 2 1 1 cos cos cos 2 1 cos cos 2 cos 2 r r a e r a r a r e V a r e a r e l e l e V θ μ = θ θ - θ = θ + - θ - = - = q r r V r r V E F E E r r = θ μ = θ - = θ μ = - = 3 3 sin cos 2 a -e1 +e2 θ r e
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Secondary Bonding (I) Secondary = van der Waals = physical (as opposite to chemical bonding that involves e - transfer) bonding results from interaction of atomic or molecular dipoles and is weak, ~0.1 eV/atom or ~10 kJ/mol. Permanent dipole moments exist in some molecules (called polar molecules ) due to the asymmetrical arrangement of positively and negatively regions (HCl, H 2 O). Bonds between adjacent polar molecules permanent dipole bonds – are strongest among secondary bonds . Polar molecules can induce dipoles in adjacent non-polar molecules and the bond is formed due to the attraction between the permanent and induced dipoles . Even in electrically symmetric molecules/atoms an electric dipole can be created by fluctuations of the electron density distribution. A fluctuating electric field in one atom is felt by the electrons of an adjacent atom, and induce a dipole momentum in this atom. This bond due to fluctuating + _ + _ - van der Waals bonding
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Induced Dipoles Electrons are typically displaced from their normal position due to external fields This charge redistribution may be considered as formation of a dipole Within limits Polarizability Two inert gas atoms will automatically induce a dipole in each other The result is a net attraction between the two neutral species E α μ I =
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Consider two inert gas atoms + _ + _ 6 2 7 2 7 2 3 3 3 recall ; ' ' ' ' ' r dr r r r dr d r f r dr d dr d f dr d a e a dr d e e f r I μ α 2245 μ α 2245 ϕ μ α 2245 μ μ α 2245 μ 2245 α = μ = = + + - = E E E E Ε E E E Again, need to consider repulsion So called L-J 6-12 potential models the behavior of inert gases well. 12
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This note was uploaded on 11/24/2009 for the course MSE 306 taught by Professor Smith during the Spring '09 term at UVA.

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MSE306Chap3Bonding-2 - Bonds Between Neutral Atoms Ionic...

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