CHM101, Section 5, Fall
2001
Third hourly exam, Tuesday 13 November 2001
Chapters 7–9
Instructions:
Read over the entire exam first. Begin
with the questions you feel most confident about. Pace yourself so that you will
have time enough to try everything. If you don’t know how to get all the way
to an answer, go as far as you can and state how you would approach the rest.
And keep your handwriting neat! Use the back of the sheets if necessary.
Definitions (2 points each; 20 points total)
1. Cathode ray—The beam of electrons that passes from the cathode to
the anode of a vacuum tube when electricity is passed through it.
2. Electromagnetic spectrum—The
complete range of wavelengths or frequencies of electromagnetic waves.
3. De Broglie equation—The relation
between the momentum of a body and its wavelength: λ = h/mv.
4. Lanthanides—The 14 elements in
which the 4f subshell is being filled.
5. Electron affinity—The change in
energy when an electron is added to a gaseous atom.
6. Aufbau principle—The hypothetical
process in which one atom is built up from the previous one in atomic number by
adding a proton to the nucleus and placing an electron in the appropriate
orbital.
7. Hund’s rule—Electrons go into
separate orbitals of the same energy as long as they are available.
8. Coordinate covalent bond—A bond
formed when one atom of the bonding pair contributes both electrons.
9. Skeletal structure—The pattern in
which atoms of a molecule or ion are connected to one another.
10. Electronegativity—The tendency of
an atom or an element to attract the electrons of a bond.
Short answers (5 points each; 30 points total)
c = 3.00x108 m s-1
1. Describe the Rutherford scattering experiment and the conclusions it led to. Very thin foils of metals such as gold, silver, or platinum were bombarded with alpha particles. Most passed through without being deflected much, but a few were deflected (scattered) through very large angles. This showed that atoms are composed of empty space with a tiny, dense nucleus at the center.
2. Explain the wave-particle duality of light and give an example of each type of behavior. Light (EM radiation) has properties of waves and of particles. Like waves, it has a frequency, amplitude, and wavelength and exists in oscillating fields. Like particles, its energy is quantized (as in the photoelectric effect).
3. Write the abbreviated electron configuration (1s22s22p4, etc.) for each of these elements and ions. Use the positional rather than the sequential option.
(a) C 1s2 2s2 2p2
(b) Ti 1s2 2s2
2p6 3s2 3p6 3d2
4s2
(c) Se 1s2 2s2
2p6 3s2 3p6 3d10
4s2 4p4
(d) Fe2+ 1s2
2s2 2p6 3s2 3p6
3d6
4. Explain how and why atomic radii vary (a) across a row of the periodic table (main groups only) and (b) down a main-group column. (a) Atomic radii decrease left to right across a row because the outer shell is being attracted by more protons (the effective nuclear charge increases). (b) Atomic radii increase down a column because successive rows have more shells.
5. Explain why there can be a compound PCl5 but not an NCl5. PCl5 uses 3d orbitals to expand P’s valence shell. NCl5 cannot do this because being in row 2, it has no d orbitals available.
6. It is observed that the greater the bond order (single, double, triple), the shorter the bond length and the greater the bond energy. Use fundamental principles of force and energy to give a reason why shorter bonds should be stronger than longer bonds. The electron in shorter bonds are closer to the protons that attract them, and so exist at a lower energy state (more energy released as they are drawn closer to the nuclei). The bonds are stronger because more energy has to be put into them in order to separate the atoms.
Problems (10 points each; 50 points total)
Planck’s constant h = 6.626x10-34 J s
1. Calculate the energy, in joules, of a photon or red light that has a frequency of 3.73 x 1014 s-1. Is this energy greater or less than that of light of 530-nm wavelength? Explain.
(a) E = hν = (6.626 x 10-34 J s)(3.73 x 1014 s-1)
= 2.47 x 10-19 J
(b) E = hc/λ = (6.626 x 10-34
J s)(3.00 x 108 m s-1)/(530 x 10-9 m) = 3.75 x
10-19 J
(a) is less than (b).
2. Draw a diagram that illustrates the aufbau principle, and show how to use it.
3. Draw the orbital diagram (with the boxes and the arrows) for the Ge atom.
4. Draw the Lewis structures for (a) PCl3 and (b) SF6.
5. Draw the Lewis resonance structures for the carbonate ion, CO32-. Describe the resulting resonance hybrid structure.
