CHEM 120 Exam Review: Atomic Structure, Periodic Trends, Bonding and Gases

Terms in this set (27)

1. Relationship between energy, frequency and wavelength of lightE = h·nu and c = lambda·nu, so E = hc/lambda. h = 6.626 x 10^-34 J·s, c = 3.00 x 10^8 m/s. Shorter wavelength means higher energy.
2. Energy of the hydrogen atom levels (Bohr)E_n = -2.18 x 10^-18 J / n^2. A photon emitted when an electron drops from n_i to n_f has energy equal to the difference E_i - E_f.
3. What does the de Broglie relation state?Any moving particle has a wavelength lambda = h/(m·v), so electrons show wave behaviour such as diffraction.
4. Heisenberg uncertainty principlePosition and momentum cannot both be known exactly at the same time: delta x · delta p >= h/(4·pi).
5. Name the four quantum numbers and their allowed values.n = 1, 2, 3...; l = 0 to n-1 (s, p, d, f); m_l = -l to +l; m_s = +1/2 or -1/2.
6. How many orbitals and electrons are in the n = 3 shell?n^2 = 9 orbitals (one 3s, three 3p, five 3d), holding up to 2n^2 = 18 electrons.
7. State the Pauli exclusion principle and Hund's rule.Pauli: no two electrons in an atom have the same four quantum numbers, so an orbital holds at most 2 electrons of opposite spin. Hund: degenerate orbitals fill singly with parallel spins before pairing.
8. Write the ground-state electron configuration of Fe.[Ar] 4s^2 3d^6.
9. Why are the configurations of Cr and Cu exceptions?A half-filled or filled d subshell is especially stable: Cr is [Ar] 4s^1 3d^5 and Cu is [Ar] 4s^1 3d^10.
10. What is effective nuclear charge (Zeff) and how does it change across a period?Zeff = Z - S, the net positive charge felt by an electron after shielding. It increases across a period, pulling electrons closer.
11. Trend in atomic radius on the periodic tableRadius decreases across a period (Zeff rises) and increases down a group (more shells).
12. Trend in first ionization energy, and one exceptionIncreases across a period and decreases down a group. Exception: O is lower than N because pairing in a 2p orbital adds repulsion (similarly B is lower than Be).
13. Ionic vs covalent bondingIonic bonds form by electron transfer between atoms with a large electronegativity difference (often metal and nonmetal). Covalent bonds form by sharing electrons between nonmetals.
14. How do you calculate formal charge?FC = (valence electrons) - (nonbonding electrons) - (1/2 x bonding electrons). The best Lewis structure minimizes formal charges and puts negative charge on the more electronegative atom.
15. What are resonance structures?Two or more valid Lewis structures differing only in electron placement. The real molecule is a hybrid, e.g. all three C-O bonds in carbonate are equivalent.
16. VSEPR: geometry of a molecule with 4 electron domains and 1 lone pairElectron geometry tetrahedral; molecular shape trigonal pyramidal (e.g. NH3), bond angle a bit less than 109.5 degrees.
17. VSEPR: shape of SF4 and XeF4SF4 (5 domains, 1 lone pair) is seesaw. XeF4 (6 domains, 2 lone pairs) is square planar.
18. What hybridization corresponds to 2, 3 and 4 electron domains?2 domains: sp (linear). 3 domains: sp2 (trigonal planar). 4 domains: sp3 (tetrahedral).
19. Sigma vs pi bondsA sigma bond forms by head-on overlap along the bond axis; a pi bond forms by side-on overlap of p orbitals. A double bond is 1 sigma + 1 pi; a triple bond is 1 sigma + 2 pi.
20. How is bond order found from MO theory, and what does it predict for O2?Bond order = (bonding e- - antibonding e-)/2. O2 has bond order 2 and two unpaired electrons in pi* orbitals, so it is paramagnetic.
21. When is a molecule with polar bonds nonpolar overall?When its geometry makes the bond dipoles cancel, e.g. CO2 (linear) and CCl4 (tetrahedral).
22. Ideal gas lawPV = nRT, where P is pressure, V volume, n moles, T temperature in kelvin, and R = 8.314 J/(mol·K) or 0.08206 L·atm/(mol·K).
23. Mini-problem: what volume does 1.00 mol of ideal gas occupy at 273.15 K and 1.00 atm?V = nRT/P = (1.00)(0.08206)(273.15)/1.00 = 22.4 L.
24. Dalton's law of partial pressuresP_total = P_A + P_B + ...; each partial pressure P_A = x_A·P_total, where x_A is the mole fraction.
25. What does kinetic molecular theory say about average kinetic energy?The average kinetic energy of gas particles depends only on absolute temperature: KE_avg = (3/2)RT per mole. At the same T, lighter molecules move faster.
26. Graham's law of effusionrate_1/rate_2 = sqrt(M_2/M_1). Lighter gases effuse faster; H2 effuses 4 times faster than O2.
27. When do real gases deviate most from ideal behaviour?At high pressure (molecular volume matters) and low temperature (intermolecular attractions matter). The van der Waals equation corrects with constants a (attraction) and b (volume).