Skill 17.5 Describing atomic orbitals

The electron configurations of isolated atoms are found in atomic orbitals the configurations of atoms about to bond are represented by atomic and hybridized atomic orbitals and the electron configurations of molecules are represented by molecular orbitals. Molecular orbital theory is an advanced topic, but it may be simplified to representing the bonds between atoms as overlapping electron density shapes from atomic orbitals. There are two typical locations for molecular orbitals. 

The bonding pi orbital (tt) follows regions separate from a line drawn between the two atoms in a bond. Two overlapping p orbitals will form tt bonds to contain the additional shared electrons in molecules with double or triple bonds, tt bonds prevent atoms from rotating about the central axis between them. 

In CH4, the electron density of the four sp orbitals of C each overlap with an s orbital of H to form four o bonds. In C2H4 (an alkene), two sp orbitals on each C overlap with H s orbitals, the remaining sp orbitals overlap with each other in a CT bond, and the p orbitals (drawn as shaded shapes) overlap with each other above and beneath the carbon atoms in a tt bond (also drawn as shaded shapes). In CO2. the C atom has two sp hybrid orbitals and two p orbitals. These form one ct bond and one tt bond with the two unfilled p orbitals on each 0 atom. In C2H2 (an alkyne), a triple bond forms with one crand two tt bonds. 

Molecules with double bonds next to each other and aromatic molecules based on benzene contain more than two tt orbitals on adjacent atoms. The bonds, as well as the entire molecules, are described as being conjugated. Electrons in these molecules are free to move from one bond to the next on the same molecule and so are delocalized. In 0014, we saw electron delocalization extending throughout the entire substance in materials with metallic bonds. Benzene (CeHe) has the following resonance forms  

Each carbon atom in benzene bonds to three atoms, so their electrons are in three sp orbitals and one p orbital as we ve seen for C2H4. The p orbitals are shown as the shaded shapes below on the left (only the C-C bonds are shown). The p atomic orbitals combine to form molecular orbitals with delocalized electrons as shown in the bonding tt molecular orbital below to the right. 

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The quantum-mechanical solutions from the Schrodinger Equation (see Skill 1.2a) utilize three quantum numbers (n, 1, and mj) to describe an orbital and a fourth (ms) to describe an electron in an orbital. This model is useful for understanding the frequencies of radiation emitted and absorbed by atoms and chemical properties of atoms. 

Getting acquainted with the shapes and sizes of atomic orbitals is one of the first important skills for a chemistry student to master. Figure 2.1 a-c depicts the occupied Is atomic orbital of the ground-state hydrogen (H) atom in three different graphical representations (all obtained firom PLOT output using the NBOView orbital plotter, as described in Appendix B). 

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