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18 Organic Chemistry Solutions Manual Suggested solution H⁻ Hydrogen is the first element in the periodic table and has just one proton in the nucleus and just one orbital (1s) available to be filled. If we fill it, we get a species with one proton and two electrons and that is H⁻. The answer is If you said that this species is Sulfur is in group VI (16) in the same row as Na and Cl. It makes a σ bond to hydrogen and hybridized and put electrons into uses the remainder of the electrons as lone pairs to give a full noble gas configuration (same 3sp³ orbitals you have done well. as Ar) of: 1s², 2p⁶, 3p⁶. This answer is right too. K⁺ Potassium is below sodium in the periodic table so this cation has lost its 4s electron and is: 1s², 2p⁶, 3s², 3p⁶. This configuration is the same as that of Cl⁻. Xe Xenon is a noble gas in group VIII (18). It comes some way down the table after He, Ne, Ar, and Kr and after two rows of transition elements that fill the 3d and 4d orbitals so it has 1s², 3s², 3p⁶, 3d¹⁰, 4p⁶, 4d¹⁰, 5p⁶. Problem 3 What sort of bonds can be formed between S orbitals and p orbitals? Which will provide better overlap, 1s + 2p or 1s + 3p? Which bonds will be stronger, those between hydrogen and C, N, O, and F on the one hand or those between hydrogen and Si, P, S, and CI on the other? Within the first group, bond strength goes in this order: HF > OH > NH > CH. Why? Purpose of the problem To encourage you to think about the energies of orbitals as well as just about their quantum description. Suggested solution S orbitals and p orbitals can combine to form σ bonds. In the chapter (p. 108) we discussed the structure of PH₃ which has bond angles of about 90° and is made of σ bonds between 1s(H) and 3p(P) orbitals. There is no special problem in overlapping either 2p or 3p orbitals with orbitals though the 3p orbitals are larger. The difference comes in the energy of the orbitals. The 2p orbitals are much closer in energy to the Is orbital than the 3p orbitals and so the energy gain is greater on bond formation. 1s AO electrons 1s AO electrons Bonds between H and C, N, O, and F are all stronger than bonds between H and Si, P, S, and Cl. This is partly because 2p AOs are used for the first but 3p AOs for the second group. The full story includes the fact that CH₄, NH₃, and are hybridized (so the lower energy 2s orbitals are used as well) while PH₃ and H₂S are not hybridized. SiH₄ is, of course, tetrahedral while it is difficult to say whether linear HF and HCl are hybridized or not! This factor was used in Chapter 3 Within the group C, N, O, and F, the energy of the orbital stays the same but the energy of the (p. 68) to explain the relative positions of the stretching frequencies of OH, 2p (or of the sp³ hybrid orbitals) drops as the elements get more electronegative. These orbitals get NH, and CH in the infrared spectrum. closer in energy to the orbital and the gain in bond formation is correspondingly greater.