16 CHAPTER 2 Bonding and Molecular Structure Problem 2.2 What type of MO results from side-to-side overlap of ans and ap orbital? The overlap is depicted in Fig.2.4.The bonding strength generated from the overlap between the +s AO Figure 2.4 Problem 2.3 List the differences betweenabond and a bond. Bond 1.Formed by head-to-head overlap of AO's. 1.Formed by lateral overlap of p orbitals (or p and dorbitals). 2.Has eylindrical charge symmetry about bond axis.2.Has maximum charge density in the 3.Dross. onal plane of the orbitals 4.Has high er en 5.Only one bondist between two atoms. 5.One r bs ben atom Problem 2.4 Show the electron distribution in MO's of (a)H()H(c)H(d)He Predict which are unstable Fill the lower-energy MO first with no more than two electrons (a)H,has atotal of two electrons,therefore: ↑↓ GG 个 Stable (excess of one bonding electron).Has less bonding strength than H (c)Hformed theoretically from H:and H.,has three electrons: 00* Stable (has net bond strength of one bonding electron).The antibonding electron cancels the bonding strength of one of the bonding electrons. (d)He,has four electrons,two from each He atom.The electron distribution is 百0* Problem 2.2 What type of MO results from side-to-side overlap of an s and a p orbital? The overlap is depicted in Fig. 2.4. The bonding strength generated from the overlap between the s AO and the  portion of the p orbital is canceled by the antibonding effect generated from overlap between the s and the portion of the p. The MO is nonbonding (n); it is no better than two isolated AO’s. 16 CHAPTER 2 Bonding and Molecular Structure Figure 2.4 Problem 2.3 List the differences between a σ bond and a π bond. σ Bond π Bond 1. Formed by head-to-head overlap of AO’s. 1. Formed by lateral overlap of p orbitals (or p and d orbitals). 2. Has cylindrical charge symmetry about bond axis. 2. Has maximum charge density in the cross-sectional plane of the orbitals. 3. Has free rotation. 3. Does not have free rotation. 4. Has lower energy. 4. Has higher energy. 5. Only one bond can exist between two atoms. 5. One or two bonds can exist between two atoms. Problem 2.4 Show the electron distribution in MO’s of (a) H2 , (b) H+ 2 , (c) H 2 , (d) He2 . Predict which are unstable. Fill the lower-energy MO first with no more than two electrons. (a) H2 has a total of two electrons, therefore: ↑↓ σ σ * Stable (excess of two bonding electrons). (b) H 2, formed from H and H·, has one electron: ↑ σ σ * Stable (excess of one bonding electron). Has less bonding strength than H2. (c) H 2, formed theoretically from H: and H·, has three electrons: ↑↓ ↑ σ σ * Stable (has net bond strength of one bonding electron). The antibonding electron cancels the bonding strength of one of the bonding electrons. (d) He2 has four electrons, two from each He atom. The electron distribution is ↑↓ ↑↓ σ σ * Not stable (antibonding and bonding electrons cancel, and there is no net bonding). Two He atoms are more stable than a He2 molecule.