结构化学(北京大学)第三章.ppt

第三章 双原子分子的结构和性质 3.1 化学键概述 Ionic Bonds As a metal and a non-metal approach one another, the valence electrons interact and the metal (indicated by the red sphere) transfers its valence electrons to the non-metal (indicated by the blue sphere). The metal becomes positively charged (through the loss of electrons) and the non-metal becomes negatively charged (through the gain of electrons). In this way, both the metal and the non-metal complete their valence shell to obtain a stable electron configuration. Covalent Bonds As two non-metals approach one another, the valence electrons interact and a covalent bond is formed between the two non-metals which share a pair of electrons so that each obtains a filled valence shell. There are two types of covalent bonds. In a non-polar covalent bond the electrons are shared equally between the two atoms. (The nuclei and core electrons are indicated by the blue spheres and the bonding electrons are indicated by the lavender dots.) 3.2 氢离子( )的结构和共价键的本质 -3- bonding wavefunction The probability distribution of the bonding wavefunction: Contour plot of electron density for bonding wavefunction: Two hydrogen 1s wavefunctions in bonding distance with subtractive linear combination antibonding wavefunction probability distribution of the antibonding wavefunction Contour plot of electron density for antibonding wavefunction ?1的能量比1s轨道低，当电子从氢原子的1s轨道进入?1 时，体系的能量降低，?1为成键轨道。相反，电子进入?2时，H2+的能量就要比原来的氢原子和氢离子的能量高，?2称为反键轨道。图3.3示出一个氢原子和一个氢离子的1s轨道叠加形成H2+的分子轨道图形。 图3.3 叠加成分子轨道等值线示意图 由上述讨论可见，当原子互相接近时，它们的原子轨道互相同号叠加，组合成成键分子轨道。当电子进入成键轨道，体系能量降低，形成稳定的分子。此时原子间形成的化学键即共价键。 从电子在分子中的分布情况，可了解共价键的成因。电子在分子中分布，可由分子中空间各点几率密度（?12）数值大小表示。分子中电子的分布和两个原子的电子分布的简单加和不同。电子云分布的差值图反映了这一结果。电子云分布的差值图，是将?12按空间各点逐点地减去处在A核位置的?a和处在B核位置?b后，绘出的差值等值线图。图3.4示出H2+ 的电子云分布的差值图。图中实线表示电子云增加的等值线，虚线表示电子云减少的等值线。 -4- 共价键的本质 由图可见，?1轨道的成键作用，实质上是将分子两端原子外侧的电子抽调到两个原子核之间，增加了核间区域的电子云。聚集在核间的电子云，同时受到两个原子核的吸引，即核间的电子云把两个原子核结合在一起，这是H2+得以形成的原因。 从能量角度看，聚集在核间运动