Sevy Lecture 19-24 PDF

When the love of God sets the tone for our own lives, for our relationships to each other and ultimately our feeling for all humankind, then old distinctions, limiting labels, and artificial divisions begin to pass away, and 1peace...

When the love of God sets the tone for our own lives, for our relationships to each other and ultimately our feeling for all humankind, then old distinctions, limiting labels, and artificial divisions begin to pass away, and 1peace increases. - Jeffrey R. Holland CHEM 111 PRINCIPLES OF CHEMISTRY Prof. Eric T. Sevy Fall 2024 The first great truth in the universe is that God loves us exactly that way—wholeheartedly, without reservation or compromise, with all of His heart, might, mind, and strength. - Jeffrey R. Holland 2 iCLICKER QUIZ How many resonance structures does NO2- have? A. 1 B. 2 C. 3 D. 4 E. What are resonance structures? 3 iCLICKER QUIZ What is the C-O bond order in carbonate, CO32-? A. 1 B. 1.33 C. 1.5 D. 1.67 E. 2 single, 1 double bond Bond order of resonance-equivalent bonds: #𝑝𝑎𝑖𝑟𝑠 𝑜𝑓 𝑏𝑜𝑛𝑑𝑖𝑛𝑔 𝑒 − 4 𝑝𝑎𝑖𝑟𝑠 = = 1.33 # 𝑜𝑓 𝑏𝑜𝑛𝑑𝑖𝑛𝑔 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛𝑠 3 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛𝑠 But resonance structures don’t always contribute equally to the delocalization… 4 CONSEQUENCES OF RESONANCE Lewis model is imperfect. None of the individual structures is fully correct. Molecule is actually an average. Bond order doesn’t have to be an integer. More equivalent structures ➔ greater stability (benzene). Resonance is a Lewis method to explain electron delocalization 5 iCLICKER QUIZ The term resonance is used to describe which of the following? A. Structures with different arrangements of atoms, but the same arrangement of electrons. B. Structures with the same arrangement of atoms, but different arrangements of electrons C. Structures with different arrangements of atoms and electrons. D. Structures with different arrangements of atoms. E. None of the above.  CARBON 6 DIOXIDE, CO2 For example: Draw the Lewis structure for carbon dioxide. Include any possible resonance structures. These resonance structures do not contribute equally to the delocalized structure of the molecule. Which best represents the molecule? How do you know? 7 FORMAL CHARGE Formal charge: # valence e– – 1/2 (bond e–) – (lone pair e–) Nonzero formal charge on an atom indicates the atom’s deviation from its typical # of bonds 0 0 0 1 0 -1 -1 0 1 The best Lewis structure… …is the one with the fewest formal charges. …puts negative charge on the most electronegative atom (when charge is unavoidable) 8 iCLICKER QUIZ When determining formal charge, one must compare the number of electrons on the free neutral atom and which of the following? A. The number of valence electrons assigned to the atom in a molecule. B. The number of lone pair electrons on the atoms. C. The number of electrons shared by the atom. D. All of the above. E. None of the above. 9 iCLICKER QUIZ Which is the most important resonance form for cyanate ion, OCN–? A B -1 0 0 0 0 -1 C +1 0 -2 Difference in formal charge is minimized, and extra -1 charge is on most electronegative atom 10 USING LEWIS DIAGRAMS TO PREDICT STRUCTURE What is the structure of N2O (laughing gas)? Is it NON? Or is it NNO? Draw 3 resonance structures for each one Determine formal charges Which one do you predict to be more stable? NON: NNO: Which NNO resonance structure(s) do you predict contribute the most to the delocalized structure? 11 RULE BREAKERS “Radicals” Draw the Lewis structure for NO Structure is reasonable, except for the unpaired e- & lack of octet on N atom :N = O: 0 0 Free Radical: Odd-electron molecule with an unpaired e- in its Lewis structure. Very reactive! 12 RULE BREAKERS Less than an octet Draw the Lewis structure for BF3 + F Less than full octets can occur for Be, B, Mg These atoms are small & not F B - very electronegative – don’t need 8 valence e- to be stable F 13 RULE BREAKERS More than an octet → “Expanded” octets Draw the Lewis structure for SF4 “Expanded” valence shell Atoms beyond the 2nd period can accommodate more than 8 valence electrons Have larger size & more + charge in the nucleus Tend to occur in molecules having central atom with Z>12 bonded to strongly electronegative elements (F, O, Cl) Expansion decreases formal charge on central atom 14 EXPANDED OCTET One way to recognize: there are not enough “shared electrons” as determined by our general recipe Valance electrons 7*4+6 = 34 Electrons needed for full octets 8*5 = 40 Shared electrons 40-34 = 6 Bonds = 6/2 = 3 Must expand the valance shell 15 EXPANDED OCTET LEWIS RECIPE 1) Count the valence e- present 2) Count the total e- needed (theoretically) Each atom needs 8 for an octet (H needs 2) 3) Find the difference (needed – present) 4) Divide by two. This is the number of bonds to make 5) If the number of bonding pairs is less than the number of connections needed, need to expand the valance on the ”central” atom 6) Predict the connectivity of the atoms 7) Draw single bonds between central and outer atoms 8) Use lone pairs to complete octets around terminal atoms, add extra electrons to the central atom 9) Check that the # of e- in your Lewis structure matches # in step 1 16 PCL3, PCL5 17 iCLICKER QUIZ Which of the following contains an atom with an expanded octet? A. MgF2 B. CO32- C. H2S D. XeF4 E. None of them 18 [ ] s2p6 GROUP 18: NOBLE GASES Nonmetals Very low boiling points Gases at room T Monatomic (He, Ne, Ar, Kr, Xr, Rn) Completely filled s & p subshells Do NOT usually gain/lose e- Exceptionally unreactive (“noble”) 19 NOBLE GASES Unreactive… or are they? Ionization energy ↓ going down the group as radius ↑ 1962 – Neil Bartlett (Canada) formed Xe compounds Xe → XeF2, XeF4, XeF6 Kr → KrF2 only known stable compound Ar → made HArF in 2000 only stable at low T Nobel gas + Halogen Excimer Lasers (KrF, XeCl…) XeF4 crystals 20 iCLICKER QUIZ Lewis works well for atoms like P, S and Xe A. True B. False 21 iCLICKER QUIZ Lewis get the magnetic character of O2 correct A. True B. False 22 iCLICKER QUIZ Lewis works well for most metal atoms A. True B. False 23 iCLICKER QUIZ Lewis’s model of bonding is useless A. True B. False 24 OCTET RULE EXCEPTIONS The Octet Rule Unfortunately, Mother Nature’s never heard of it → There are many exceptions to the octet rule Lewis structures & Octet Rule are most useful for period 2 p-block elements → C, N, O, H Organic Chemistry! 25 iCLICKER QUIZ Which of the following statements is correct? A. Lewis structures can be used to explain bonding in all molecules. B. All atoms (except H) obey the octet rule. C. There is never more than one correct Lewis structure of a molecule. D. The largest separation of formal charge is the most favorable molecular configuration. E. Lewis structures are useful, even if they are not correct description of bonding. 26 DRAWING MOLECULES Lewis structure But this gets unwieldy Condensed structure C C Line structure C C C Lines are bonds, Ends/kinks are carbons, H’s are implied (assume each C has 4 bonds) 27 LINE STRUCTURES ARE YOUR FRIEND NASONEX ‘3D’ Atoms TYLENOL CAFFEINE But remember to keep track of ‘implied’ atoms: How many C atoms are in acetaminophen? 8 How many H atoms are in Caffeine? 10 28 MOLECULES HAVE SHAPES Structure - function relationship 29 THE SHAPE OF MATTER MATTERS 30 MOLECULAR SHAPE Chemical/physical properties are often related to the shape of a molecule Lewis Structures show atoms and bonds, but not 3D spatial orientations Bond angle often differs from Lewis structure depiction Need a way to predict 3D shape 31 VSEPR Valence Shell Electron Pair Repulsion Theory “The best arrangement of electrons is the one that minimizes the repulsions among them.” Most important factor = electrons keep as far apart as possible Sort of like balloons repelling Helps you predict the arrangement of valence e- around a central atom that produces the most likely (lowest energy) bonding orientation 32 MOLECULAR GEOMETRY Steps for determining molecular geometry using VSEPR: 1) Draw the Lewis structure 2) Determine the steric number (SN) of the central atom: (SN = # of electron groups around an atom) 3) Arrange the electron groups to minimize repulsion between them → electron group geometry 4) Ignore lone pairs when naming the final shape of the molecule → molecular geometry