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General Chemistry Principles, Patterns, and Applications, 2011

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25. How does electron screening affect the energy difference between the 2s <strong>and</strong> 2p atomic orbitals of the<br />

period 2 elements? How does the energy difference between the 2s <strong>and</strong> 2p atomic orbitals depend on the<br />

effective nuclear charge?<br />

26. For σ versus π, π versus σ*, <strong>and</strong> σ* versus π*, which of the resulting molecular orbitals is lower in energy?<br />

27. The energy of a σ molecular orbital is usually lower than the energy of a π molecular orbital derived from the<br />

same set of atomic orbitals. Under specific conditions, however, the order can be reversed. What causes this<br />

reversal? In which portion of the periodic table is this kind of orbital energy reversal most likely to be<br />

observed?<br />

28. Is the s 2pz molecular orbital stabilized or destabilized by interaction with the σ 2 smolecular orbital in N 2 ?<br />

in O 2 ? In which molecule is this interaction most important?<br />

29. Explain how the Lewis electron-pair approach <strong>and</strong> molecular orbital theory differ in their treatment of<br />

bonding in O 2 .<br />

30. Why is it crucial to our existence that O 2 is paramagnetic?<br />

31. Will NO or CO react more quickly with O 2 ? Explain your answer.<br />

32. How is the energy-level diagram of a heteronuclear diatomic molecule, such as CO, different from that of a<br />

homonuclear diatomic molecule, such as N 2 ?<br />

33. How does molecular orbital theory describe the existence of polar bonds? How is this apparent in the<br />

molecular orbital diagram of HCl?<br />

A N S W E R S<br />

1. An atomic orbital is a region of space around an atom that has a non-zero probability for an electron with a<br />

particular energy. Analogously, a molecular orbital is a region of space in a molecule that has a non-zero<br />

probability for an electron with a particular energy. Both an atomic orbital <strong>and</strong> a molecular orbital can<br />

contain two electrons.<br />

15 No. Because an np x orbital on one atom is perpendicular to an np y orbital on an adjacent atom, the net<br />

overlap between the two is zero. This is also true for np y <strong>and</strong> np z orbitals on adjacent atoms.<br />

N U M E R I C A L PR O BL E M S<br />

1. Use a qualitative molecular orbital energy-level diagram to describe the bonding in S 2 2− . What is the bond<br />

order? How many unpaired electrons does it have?<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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