5: Building Bonds in .NET framework

Print EAN 13 in .NET framework 5: Building Bonds

180

120

1095

Bent (V shaped)

How can hydrogen atoms form four identical bonds with carbon Conceptually, two things happen to produce the four equivalent orbitals necessary for methane First, one of carbon s 2s electrons is promoted (sent to a higher-energy orbital) to the empty 2p orbital This promotion results in four half-filled valence orbitals; the 2s orbital and three 2p orbitals now each contain one electron Second, the single 2s orbital is mixed with the three 2p orbitals to create four identical sp3 hybrid orbitals The fact that each sp3 orbital is identical is important because VSEPR theory can now explain the symmetrical shape of methane: the tetrahedron So, the shapes of real molecules emerge from the geometry of valence orbitals the orbitals that bond to other atoms Here s how to predict this geometry:

molecule You can do this by looking at the Lewis structure In formaldehyde (CH2O), for example, carbon bonds with two hydrogen atoms and double bonds with one oxygen atom So, carbon effectively has three valence orbitals

(like s and p) that the valence electrons occupy Carbon has four valence electrons in 2s22p2 configuration Two valence electrons occupy an s orbital, and one electron occupies each of two identical p orbitals The s orbital isn t equivalent to the p orbitals So, we mix the s orbital with the two p orbitals to create three identical sp2 hybrid orbitals Note that the total number of orbitals doesn t change; in the example, formaldehyde has three valence orbitals before mixing and still has three valence orbitals after mixing VSEPR theory predicts that electrons in three identical orbitals mutually repel to create a trigonal planar geometry the three orbitals splay out in a plane with 120 degrees between each orbital The shape of the formaldehyde molecule is trigonal planar Different combinations of orbitals produce different hybrids One s orbital mixes with two p orbitals to create three identical sp2 hybrids One s orbital mixes with one p orbital to create two identical sp hybrids Centers with sp3, sp2, and sp hybridization tend to possess tetrahedral, trigonal planar, and linear shapes, respectively Lewis structures provide a good starting place for roughly estimating the shapes of molecules If a central atom has two bonding partners, the shape of the molecule around that center will likely be fairly linear If the central atom has three bonding partners, the shape will likely be close to trigonal planar If the central atom has four bonding partners, the shape will likely be close to tetrahedral The actual shapes may vary from these rough estimates depending on other factors, such as whether the central atom has lone pairs For example, the oxygen atom of water has two lone pairs in addition to two bonding orbitals In all, then, the electrons in the orbitals surrounding oxygen create a tetrahedral-like geometry But, because only two hydrogen atoms are actually bound, the shape of the molecule is bent

Methane, CH4, has four hydrogen atoms bonded to a central carbon atom Ammonia, NH3, has three hydrogen atoms bonded to a central nitrogen atom Using VSEPR theory, compare and contrast the orbital geometry and molecular shape of these two molecules Carbon and nitrogen are both sp3 hybridized in methane and ammonia, respectively Carbon contributes a single electron in each hybrid orbital to a covalent bond, the second electron in each bond coming from a hydrogen Nitrogen has one more valence electron than

carbon Therefore, one of the hybrid orbitals of nitrogen contains two electrons and can t receive a bonding electron from hydrogen Although ammonia has only three bonded hydrogen atoms for this reason, the central nitrogen has a lone pair of electrons in the remaining sp3 orbital These electrons still repel other electron pairs So, the ammonia molecule has a nearly tetrahedral orbital geometry, similar to that of methane However, because one of the orbitals contains a lone pair (not a bonded atom) the molecular shape of ammonia is trigonal pyramidal

What s the hybridization of carbon in carbon dioxide (CO2) In formaldehyde (CH2O) In methyl bromide (H3CBr)

Use Lewis structures and VSEPR theory to predict the shape of water (H2O), ethyne (C2H2), and carbon tetrachloride (CCl4)