CHEM 201 Exam Review: Electronic Structure, Lewis Structures, VSEPR and Molecular Interactions

Terms in this set (32)

1. How are wavelength, frequency and energy of light related?c = λ·v and E = h·v. Higher frequency means shorter wavelength and higher photon energy, so UV photons carry more energy than visible photons, and visible more than infrared.
2. What is the energy of a single photon of 500 nm light?E = hc/λ = (6.626 x 10^-34 J·s)(3.00 x 10^8 m/s)/(500 x 10^-9 m) = 3.98 x 10^-19 J.
3. Rydberg equation for hydrogen emission lines1/λ = R_H (1/n1^2 - 1/n2^2), where n2 > n1 and R_H = 1.097 x 10^7 m^-1. Transitions down to n1 = 2 give the visible Balmer series.
4. Why do atoms give line spectra instead of continuous spectra?Electron energies are quantized, so only specific energy differences between levels are possible. Each transition emits or absorbs a photon of one specific wavelength.
5. How do you count the nodes in an atomic orbital? Apply it to a 3p orbital.Total nodes = n - 1, angular nodes = l, radial nodes = n - l - 1. A 3p orbital (n = 3, l = 1) has 2 nodes in total: 1 angular and 1 radial.
6. What ml values are allowed for l = 2, and how many d orbitals is that?ml = -2, -1, 0, +1, +2, so there are 5 d orbitals in any shell with n ≥ 3.
7. Is the quantum number set n = 2, l = 2 allowed?No. l must range from 0 to n - 1, so for n = 2 only l = 0 (2s) or l = 1 (2p) is allowed. A 2d subshell does not exist.
8. Why is 2s lower in energy than 2p in multi-electron atoms?A 2s electron penetrates closer to the nucleus than a 2p electron, so it is less shielded and feels a higher effective nuclear charge. In hydrogen, with one electron, they are degenerate.
9. Paramagnetic vs diamagnetic: classify a ground-state N atom.Paramagnetic species have unpaired electrons and are attracted to a magnetic field; diamagnetic species have all electrons paired. N (1s2 2s2 2p3) has 3 unpaired 2p electrons, so it is paramagnetic.
10. What is electron affinity, and what is its general periodic trend?The energy change when a gaseous atom gains an electron. It generally becomes more negative (more favourable) from left to right across a period, with halogens the most favourable. Noble gases and group 2 have unfavourable values.
11. Steps for drawing a Lewis structureCount total valence electrons (adjusting for charge), put the least electronegative atom in the centre (never H), connect atoms with single bonds, complete outer octets, place leftover electrons on the central atom, then form multiple bonds if the central atom lacks an octet.
12. What is the formal charge on N in NH4+?FC = 5 valence - 0 nonbonding - (1/2)(8 bonding) = +1. This accounts for the overall +1 charge of the ion.
13. How are bond order, bond length and bond energy related?A higher bond order means a shorter, stronger bond. For carbon-carbon bonds, C≡C is shorter and stronger than C=C, which is shorter and stronger than C-C.
14. What is the C-O bond order in the carbonate ion, and why are all three bonds the same?CO3^2- has three equivalent resonance structures, each with one C=O and two C-O bonds. The real ion is their average, so each bond has a bond order of 4/3 and all three are identical.
15. Electron-domain geometry vs molecular geometry, using NH3Electron-domain geometry counts all bonding and lone-pair domains; molecular geometry describes only the positions of the atoms. NH3 has a tetrahedral electron geometry but a trigonal pyramidal shape, with angles of about 107°.
16. Give the molecular shapes of ClF3, BrF5 and I3-.ClF3: T-shaped (5 domains, 2 lone pairs). BrF5: square pyramidal (6 domains, 1 lone pair). I3-: linear (5 domains, 3 equatorial lone pairs).
17. Why is CH2Cl2 polar while CCl4 is nonpolar?Both are tetrahedral. In CCl4 the four identical C-Cl dipoles cancel. In CH2Cl2 the C-H and C-Cl bonds have different polarities, so the dipoles do not cancel and there is a net dipole moment.
18. Sigma vs pi bonds in valence bond theoryA sigma bond forms by head-on orbital overlap along the line between the nuclei and allows free rotation. A pi bond forms by side-by-side overlap of p orbitals above and below that line and prevents rotation.
19. What is the hybridization of carbon in methane, ethene and ethyne?Methane: sp3 (tetrahedral, 109.5°). Ethene: sp2 (trigonal planar, 120°, one pi bond). Ethyne: sp (linear, 180°, two pi bonds).
20. How many sigma and pi bonds are in acetonitrile, CH3CN?5 sigma bonds (three C-H, one C-C and one in the C≡N) and 2 pi bonds (both in the C≡N triple bond).
21. Use MO theory to explain why He2 does not exist.He2 would have 2 electrons in sigma1s (bonding) and 2 in sigma*1s (antibonding), so bond order = (2 - 2)/2 = 0 and no stable bond forms.
22. Compare N2, O2 and O2+ using MO theory.N2: bond order 3, all electrons paired (diamagnetic). O2: bond order 2, two unpaired pi* electrons (paramagnetic). O2+: one pi* electron removed, so bond order 2.5 and a shorter, stronger bond than O2.
23. Why does the MO energy ordering differ for B2, C2 and N2 compared with O2 and F2?In the lighter molecules, mixing between 2s and 2p orbitals (s-p mixing) pushes sigma2p above pi2p. In O2 and F2 the 2s-2p energy gap is larger, so sigma2p lies below pi2p.
24. Constitutional isomers vs stereoisomers, with a cis/trans exampleConstitutional isomers differ in how the atoms are connected (butane vs isobutane). Stereoisomers have the same connectivity but a different 3D arrangement. Cis- and trans-2-butene exist because the C=C pi bond prevents rotation.
25. Aldehyde vs ketoneBoth contain a carbonyl group (C=O). In an aldehyde the carbonyl carbon is bonded to at least one H, so it sits at the end of a chain (R-CHO). In a ketone the carbonyl carbon is bonded to two carbon groups (R-CO-R').
26. Why does ethanol boil far higher than dimethyl ether, even though both are C2H6O?Ethanol has an O-H group, so its molecules hydrogen bond with one another. Dimethyl ether has no H bonded to O, so it has only dipole-dipole and dispersion forces. Ethanol boils at about 78 °C; dimethyl ether at about -24 °C.
27. What forces let NaCl dissolve in water?Ion-dipole attractions: the partially negative O ends of water surround Na+ and the partially positive H ends surround Cl-. Together with the gain in entropy, this offsets the lattice energy.
28. Why does n-pentane boil higher than neopentane, its isomer with the same molar mass?The long, straight n-pentane molecule has more surface contact with its neighbours, so its London dispersion forces are stronger than those of compact, nearly spherical neopentane.
29. What does "like dissolves like" mean?Polar and ionic substances dissolve well in polar solvents (for example sugar in water). Nonpolar substances dissolve in nonpolar solvents (for example oil in hexane). Solubility is favoured when solute and solvent have similar intermolecular forces.
30. What controls lattice energy? Compare MgO and NaCl.By Coulomb's law, lattice energy is proportional to (q1·q2)/r. MgO (charges +2 and -2, small ions) has a much larger lattice energy than NaCl (charges +1 and -1), so it has a much higher melting point.
31. How does band theory distinguish conductors, semiconductors and insulators?In metals the valence band and conduction band overlap or the valence band is partly filled, so electrons move freely. Semiconductors have a small band gap that electrons can cross with heat or doping. Insulators have a large band gap.
32. Diamond vs graphiteBoth are pure carbon. Diamond is a 3D network of sp3 carbons: very hard and an electrical insulator. Graphite is layers of sp2 carbons with delocalized pi electrons: soft and slippery, and it conducts electricity along its layers.