Tetrahedral methane pyramidal

Ammonia, (NH3) has a pyramidal shape. Methane, (CH4) has a tetrahedral shape. 

The same kind of sp3 hybridization that describes the bonds to carbon in the tetrahedral methane molecule also describes bonds to nitrogen in the trigonal pyramidal ammonia molecule, to oxygen in the bent water molecule, and to all other atoms that VSEPR theory predicts to have a tetrahedral arrangement of four charge clouds. 

The same procedure can be applied to molecules of the type AH , where A is an element from the second or third row of the periodic table. In particular, starting from tetrahedral methane, CH4, the archetype of organic molecules, one can obtain the organic fragments pyramidal CH3(C3v), bent CH2(C2v), and CH(Coov), by removing one, two, or three atoms ofhydrogen (5-2). 

The modem chemist could not reasonably have expected either planar or pyramidal CH4 to be potentially isolable molecules, i.e. to be potential energy surface minima, and indeed as we have seen calculation indicates that they are not (and the inversion transition state for tetrahedral methane is not planar Chapter 1). Consider however the simple artifice of anchoring the basal bonds of pyramidal methane to a... 

Four electron pairs, all bonding, yield the same electron-pair and molecular geometries tetrahedral (Line 4). The tetrahedral methane molecule, CH4, looks like a tall pyramid with a triangular base (Fig. 13.5[a]). Each bond angle is 109.5°—the tetrahedral angle. 

Water, ammonia, and methane share the common feature of an approximately tetrahedral ariangernent of four electron pairs. Because we describe the shape of a molecule according to the positions of its atoms rather than the disposition of its electron pairs, however, water is said to be bent, and ammonia is trigonal pyramidal. 

Figure 1.10 The tetrahedral, trigonal pyramidal, and angular geometries of the methane, ammonia, and water molecules based on the tetrahedral arrangement of four electron pairs.

Because valence electron octets are so common, particularly for second-row elements, the atoms in a great many molecules have shapes based on the tetrahedron. Methane, for example, has a tetrahedral shape, with H-C-H bond angles of 109.5°. In NH3, the nitrogen atom has a tetrahedral arrangement of its four charge clouds, but one corner of the tetrahedron is occupied by a lone pair, resulting in a trigonal pyramidal shape for the molecule. Similarly, H20 has two corners of the tetrahedron occupied by lone pairs and thus has a bent shape. 

A methane molecule is tetrahedral, with bond angles of 109.5°. H 109.5° -P H > P Molecular geometries are / V / described by the relative positions of the nuclei, not the location of electron clouds. NH3 is trigonal pyramidal, not tetrahedral. 

H2S (hydrogen sulfide) is bent, CaCl2 (calcium chloride) is linear, CH4 (methane) is a tetrahedral, BF3 (boron trifluoride) is a trigonal pyramid, and KBr (potassium bromide) is linear. 

Drawing the Lewis structure of a molecule can help you determine the molecule s shape. In Figure 3.30, you can see the shape of the ammonia, NH3, molecule. The ammonia molecule has three bonding electron pairs and one lone pair on its central atom, all arranged in a nearly tetrahedral shape. Because there is one lone pair, the molecule s shape is pyramidal. The molecule methane, CH4, is shown in Figure 3.31. This molecule has four bonding pairs on its central atom and no lone pairs. 

For example, hh a disubstituted compound CH Rj (Fig. 8) (1) if the molecule is planar, then two isomers are possible. This planar configuration can be either square or rectangular in each case, there are two isomers only. (2) If the molecule is pyramidal, then two isomers are also possible. There are only two isomers, whether the base is square or rectangular. (3) If the molecule is tetrahedral, then only one form is possible. The carbon atom is at the center of the tetrahedron. In actuality, only one disubstituted isomer is known. Therefore, only the tetrahedral model for a disubstituted methane agrees with the evidence of the isomer number. 

Figure 7 Spatial models for methane where the four hydrogen atoms are equivalent. I, planar II, pyramidal III, tetrahedral.

Figure 8 Spatial models for a disubslituted methane. Top, planar middle, pyramidal bottom, tetrahedral.

For tetrasubstituted compounds of the type CRjR2R3R4 (Fig. 9) (1) if the molecule is planar, then three isomers are possible. (2) If the molecule is p3uramidal, then six isomers are possible. Each of the forms in Fig. 9, top, drawn as a pyramid, is not superimposable on its mirror image. Thus, three pairs of enantiomers are possible (one of which is shown in Fig. 9, middle). (3) If the molecule is tetrahedral, two isomers are possible, related to one another as object to mirror image. In actuality, only two tetrasubsti-tuted isomers of methane are known (pair of enantiomers). This is strong evidence for the tetrahedral model for the carbon atom. Similar reasoning leads to the same conclusion for trisubstituted methanes. 

Both the water molecule and the nitrate ion are planar all the atoms lie in the same plane. You could imagine these species lying flat on a tabletop, but most molecules are not planar. The shape of the methane molecule, CH4, is tetrahedral. Connecting the four hydrogen atoms forms a pyramid with four identical sides, thus the name tetrahedral. All H-C-H bond angles are the same, 109.5°, the tetrahedral angle, and all C-H bond lengths are the same. 

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