Review of bond types. Sharing Electrons to Form Covalent Bonds. What is the covalent bond? Patterns in nonmetal - nonmetal chemical reactions:

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1 Review of bond types To find truth you have to try and you have to persist in trying. Sometimes it s fun. Sometimes it s hard or boring. But it s always worth it.... The Creator of the universe has implanted a message in every created thing. Geology, astronomy, physics all science is really nothing more than an effort to read those messages. ~ Henry Eyring PS 110A Hatch Ch Metallic bonding: many atoms collectively share orbitals and electrons Ionic bonding: metals easily give up electrons to nonmetals, which want them Today we discuss covalent bonding and intermolecular interactions. Living things are made of covalentlybonded PS 110A Hatch Ch What is the covalent bond? Sharing of two electrons (a pair) by two atoms to complete a (sub)shell. Represent the electron sharing by dots or single heavy line Sharing Electrons to Form Covalent Bonds Resulting molecular orbitals are lower in energy than atomic orbitals ften corresponds to filled shells for each atom in bond Result: usually rather than network structures. Standard Example: H + H H 2 : fills in lower molecular orbitals. Fairly strong bond! PS 110A Hatch Ch PS 110A Hatch Ch Patterns in nonmetal nonmetal chemical reactions: chemical reactions: Covalent Bond Prefixes Some of the most important compounds in nature combine nonmetals with nonmetals. Examples: H 2 (water) C 2 (carbon dioxide) CH 4 (methane) NH 3 (ammonia) Demo: water, ammonia, NH + 4 (ammonium ion) N 3 (nitrate ion) methane models (sulfate ion) S 4 2 PS 110A Hatch Ch Use of prefixes 1 mon 2 di 3 tri 4 tetra Examples: C carbon monoxide C 2 carbon dioxide CCl 4 carbon tetrachloride PS 110A Hatch Ch. 21 7

2 Molecular ions Combinations of atoms which often occur together in compounds carbonate (C 3 ) nitrate (N 3 ) sulfate (S 4 ) Sharing of More Than ne Electron Pair: Multiple Bonds Multiple bonds are stronger than single bonds (although a double bond is not twice as strong as a single bond) By convention we represent single bonds with a single line, double bonds with two lines, and triple bonds with three lines. High electron density between multiplybonded atoms makes them relatively reactive P1: Name the following: Be, CCl 4, LiF, Na 2 S, CaC 3 These are examples of hydrocarbons. PS 110A Hatch Ch PS 110A Hatch Ch Demonstration of covalent bonding: Nylon formation Common of Covalent Compounds carboxyl The preferred fabric for parachutes and women s stockings prior to WWII was silk. This was running in short supply, so synthetic fibers were developed. amine exist as, atoms don t separate usually weak interactions between gases or liquids, usually most likely colorless nonconductors often toxic (some important exceptions) PS 110A Hatch Ch PS 110A Hatch Ch Important Rule Rule of thumb: (ctet Rule) Atoms want to obtain the same number of valence electrons as the nearest noble gas. (So they end up with 8 electrons in the subshell.) PS 110A Hatch Ch Example: Fluorine P2: How many valence electrons does F (fluorine) have? How many electrons would fluorine like to gain? How many would fluorine then have? What is the principle oxidation state (number) of fluorine? (It s charge when it is an ion.) How does F do it if it has only another F to react with? Fluorines share (two)! ( Covalent Bond ) F F F F Demo: F 2 PS 110A Hatch Ch

3 xygen How many valence electrons hath (oxygen)? How many electrons would oxygen like to gain? How many would oxygen then have? What is the principal oxidation state (number) of oxygen? How does oxygen do it? Double Covalent Bond (4 shared electrons) Demo: 2 PS 110A Hatch Ch Two more important cases... N + N N N Triple Covalent Bond N 2 or N N Demo: N 2 H + H H H H 2 or H H PS 110A Hatch Ch Nature likes F F (single covalent bond) (double covalent bond) N N (triple covalent bond) Does nature like? C C? C C (quadruple covalent bond?) Will not form a small molecule! Pure carbon finds other structural ways to combine. PS 110A Hatch Ch Equal Sharing of Electrons H + H H 2 : fills in lower molecular orbitals. The two atoms are identical so electrons are not drawn toward one more than the other equal sharing of electrons. PS 110A Hatch Ch Electron sharing is not equal for unlike atoms Different elements have different ability to attract electrons ( electronegativity ) Electronegativity increases toward the upper right of periodic table (ignoring noble gases) Polar Molecules Charge map for H 2 For a molecule to be polar, Bonds must have unequal sharing: dipoles Bond dipoles must not cancel Water is polar (demo), C 2 is not. H 2 C Electronegativity Increases Electronegativity Increases PS 110A Hatch Ch Formaldehyde PS 110A Hatch Ch

4 Which of the fundamental interactions is important for forces between? P3:Which of the following are important for determining the strength of electromagnetic forces? a) The weak nuclear interaction b) The strong nuclear interaction c) The electromagnetic interaction d) The gravitational interaction a) The distance between the charges b) The magnitude of the charges c) The mass of the particles d) Both a) & b) PS 110A Hatch Ch PS 110A Hatch Ch P4:Which electric forces do you expect to be greatest? a) Bonding forces between atoms within a molecule b) Attractive forces between different c) There is no way to know Intermolecular Forces Between different Caused by permanent or temporary charges on Much weaker than covalent bonding interactions Wide range of strengths explains wide range of boiling, melting points of covalent materials PS 110A Hatch Ch PS 110A Hatch Ch Hydrogen Bonding H bound to N,, or F. The resulting polarity causes an attraction between. Among strongest intermolecular interactions Happens because H is small and has only 1 electron, and atom it is bound to is quite electronegative. H is essentially a bare proton The most important Hbond: water Dipoledipole interactions Formaldehyde H 2 C PS 110A Hatch Ch PS 110A Hatch Ch

5 Dispersion forces arise from temporary dipoles due to fluctuations in the electron distribution Electron distribution not static Formation of temporary dipoles +/ combination is favored Very weak Name: dispersion or van Der Waals interactions Since carbon dioxide has no net dipole, it reacts only through these dispersion forces Relative Strengths of Intermolecular Interactions PS 110A Hatch Ch PS 110A Hatch Ch P5: What causes nitrogen (N 2 ) gas to liquefy? Example: Nitrogen a) Covalent bonds between the b) Hydrogen bonds between the c) Dispersion forces between the d) Attraction of the permanent dipole in one molecule to the dipole in another PS 110A Hatch Ch Chemically unreactive Boiling point 77 K (196 C; 321 F) Colorless Good insulator Triple bond Electron sharing is equal, so no dipole What is strongest intermolecular force here? PS 110A Hatch Ch What is the best explanation for the high freezing point of water (in comparison to Nitrogen)? a) Covalent bonds between the b) Hydrogen bonds between the c) Dispersion forces between the d) Attraction of the permanent dipole in one molecule to the dipole in another Example: Water Fairly reactive, universal solvent Boils at 273 K (0 C; 32 F) More dense as liquid than solid Can react with itself: 2H 2 = H H Water conducts electricity (but very poorly) This reaction goes only to a very small extent (1 molecule in 10 million!) Highly polar, so dissolves ionic materials Extensive hydrogen bonding interactions dissolve nonionic materials like sugars, lead to high boiling point, unusual crystal structure PS 110A Hatch Ch PS 110A Hatch Ch

6 Acids and bases Solutions with large hydronium (H 3 + ) ion concentration are acids. Solutions with a small concentration of hydronium and large concentrations of hydroxide (H ) are bases ph measures acidity 7 is neutral Less than 7 is acidic, more than 7 is basic It is a log scale: change of one in ph is 10 1 change in concentration. Change of 2 in ph is 10 2 change in concentration. Solubility often dramatically increases for acids. Toothpaste has ph of about 8. Is it an acid or base? Glucose (a Kind of Sugar) C 6 H 12 6 Crystalline, molecular solid Melts, then decomposes on heating Sticky Many hydrogen bonds hold together in crystal Many Hbonds add up, so melting point is relatively high (compared to water, for example) Hbonds make sticky PS 110A Hatch Ch PS 110A Hatch Ch Diamond (Pure Carbon) Very hard Crystalline Colorless Decomposes rather than melting Covalent network material; like a giant molecule all held together by strong bonds Makes diamond hard Prevents melting No lowlying unoccupied molecular orbitals (unless impurities are present) Graphite (Another form of Pure Carbon) Soft Crystalline Dark color Planes of strongly bound carbon atoms Planes held to each other by very weak intermolecular forces Many lowenergy unoccupied molecular orbitals PS 110A Hatch Ch PS 110A Hatch Ch Buckminsterfullerene (a Third Form of Pure Carbon) Discoverers won Nobel Prize in Chemistry Molecular Ions Covalent bonding within the ion Stronger covalent bonds if number of electrons doesn t match total nuclear charge resulting molecule is charged These charged assemble together in crystal lattice like ionic materials Examples: nitrate, silicate 3 neighbors PS 110A Hatch Ch PS 110A Hatch Ch

7 Does a covalently bonded compound form? Consider the electron structure 1. Find total number of valence electrons available. (n A ) 2. Find total number of valence electrons needed. (n N ) (octet rule) 3. Find number of valence electrons that must be shared. (n S = n N n A ) 4. Is it an even number to form pairs? Forming Molecular Ions Form ion using covalent bonds Bond with other ions using ion bonds Examples Nitrates (N 3 ) NaN 3 HN 3 Mg(N 3 ) 2 gypsum (CaS4 ), calcite (CaC 3 ) PS 110A Hatch Ch PS 110A Hatch Ch Complex Molecular Ions: Note: use extra electrons as valence N 3 versus N 3 electrons 1. # valence e s? (5 + 3*6) (5 + 3*6 + 1) # e s wanted? (4*8) (4*8) # e s to be shared? (3223) (3224) Even Number? No! Yes! Can it be arranged to give all atoms an octet of es and have electronpair sharing for covalent bonding? nitrate ion N N Yes for N 3 but No for N 3 PS 110A Hatch Ch PS 110A Hatch Ch Ionic Bonds and Covalent Bonds How about... NaN 3 dissolved in H 2 Na + + N 3 nitrate ion acts as a unit P6: How many nitrate ions do you need for an ionic bond with the metal magnesium (Mg)? What is the formula for the compound most likely formed? ammonium sulfate: NH 4+ with S 4 2 (NH 4 ) 2 S 4 P7: What are the primary chemical characteristics of this compound? (Remember this is ionic bonding between the two complex ions.) Demo: reagent jar and H 2 S 4 model PS 110A Hatch Ch PS 110A Hatch Ch

8 Consider.. Minerals CaCl 2 + Na 2 C 3 CaC 3 +2NaCl Balanced? C 3 2 C ver 90% of the earth s crust is made up of silicates. The basic building block is the molecular ion Si 4 4. It is tetrahedral in shape. Example: livine is a common mineral with the formula: (Mg, Fe) 2 Si 4 (Magnesium and Iron are nearly the same size and can substitute for each other.) Si Demo: ionic bond of C 2 3, CaC 3 precipitate, real crystal PS 110A Hatch Ch PS 110A Hatch Ch The ctet Rule for 2 How many total valence electrons? 12 If each had 8 electrons, how many are needed? 16 How many must be shared? (covalent bonding, must be pairs of electrons!) 4 Can we arrange this? yes C 2 Example Carbon + xygen to form C 2? 1. How many total valence electrons are actually present? How many are needed? 3 x 8 = How many must be shared? = 8 4. Can this be arranged? C C PS 110A Hatch Ch PS 110A Hatch Ch Covalent bonding Practice on this one: NH 3 PS 110A Hatch Ch