Chapter 4

Draw Lewis structures for the following molecules and ions, and in each case identify the bond order. (Section 4.2 ) \\[\mathrm{Cl}_{2} \quad \mathrm{Se}_{2} \quad \mathrm{HBr} \quad \mathrm{ClO}^{-}\\]

Use valence bond theory to describe the bonding in the cyanide anion, CN. What orbitals interact to form the bonds between the atoms? (Sections 4.4,4.5 )

Use the linear combination of atomic orbitals approach to write expressions for the wavefunctions for the in-phase and out-of-phase combinations of two lithium 2s orbitals. Which molecular orbitals in \(\mathrm{Li}_{2}\) do these combinations correspond to? (Sections \(4.6-4.8\))

The mass spectrum of a sample of beryllium contains an intense peak at \(m / z 9\) and a less intense peak at \(m / z 18\) Identify the species involved and use molecular orbital theory to support the existence of the species giving rise to the peak of lower intensity. (Sections \(4.6-4.8\) )

Which of the following pairs of molecules or ions are isoelectronic with each other? (Sections \(4.9-4.10\) ) (a) \(\mathrm{CO}\) and \(\mathrm{NO}^{-}\) (b) \(\mathrm{CN}^{-}\) and \(\mathrm{NO}^{+}\) (c) \(\mathrm{N}_{2}^{-}\) and \(\mathrm{O}_{2}^{-}\)

Draw a labelled molecular orbital energy level diagram for the acetylide dianion \(\mathrm{C}_{2}^{2-}\) and use it to explain why the bond length \(\operatorname{in} \mathrm{C}_{2}^{2-}(119 \mathrm{pm})\) is less than that in \(\left.\mathrm{C}_{2}(124 \mathrm{pm}) . \text { (Section } 4.11\right)\)

Give examples of neutral homonuclear and heteronuclear diatomic molecules that are isoelectronic with \(\mathrm{C}_{2}^{2-}\). (Section 4.11 ).

Construct a labelled molecular orbital energy level diagram for Si \(_{2}\), stating any assumptions you have made. Measurements suggest that \(\mathrm{Si}_{2}\) is diamagnetic in the gas phase. Is this consistent with your diagram? If not, what changes could be made to the order of the molecular orbitals? How could these be justified? (Section 4.11).