Presentation on theme: "Q. Are we ready for complex covalent molecules yet? A. Almost."— Presentation transcript:
1 Q. Are we ready for complex covalent molecules yet? A. Almost. Astaxanthin, a member of the carotenoid family, is part of a nutritious diet—for lobsters and shrimp. These crustaceans bind astaxanthin to the protein β-crustacyanin, which changes the shape of the astaxanthin molecules, turning them a gray-blue color. Cooking releases the molecules, returning them to their original bright red color.cook
2 So far, covalent bonding has been described as totally equal sharing So far, covalent bonding has been described as totally equal sharing. Sometimes, sharing is orClH+Cl kinda hogsthe electron.H gets 2 (not 8because H can’t handle 8)ClHHydrogen doesn’t have to complete an octet, only a doublet.
3 Many bonds are polarized. Electrons preferentially circulate around the chlorine atom, taking up more space there.d+Hd-ClThink lurid!What about the sex life of bowling pins?
5 Covalent molecules with more than 2 atoms are slightly more complex Covalent molecules with more than 2 atoms are slightly more complex. Example: CO2Bonds: polarMolecule: not polar.
6 The shape of a molecule is determined by electron-electron repulsion. 120 degreesThe first bond (gray) is normal; the second one (orange: it’s one bond in two parts) lies above & below the first bond—think ofp -type orbitals.All there is to VSEPR theory:How can electrons be asfar apart as possible?
8 If there are more than 2 atoms in the molecule, the existence of polarized bonds does not necessarily make the whole molecule polarized. It depends on symmetry.
9 The individual C-O bonds are polar, but the effect cancels due to symmetry when we consider the whole CO2 molecule. The CO2 molecule is not polar. Despite pretty high mass (44x hydrogen) it is still a gas at normal temperatures.
10 If there are more than 2 atoms in the molecule, the existence of polarized bonds does not necessarily make the whole molecule polarized.F1. Are the bonds polar?2. Is the molecule polar?Always 2questions:120oBFFThis is configuration of atoms is called trigonal planar.BF3 is a toxic gas, useful for synthesizing various chemicals.This molecule violates the octet rule. Oh, well.
14 Tetrahedron: to draw this thing, we learn to draw a cube first Tetrahedron: to draw this thing, we learn to draw a cube first. Then locate the lower and upper “crossed” vertices. This shape is important!HHCH109.5 degrees in 3D gives the electrons more space than 90 degrees in 2DH
16 What is the shape of water? Nonbonding e- cloudHOHRight idea, but we forgot an important detail. Nothing constrains those electron clouds to exist in 2D!
17 Tetrahedron: to draw this thing, we learn to draw a cube first Tetrahedron: to draw this thing, we learn to draw a cube first. Then locate the lower and upper “crossed” vertices. This shape is important!OH109.5 degrees gives morespace to the electronsH
18 So water isn’t straight! Electronscomin’ at ya!ElectronsGoing away from you!OElectrons in the planeof the blackboard…umm…screen.HH109.5 degrees
19 It’s great not to be straight!* Despite its low mass (18 x hydrogen) water is a liquid at normal temperatures. Dipole-dipole forces!Explains: high boilinghigh heat capacity,surface tension.HOVector addition. Break some vectors down into x and y components, then add them for practice.*This is not a social commentary.
21 See also: http://www.youtube.com/watch?v=RqrdcVCZ794 Let’s do a thought experiment (gedanken experiment). What happens if three people aim water cannons at a ball? Can the ball be suspended in mid-air?The purpose of this is to illustrate symmetry of forces—relating to polarity of molecules. Gotta keep trying!See also:
23 Electronegative elements (Northeast elements) Suck Electrons This concept map addresses the following question: Are molecules polar?Electronegative elements (Northeast elements) Suck ElectronsAtoms identicalAtoms NOT identicalBond Not PolarBond PolarSymmetrical?Yes NoMoleculenot polarMoleculepolarExample: H2Example: CH4Example: CFH3Why: all 4 H atoms pull electrons equally and Symmetrically from C so forces balanceWhy: the 3 H atoms pull electrons down, but not as hard as the F atom pulls up.Result: CFH3 has higher melting point (-142) than CH4 (-182) than H2 (-253).
24 Happy St. Patrick’s Day (Late) Green: l ~ 530 nm
27 Some covalent structures make ions; after that, treat them as ions. So far, we have seen only atomic ions Na2O, NaCl, CaCl2There are also "molecular ions" - covalent moleculescan have a charge !e.g. Sulfate ionSO42-How many valence electrons ?S : 6 x 1 = 6O : 6 x 4 = 24Charge : 2- = 232 e- totalIf it looks like a duck,swims like a duck,and quacks like a duck,then it probably is a duck.Molecular ions pass the duck test.
28 What does this molecular ion look like? You need to be told that the sulfur is in center. (When in doubt, assume the least prevalent atom is centered.)put electrons in to bond O to S (takes 8)satisfy octet for oxygen (takes 24)total used: 32 electrons = OK
29 Treat the molecular ion similar to an atomic one Treat the molecular ion similar to an atomic one. Sulfate has –2 charge, so behaves like O2-
30 Do another: phosphate Let's do another: phosphate PO43- P : 5 valence e- 4 x O : 24 valence e-charge : 3 valence e-32 total electrons
31 Why is phosphate –3 while sulfate is only –2? Answer: One less proton:compare P (z = 15) to S (z = 16)
33 Formal charge is not used much in this class, but it is described in the notes on the web. You can get a formal charge for each atom in the molecule.FC = Number of Protons in atom- electrons in inner core of atom- Half the Number of electrons in bonds- any nonbonded electrons.Nature hates to produce charges, so the “best” Lewisstructure has low formal charge.
34 Return of our old nemesis: Nonmolecular vs. Molecular Compounds …is largely the difference between ionic and covalent!Covalent: mutual attraction to shared ion cloud.Ionic: One has electrons, the other gave electrons.
35 Liquids are a very complex and rare* state of matter. Biggest difference: cohesion.Why cohesion?Permanent dipoles—e.g. water.Induced dipoles—e.g. Cl2 or He (at very cold temps!)*The liquid state is remarkably rare; in particular,few planets have liquid water. Some have liquid methane.
36 Nonpolar molecules and atoms generate temporary dipoles Nonpolar molecules and atoms generate temporary dipoles. The associated field travels to a neighbor almost instantaneously, causing it to form a complementary dipole. This leads to a weak attraction, visible at low temperatures.__d+dd+dThis plus charge is only a partial charge AND it lasts just a very, very short time, but that is long enough to induce a neighbor to generate a charge. The two dipoles then attract, very weakly.
38 Water is a strange, strange liquid! Very high melting and boiling points for such a light molecule (due to high polarity and H bonds)Solid density is less than liquid density. Very unusual!High heat capacity: it takes a lot of energy to raise the temperature of water. This is important determining the weather and stability of ocean temperature, etc.High heat of vaporization: boiling water takes lots of energy; it really is attracted to itself! Can you think of social groups that are like this?
39 Other solids lighter than their corresponding liquid, like water. CAUTION: This is just a Yahoo Answers List!== Less Dense in Solid Form ==confirmed as solid less dense than liquid:gallium (solid) vs (liquid)bismuth (solid) vs (liquid)germanium (solid) vs 5.60 (liquid)silicon (solid) vs 2.57 (liquid)water (solid) vs (liquid)claimed but probably false:acetic acid (solid) vs (liquid)antimony (solid) vs 6.53 (liquid) (this "error" is repeated in many places, inc wikipedia) Water is not always less dense in solid form. Depending on how the water crystals are formed, it may actually be more dense. Examples include HDA and VHDA.
40 More about water Figure 7.9 Surface Tension High surface tension. The amount of energy required to expand the surface area by one unit of area is called the surface tension.Figure 7.9 Surface Tension
41 Hydrogen “bonds”—how real? Things containing-OH (e.g. CH3OH)-NH (e.g. CH3NH2)HF
42 H-bonding & permanent dipoles make water very high-boiling.
44 Solids --crystalline: a regular array of atoms or molecules exists We see x-ray diffractionexamples: sugar, salt, ice--amorphous:atoms or molecules in no real order, yet more or less immobilized.examples: glass, very rapidly cooled water or rapidly cooled polymers
45 Metals: things with distributed electrons (not just shared—but smeared out!) Metals can be liquid (mercury) or solid (gold).Usually elemental, but covalent molecular metals and "near-metals" do exist.The nuclei of elemental metals are usually said to be afloat in a "sea of electrons".
46 Metallic Properties: all related to that sea of electrons Luster--they shine!High electrical conductivity--use metals for wires.High thermal conductivity--cold metal will feel much colder than cold wood, for example. The heat rushes out of metal quickly. You may see someone walk on fiery hot coals, but not on sunbaked train tracks!Ductility and malleability: metals can be drawn (into wire) or hammered (into leaf).Insolubility in water. Well...this is a tricky one. Metals do not dissolve with water--unless they react with it to produce a metal salt.
47 Semiconductors are metalloids with deliberate impurities.
48 Before semiconductors, we used tubes to control the flow of electrons.
49 Transistors do the same thing as tubes, but we can make them much smaller and at lower cost.
50 The Eniac (Electronic Numerical Integrator and Computer)
51 Supercritical fluids ... are sort of half-gas/half-liquid substances. Special types of materials—a little bit more is said in the notes on-line. Supercritical fluids ... are sort of half-gas/half-liquid substances.Gels ... are mechanical solids made mostly from liquids! AerogelsSuperconductorsLiquid Crystals