Lecture_19 - Today's Lecture Bonding General Concepts 13.1...

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1 Bonding: General Concepts Today’s Lecture 13.1 Types of Chemical Bonds 13.2 Electronegativity 13.3 Bond Polarity and Dipole Moments 1 Chemical Bonds ± chemical bonds are forces that hold groups of atoms together in a molecule ± bonding is a model used to explain molecular stability ± the system strives to achieve the lowest possible energy ± e.g. in an ionic compound, there are very strong attractions between closely packed, oppositely charged ions 2 ± ionic compoundsresult from thereact ionof a metal and a nonmetal (metals tend to lose electrons to form cations, nonmetals tend to gain electrons to form anions) e.g. 2 Na (s) + Cl 2(g) 2 NaCl (s) ± redox reaction - electrons are transferred from Na atoms to Cl atoms to give Na + and Cl - ions
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2 Coulomb’s Law ± Coulomb’s law can be used to calculate the energy of interaction between a pair of ions: ± where: ± V, the energy of interaction in J = πε = r Q Q nm J 10 x 2.31 r 4 Q Q V 2 1 19 - 2 1 0 3 ± r, the distance between the ion centres in nm ± Q 1 and Q 2 are the numerical ion charges ± ε 0 , vacuum permittivity Coulomb’s Law Applied to NaCl ± in solid NaCl: Q 1 =+1 ,Q 2 = -1, r = 0.276 nm ± V=-8 .37x10 -19 J (this energy refers to one Na + Cl - ion pair in the gas phase) ± V = -540 kJ/mol (this energy refers to one mole of Na + Cl - ion pairs in the gas phase) ± negative energy indicates that the ion pair has lower energy than the separated ions 4 ± note that in crystalline NaCl, each Na + is surrounded by 6 Cl - and each Cl - is surrounded by 6 Na + , so the energy associated with ionic bonding is much greater than 540 kJ/mol = Cl = Na + Sodium chloride
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3 ± what happens when two identical hydrogen atoms are brought close together? Chemical Bonds – Covalent Bonds ± consider the types of interactions: ± repulsive 5 ± attractive Figure 13.1(a) Bond Formation at an Optimal Distance ± system will adjust to minimize the sum of the positive
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This note was uploaded on 06/27/2009 for the course CHEM CHEM 121 taught by Professor Peggy during the Spring '09 term at Simon Fraser.

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Lecture_19 - Today's Lecture Bonding General Concepts 13.1...

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