Four Electron Groups Tetrahedral Geometry

Chapter 10 Chemical Bonding II Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory 

Methane is an example of a molecule with four electron groups around the central atom  

What is the geometry of the HCN molecule The Lewis structure of HCN is H—C=N. (a) linear (b) trigonal planar (c) tetrahedral 

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A steric number of 4 identifies four electron groups that must be separated in three-dimensional space. Four groups are as far apart as possible in tetrahedral electron group geometry. 

According to the VSEPR model developed in Chapter 9, an inner atom with a steric number of 4 adopts tetrahedral electron group geometry. This tetrahedral arrangement of four electron groups is very common, the only important exceptions being the hydrides of elements beyond the second row, such as H2 S and PH3. Thus,... 

The Lewis structure of NC13 has three Cl atoms bonded to N and one lone pair attached N. These four electron groups around N produce a tetrahedral electron-group geometry. The fact that one of the electron groups is a lone pair means that the molecular geometry trigonal pyramidal. 

B The Lewis structure of POCl3 has three single P-Cl bonds and one P-0 bond. These four electron groups around P produce a tetrahedral electron-group geometry. No lone pairs are attached to P and thus the molecular geometry is tetrahedral. 

The water molecule s electron group geometry, which is determined hy the arrangement of its four electron groups around the oxygen atom, is tetrahedral (Figure 12.7). 

VSEPR theory predicts that four valence shell electron groups are directed toward the corners of a regular tetrahedron. That shape gives the maximum separation for four electron groups around one atom. Thus, VSEPR theory predicts tetrahedral electronicgecmetry for an AB4 molecule that has no unshared electrons on A. There are no lone pairs of electrons on the central atom, so a bonded atom is at each comer of the tetrahedron. VSEPR theory predicts a tetmhedml molecular geometry for each of these molecules. 

When there are four electron groups around the central atom, it is sp hybridized. AB4 molecules and ions with no lone pairs on the central atom have tetrahedral electronic geometry, tetrahedral molecular geometry, and sp hybridization on the central atom. 

ILach C atom in C2H5 has four electron groups. The VSEPR theory tells us that each C atom has tetrahedral electronic geometry the resulting atomic arrangement around each C atom has one C and three H atoms at the comers of this tetrahedral arrangement. The VB interpretation is that each C atom is sp hybridized. The C — C bond is formed by overlap of a half-filled sp hybrid orbital of one C atom with a half-filled sp hybrid orbital of the other C atom. Each C — H bond is formed by the overlap of a half-filled sp hybrid orbital on C with the half-filled Ir orbital of an H atom. 

Tetrahedral A term used to describe the electronic geometry around a central atom that has four electron groups. Also used to describe the molecular geometry of a molecule or polyatomic ion that has one atom in the center bonded to four atoms at the corners of a tetrahedron (AB4). 

If a molecule has four electron groups arotmd the central atom, as CH4, does, it has a tetrahedral geometry with bond angles of 109.5°. 

The mutual repulsion of the four electron groups causes the tetrahedral shape— the tetrahedron allows the maximum separation among the four groups. When we write the structure of CH4 on paper, it may seem that the molecule should be square planar, with bond angles of 90°. However, in three dimensions the electron groups can get farther away from each other by forming the tetrahedral geometry. 

The four electron groups (one lone pair and three bonding pairs) get as far away from each other possible. If we look only at the electrons, we find that the electron geometry— the geometrical arrangement of the electron groups—is tetrahedral. 

The electron geometry is tetrahedral (four electron groups), and the molecular geometry—the shape of the molecule— is trigonal pyramidal (four electron groups, three bonding groups, and one lone pair). 

Since ammonia has a total of four electron groups around its central atom, the electron geometry is again tetrahedral. 

In a molecule of methane, CH4, the central C atom is bonded to four H atoms. From the electron-dot formula, you may think that CH4 is planar with 90° bond angles. However, the best geometry for minimum repulsion is tetrahedral, which places the four electron gronps at the comers of a tetrahedron, giving bond angles of 109°. When there are four atoms attached to four electron groups, the shape of the molecule or polyatomic ion is tetrahedral. 

In the electron-dot formula of water, H2O, there are also four electron groups, which have minimal repulsion when the electron-group geometry is tetrahedral. However, in H2O, two of the electron groups are lone pairs of electrons. Because the shape of H2O is determined by the two H atoms bonded to the central O atom, the H2O molecule has a bent shape with a bond angle of 109°. Table 10.3 gives the molecular shapes for molecules with two, three, and four bonded atoms. 

In the H2O molecule, two of the four electron groups are bond pairs and two are lone pairs. The molecular shape is obtained by joining the two H nuclei to the O nucleus with straight lines. For H2O, the electron-group geometry is tetrahedral and the molecular geometry is V-shaped, or bent. In the diagram below, the Lewis structure for water is drawn in two ways. 

Tetrahedral geometry may be the most common shape in chemistry, but several other shapes also occur frequently. This section applies the VSEPR model to four additional electron group geometries and their associated molecular shapes. 

Both inner atoms have steric numbers of 4 and tetrahedral electron group geometry, so both can be described using s p hybrid orbitals. All four hydrogen atoms occupy outer positions, and these form bonds to the inner atoms through 1 s-s p overlap. The oxygen atom has two lone pairs, one in each of the two hybrid orbitals not used to form O—H bonds. 

The 2 1 species are known as cuprates and are the most common synthetic reagents. Disubstituted Cu(I) species have the 3c 10 electronic configuration and would be expected to have linear geometry. The Cu is a center of high electron density and nucleophilicity, and in solution, lithium dimethylcuprate exists as a dimer [LiCu(CH3)2]2.3 The compound is often represented as four methyl groups attached to a tetrahedral cluster of lithium and copper atoms. However, in the presence of Lil, the compound seems to be a monomer of composition (CH3)2CuLi.4... 

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