CHEM 131 Name __________________________________
Quiz 2 – Jan. 27, 2012
Complete the following problems. You must show your work to receive full credit. Show your
answers to the correct number of significant figures with the correct units.
1. There are three possible Lewis structures for the compound OCS. Draw the three structures below and
identify which structure, if any, you would expect to most closely describe the real structure of OCS. Justify
your answer. (8 pts)
FC: 0 0 0 FC: +1 0 -1 FC: -1 0 +1
Since the structure on the left has formal charge of zero on all atoms, it would be the
preferred structure.
2. Concisely explain the following observations: (8 pts)
a. Sodium has an atomic radius of 186 pm, while chlorine atoms and sodium ions both have radii of 99
pm.
Sodium cation is smaller than sodium atom because the effective nuclear charge increases
when losing an electron. Also, in Na+ the largest orbital occupied is of a smaller principle
quantum number.
Sodium cations and chlorine atoms have the same electron configuration, with the 3p orbital
completely full. Effective nuclear charge has only minimal effect on size across the p orbital, so
it is reasonable that Na+ and Cl0 have similar size.
b. The first ionization energy of magnesium is much larger than the first ionization energy of aluminum,
which is larger than the ionization energy of sodium.
Ionization of Mg requires removal of an electron from a filled 3s orbital, which requires
significant energy due to the stability of filled orbitals. Ionization of Al requires the loss
of an electron from a partially filled orbital and results in a filled orbital, while ionization
of Na results in a filled valence shell, requiring the least energy of the three.
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.. CS
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O
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O
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.. CS
O
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