Have you read chapter 23? a) Yes, b) No, I m conserving. brain storage.

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1 How do nonmetal atoms bond to each other? What are the properties of these bonds? How does this affect properties of the material? What forces exist between molecules? How do these forces influence physical properties such as melting & boiling points, hardness, electrical conductivity, etc.? 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

2 Have you read chapter 23? a) Yes, b) No, I m conserving brain storage.

3 Review of Bonding Motifs Metallic bonding: many atoms collectively share orbitals and electrons Ionic bonding: metals easily give up electrons to nonmetals, which want them Today add a 3rd type: covalent bonding

4 Review of Bonding Empty orbitals Empty orbitals Empty orbitals Full shells Band gapfull shells Full shells

5 Molecular Orbitals & Bonding When atoms get close, MO s can form If the electrons are lower in energy in the resulting MO s than they were in the atomic orbitals, bonds form Energy + H H H H or H 2

6 Sharing Electrons to Form Covalent Bonds A few molecular orbitals are lower in energy than atomic orbitals Typically correspond to filled shells for each atom in bond Result: molecules! Orbitals are at specific energies between specific atoms. Example: H + H H 2 Demonstration: covalent bond formation in nylon

7 the code for covalent bonds In covalent bonds, electrons are shared so that each atom has access to enough electrons to fill its shell. Electrons are always shared in pairs. (An orbital holds how many electrons?) Each pair is represented by a single line in a diagram. Two atoms can share up to 3 pairs of electrons.

8 A few questions If you have more electrons at low energy levels, what happens to the strength of the bond? A. It increases B. It decreases If you have more electrons packed close together what happens to the electric force? A. You get a larger attractive force B. You get a smaller attractive force C. You get a larger repulsive force D. You get a smaller repulsive force

9 Sharing of More Than One Electron Pair: Multiple Bonds Multiple bonds are stronger High electron density between multiplybonded atoms makes them relatively reactive.

10 Basic units metals: individual + ions stacked in negative overlapping electron cloud ionic compounds: oppositely charged ions stacked covalent compounds: individual molecules (even in solid)

11 What does the energy well look like? Just a few low energy molecular orbitals that are full, and lots of individual empty atomic an molecular orbitals that are sometimes stacked pretty close together. Empty orbitals Full shells

12 Characteristics of materials that have covalent bonds. Would they be transparent or opaque? Will they conduct? Can you explain why? Would they have high or low melting and boiling points? What determines the melting and boiling points?

13 Forces Between molecules Things like phase changes and brittleness are determined by the forces between molecules. What types of fundamental interactions can you have at a microscopic level? Can you have electric forces when the total charge is zero?

14 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) Electronegativity Increases Electronegativity Increases

15 Polar Molecules Charge map for H 2 O For a molecule to be polar, Bonds must have unequal sharing: dipoles Bond dipoles must not cancel Water is polar (demo) 2δ- δ+ δ+ No net dipole!

16 Which of the fundamental interactions is important for forces between molecules? 1. The weak nuclear interaction 2. The strong nuclear interaction 3. The electromagnetic interaction 4. The gravitational interaction

17 Which of the following are important for determining the strength of electromagnetic forces? A. The distance between the charges B. The magnitude of the charges C. The mass of the particles D. Both A & B

18 Comparing the distances between two bound atoms in a molecule and the distances between molecules, which is greater (on average)? A. The distance between bound atoms B. The distance between molecules C. Neither

19 Which forces do you expect to be greatest? 1. Bonding forces between atoms within a molecule 2. Attractive forces between different molecules 3. There is no way to know

20 Intermolecular Forces Between different molecules Caused by permanent or temporary charges or asymmetric charge distributions Much weaker than covalent bonding interactions Wide range of strengths explains wide range of boiling, melting points of covalent materials

21 Types of forces in chemistry Bonds: Metalic Ionic Covalent Intermolecular forces Hydrogen bonding Dipole-dipole bonding Dispersion forces Only pertinent for what type of matter?

22 Hydrogen Bonding The ultimate polar bond. H bound to N, O, or F Among strongest intermolecular interactions H is small and has only 1 electron. If the atom it is bound to is quite electronegative, H is essentially a bare proton The most important H-bond: water δ+ δ+ 2δδ+ δ+ 2δ- 2δδ+ δ+

23 Even Nonpolar Molecules Are Sticky Electron distribution not static Formation of temporary dipoles +/- combination is favored Very weak Name: dispersion or van Der Waals interactions

24 Relative Strengths of Intermolecular Interactions

25 Nitrogen Properties Chemically unreactive Boiling point 77 K (-196 C; -321 F) Colorless Why? Triple bond What are strongest intermolecular forces between N 2 molecules? No low-lying orbitals interconnecting the N 2. N N

26 What causes nitrogen (N 2 ) molecules to liquefy? 1. Covalent bonds between the molecules 2. Hydrogen bonds between the molecules 3. Dispersion forces between the molecules 4. Attraction of the permanent dipole in one molecule to the dipole in another

27 Water Properties Fairly reactive, universal solvent Boils at 373 K (100 C; 212 F) More dense as liquid than solid Can react with itself: 2H 2 O = H 3 O + + OH - Water conducts electricity (but not well) This reaction goes only to a very small extent (1 molecule in 10 million!) Why? Highly polar, so dissolves ionic materials Extensive hydrogen bonding interactions dissolve nonionic materials like sugars, lead to high boiling point, unusual crystal structure

28 What is the best explanation for the high freezing point of water? 1. Covalent bonds between the molecules 2. Hydrogen bonds between the molecules 3. Dispersion forces between the molecules 4. Attraction of the permanent dipole in one molecule to the dipole in another

29 What happens when you mix two polar materials? They will mix together (dissolve) Why? What about a non-polar material mixed with a polar material? What force holds non-polar atoms together? What determines the size of this force? Compare the size of the force holding the polar molecules to non-polar ones to the forces that holds the same type of molecule.

30 Glucose (a Kind of Sugar) Properties Crystalline, molecular solid Melts, then decomposes on heating Sticky Why? Many hydrogen bonds hold molecules together in crystal Many H-bonds add up, so melting point is relatively high (compared to water, for example) H-bonds make molecules sticky Alternating + & - is like what other type of bond?

31 Diamond (Pure Carbon) Properties Very hard Crystalline Colorless Decomposes rather than melting Why? Covalent network material; like a giant molecule all held together by strong bonds Makes diamond hard Prevents melting No low-lying unoccupied molecular orbitals (unless impurities are present)

32 Properties Soft Graphite (Another form of Pure Carbon) Crystalline Dark color Why? Planes of strongly bound carbon atoms Planes held to each other by very weak intermolecular forces Many low-energy unoccupied molecular orbitals

33 Buckminsterfullerene (a Third Form of Pure Carbon) Discoverers won Nobel Prize in Chemistry

34 Molecular Ions Covalent bonding within the ion If the number of electrons available doesn t match the number of electrons the atoms want, the molecule will tend to behave like a non-metal (or occasionally a metal) and steal (or give away) electrons. These charged molecules assemble together in crystal lattice like ionic materials Examples: nitrate, silicate