orbitals-universityofnorthcarolinawilmington軌道-北卡羅來那大學wilming(編輯修改稿)
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【文章內(nèi)容簡介】 e dot, representing its 1s electron. ? Carbon has four dots (2s2 2p2). ? Stable molecule results in a pleted shell, an octet (eight e) for maingroup atoms (two for hydrogen). 16 Number of Covalent Bonds to an Atom ? Atoms with one, two, or three valence electrons form one, two, or three bonds. ? Atoms with four or more valence electrons form as many bonds as they need electrons to fill the s and p levels of their valence shells to reach a stable octet. ? Carbon has four valence electrons (2s2 2p2), therefore forms four bonds (CH4). 17 Valence of Oxygen and Nitrogen ? Oxygen has six valence electrons (2s2 2p4), so it forms two bonds (H2O). 18 Valence of Nitrogen ? Nitrogen has five valence electrons (2s2 2p3), and it forms three bonds (NH3). 19 Nonbonding electrons ? Valence electrons not used in bonding are called nonbonding electrons, or lonepair electrons. ? Consider the nitrogen atom in ammonia (NH3): ? N shares six valence electrons in three covalent bonds。 the remaining two valence electrons are a nonbonding (lone) pair. 20 Valence Bond Theory ? Covalent bond forms when two atoms approach each other closely so that a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom. ? Electrons are paired in the overlapping orbitals and are attracted to nuclei of both atoms. ? The H–H bond results from the overlap of two singly occupied hydrogen 1s orbitals. ? The HH bond is cylindrically symmetrical, sigma (s) bond. 21 Bond Energy ? The reaction 2 H ? H2 releases 436 kJ/mol. ? The product has 436 kJ/mol less energy than two H atoms: H–H has bond strength of 436 kJ/mol. (1 kJ = kcal。 1 kcal = kJ). 22 Bond Length ? Distance between nuclei that leads to maximum stability. ? If too close, they repel because both nuclei are positively charged. ? If nuclei are too far apart, bonding is weak. 23 Hybridization: sp3 Orbitals and the Structure of Methane ? Carbon has 4 valence electrons (2s2 2p2) ? In CH4, all C–H bonds are identical (tetrahedral) ? How can this be explained ?? 24 Hybridization: sp3 Orbitals and the Structure of Methane ? sp3 hybrid orbitals: s orbital and three p orbitals bine to form four equivalent, unsymmetrical, tetrahedral orbitals (s+p+p+p = sp3), Pauling (1931) 25 Tetrahedral Structure of Methane ? sp3 orbitals on C overlap with 1s orbitals on 4 H atoms to form four identical CH bonds ? Each C–H bond has a strength of 438 kJ/mol and length of 110 pm ? Bond angle: each H–C–H is 176。, the tetrahedral angle. 26 Hybridization: sp3 Orbitals and the Structure of Ethane ? Two C’s bond to each other by s overlap of an sp3 orbital from each C. ? The other three sp3 orbitals on each C overlap with H
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