Trigonal planar shape hybrid orbitals

The carbon atom in a typical carbocation is sp hybridized. The pz orbital is empty and is perpendicular to the plane of the other three bonds. Thus, carbocation adopts a trigonal planar shape. 

The hybridisation between one s orbital and two p orbitals results in three sp hybrid orbitals, which form a trigonal planar shape. In this scenario, the third p orbital is left unhybridised, and is at... 

An sp hybridized nitrogen centre is consistent with the trigonal planar shape of [N03]. Allow the hybrid orbitals to overlap with suitable orbitals from oxygen a choice of sp hybridization on the O atom provides suitable orbitals to accommodate the oxygen lone pairs. Occupation of each bonding orbital by a pair of electrons gives three equivalent N—O f7-bonds ... 

Look again at Table 9-3 on the previous page. Notice that the number of atomic orbitals mixed to form the hybrid orbital equals the total number of pairs of electrons. In addition, the number of hybrid orbitals formed equals the number of atomic orbitals mixed. For example, AICI3 has a total of three pairs of electrons and VSEPR predicts a trigonal planar molecular shape. To have this shape, one s and two p orbitals on the central atom A1 must mix to form three identical sp hybrid orbitals. 

There are three bonding pairs and no lone pair on the A1 atom. From Table 10.1 we see that the shape of three electron pairs is trigonal planar, and from Table 10.4 we conclnde that A1 mnst be ip -hybridized in AII3. The orbital diagram of the gronnd-state A1 atom is... 

Plan We note, as in Sample Problem 11.1, the shape around each central atom to postulate the type of the hybrid orbitals, paying attention to the multiple bonding of the C and O bond. Solution In Sample Problem 10.8, we determined the shapes around the three central atoms of acetone tetrahedral around each C of the two CH3 (methyl) groups and trigonal planar around the middle C atom. Thus, the middle C has three sp orbitals and one unhybridized p orbital. Each of the two methyl C atoms has four sp orbitals. Three of these sp orbitals overlap the U orbitals of the H atoms to form a bonds the fourth overlaps an sp orbital of the middle C atom. Thus, two of the three sp orbitals of the middle C form CT bonds to the other two C atoms. 

As you review each type of hybrid orbital in the following subsections, take note of (1) the number and types of atomic orbitals that were combined to make the hybrid orbitals, (2) the number of orbitals that remain uncombined, and (3) the three-dimensional arrangement in space of the hybrid orbitals and any uncombined p orbitals. In particular, you will find that these three-dimensional arrangements will retain the names (tetrahedral, trigonal planar, linear) and bond angles (109.5°, 120°, and 180°) used to describe the shapes of molecules in our section on VSEPR (Section 1.3). 

FIGURE 13.25 The three sp hybrid orbitals are formed by the combination of one s and two p atomic orbitals. They are arranged in a plane and have the shape of an equilateral triangle. These hybrid orbitals are used to explain the trigonal planar structure of molecules like BF3. 

The notation sp indicates that the hybrids are mixtures of one s orbital and two p orbitals. The shapes of the sp hybrid orbitals are shown in Figure 10.8 . Notice that the three hybrid orbitals have a trigonal planar geometry with 120° angles between them. The unhybridized p orbital is oriented perpendicular to the three hybridized orbitals. 

More detail about orbital hybridization than provided above is given in Sections 1.9 1.15 of Organic Chemistry. With that greater detail it will be apparent from consideration of orbital hybridization that for three groups of valence electrons the ideal separation is 120° (trigonal planar), and for two groups of valence electrons the ideal separation is 180° (linear). VSEPR theory allows us to come to essentially the same conclusion as by the mathematical hybridization of orbitals, and it will serve us for the moment in predicting the three-dimensional shape of molecules. 

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