CHEM 6A Lecture Notes - Lecture 21: Lithium Chloride, Negative Energy, Periodic Trends
3 Jun 2018
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Energy of an Ion Pair (*Why atoms form ionic bonds?)
● Consider Na(g) + Cl(g) → Na+Cl-(g)
â—Ź First: Electron transfer occurs to form ions
○ Na(g) → Na+(g) + e-First ionization energy of Na - IE1IE1 = 0.824 aJ
Cl(g) + e- → Cl-(g) EA1EA1 = -0.580 aJ
○ Na(g) + Cl(g) → Na+(g) + Cl-(g) electrons cancel out
â– Erxn = 0.244 aJ
Then: Ions move together, separated by equilibrium bond distance
● Energy change defined by Coulomb’s law
â—Ź Q1, Q2 = integer charges of ions (E proportional to Q)
â—Ź d = distance (in pm) between ions (E proportional 1/d)
â—Ź ECoulomb = k = (231 aJ pm)
â—Ź For Na+Cl-
â—Ź D = rNa+ + rCl-
â—‹ = 102 pm + 181 pm (Table 6.4)
â—‹ 283pm
â—Ź ECoulomb = (231 aJ pm) = -0.816 aJ
● Na(g) + Cl(g) → Na+(g) + Cl-(g) Erxn = 0.244 aJ
● Na+(g) + Cl-(g) → Na+(g)Cl-(g) ECoulomb = -0.816aJ
● Na(g) + Cl(g) → Na+Cl-(g) Erxn = -0.572 aJ
â—Ź Overall lowering of energy
â—Ź Bond formation is favorable
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Document Summary
Energy of an ion pair (*why atoms form ionic bonds?) First: electron transfer occurs to form ions. Na(g) na+(g) + e- first ionization energy of na - ie1. Na(g) + cl(g) na+(g) + cl-(g) electrons cancel out. Then: ions move together, separated by equilibrium bond distance. Q1, q2 = integer charges of ions. D = distance (in pm) between ions (e proportional 1/d) Ecoulomb = k = (231 aj pm) = 102 pm + 181 pm (table 6. 4) Ecoulomb = (231 aj pm) = -0. 816 aj. Na(g) + cl(g) na+(g) + cl-(g) erxn = 0. 244 aj. Na+(g) + cl-(g) na+(g)cl-(g) ecoulomb = -0. 816aj. Na(g) + cl(g) na+cl-(g) erxn = -0. 572 aj. Comparison between different ionic compounds can be estimated by ecoulomb. Ecoulomb = k ionic bond strength proportional to q1,q2 and 1/d. For mgcl2 increase in q1, q2 (larger charge) Leads to increase in energy (more negative) A covalent bond consists of a shared electron pair.
