In this theory, electrons are potrayed to move in waves. When more than one of these waves comes close together, the in-phase combination of these waves produces an interaction that leads to a species that is greatly stabilized. Viva Differences. Bonding molecular orbitals can be described as type of molecular orbitals that take part in the formation of a chemical bond. Antibonding molecular orbitals can be described as orbitals containing electrons outside the region between two atomic nuclei.
Bonding molecular orbitals are more stable than both antibonding molecular orbitals and parent atomic orbitals. Antibonding molecular orbitals are less stable than both bonding molecular orbitals and parent atomic orbitals. The geometry of a molecule is represented by the spatial arrangement of bonding molecular orbitals.
The length of the carbon-hydrogen bonds in methane is pm. While previously we drew a Lewis structure of methane in two dimensions using lines to denote each covalent bond, we can now draw a more accurate structure in three dimensions, showing the tetrahedral bonding geometry.
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In this convention, a solid wedge simply represents a bond that is meant to be pictured emerging from the plane of the page. A dashed wedge represents a bond that is meant to be pictured pointing into, or behind, the plane of the page.
Normal lines imply bonds that lie in the plane of the page. Exercise 2. In the images below, the exact same methane molecule is rotated and flipped in various positions. Draw the missing hydrogen atom labels. It will be much easier to do this if you make a model. Solutions to exercises. How does this bonding picture extend to compounds containing carbon-carbon bonds?
In ethane CH 3 CH 3 , both carbons are sp 3 -hybridized, meaning that both have four bonds with tetrahedral geometry. The carbon-carbon bond, with a bond length of pm, is formed by overlap of one sp 3 orbital from each of the carbons, while the six carbon-hydrogen bonds are formed from overlaps between the remaining sp 3 orbitals on the two carbons and the 1 s orbitals of hydrogen atoms.
Bonding molecular orbital
All of these are sigma bonds. Because they are formed from the end-on-end overlap of two orbitals, s igma bonds are free to rotate. This means, in the case of ethane molecule, that the two methyl CH 3 groups can be pictured as two wheels on an axle, each one able to rotate with respect to the other.
The sp 3 bonding picture is also used to described the bonding in amines, including ammonia, the simplest amine. Just like the carbon atom in methane, the central nitrogen in ammonia is sp 3 - hybridized. With nitrogen, however, there are five rather than four valence electrons to account for, meaning that three of the four hybrid orbitals are half-filled and available for bonding, while the fourth is fully occupied by a nonbonding pair lone pair of electrons.
The bonding arrangement here is also tetrahedral: the three N-H bonds of ammonia can be pictured as forming the base of a trigonal pyramid, with the fourth orbital, containing the lone pair, forming the top of the pyramid. The bonding in water results from overlap of two of the four sp 3 hybrid orbitals on oxygen with 1 s orbitals on the two hydrogen atoms. The two nonbonding electron pairs on oxygen are located in the two remaining sp 3 orbitals.
The valence bond theory, along with the hybrid orbital concept, does a very good job of describing double-bonded compounds such as ethene. Three experimentally observable characteristics of the ethene molecule need to be accounted for by a bonding model:. Clearly, these characteristics are not consistent with an sp 3 hybrid bonding picture for the two carbon atoms. Instead, the bonding in ethene is described by a model involving the participation of a different kind of hybrid orbital. Three atomic orbitals on each carbon — the 2 s , 2 p x and 2 p y orbitals — combine to form three sp 2 hybrids , leaving the 2 p z orbital unhybridized.
Lecture Molecular Orbital Model of Chemical Bonding
The unhybridized 2 p z orbital is perpendicular to this plane in the next several figures, sp 2 orbitals and the sigma bonds to which they contribute are represented by lines and wedges; only the 2 p z orbitals are shown in the 'space-filling' mode. The carbon-carbon double bond in ethene consists of one sigma bond, formed by the overlap of two sp 2 orbitals, and a second bond, called a pi bond , which is formed by the side-by-side overlap of the two unhybridized 2 p z orbitals from each carbon.
Unlike a sigma bond, a pi bond does not have cylindrical symmetry. If rotation about this bond were to occur, it would involve disrupting the side-by-side overlap between the two 2 p z orbitals that make up the pi bond. This argument extends to larger alkene groups: in each case, six atoms lie in the same plane. A similar picture can be drawn for the bonding in carbonyl groups, such as formaldehyde. In this molecule, the carbon is sp 2 -hybridized, and we will assume that the oxygen atom is also sp 2 hybridized.
The carbon has three sigma bonds: two are formed by overlap between sp 2 orbitals with 1 s orbitals from hydrogen atoms, and the third sigma bond is formed by overlap between the remaining carbon sp 2 orbital and an sp 2 orbital on the oxygen. The two lone pairs on oxygen occupy its other two sp 2 orbitals. The pi bond is formed by side-by-side overlap of the unhybridized 2 p z orbitals on the carbon and the oxygen. Just like in alkenes, the 2 p z orbitals that form the pi bond are perpendicular to the plane formed by the sigma bonds.
Recall that carbocations are transient, high-energy species in which a carbon only has three bonds rather than the usual four and a positive formal charge. We will have much more to say about carbocations in this and later chapters. For now, though, the important thing to understand is that a carbocation can be described as an sp 2 -hybridized carbon with an empty p orbital perpendicular to the plane of the sigma bonds.
Finally, the hybrid orbital concept applies as well to triple-bonded groups, such as alkynes and nitriles. Consider, for example, the structure of ethyne common name acetylene , the simplest alkyne. Both the VSEPR theory and experimental evidence tells us that the molecule is linear: all four atoms lie in a straight line. In the hybrid orbital picture of acetylene, both carbons are sp- hybridized.
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The 2 p y and 2 p z orbitals remain unhybridized, and are oriented perpendicularly along the y and z axes, respectively. The carbon-carbon sigma bond, then, is formed by the overlap of one sp orbital from each of the carbons, while the two carbon-hydrogen sigma bonds are formed by the overlap of the second sp orbital on each carbon with a 1 s orbital on a hydrogen. Each carbon atom still has two half-filled 2 p y and 2 p z orbitals, which are perpendicular both to each other and to the line formed by the sigma bonds.
Valence Bond Theory: A Localized Bonding Approach
These two perpendicular pairs of p orbitals form two pi bonds between the carbons, resulting in a triple bond overall one sigma bond plus two pi bonds. Identify the hybridization of all carbon atoms in the molecule. The hybrid orbital concept nicely explains another experimental observation: single bonds adjacent to double and triple bonds are progressively shorter and stronger than single bonds adjacent to other single bonds.
Consider for example, the carbon-carbon single bonds in propane, propene, and propyne. All three are single sigma bonds; the bond in propyne is shortest and strongest, while the bond in propane is longest and weakest. The explanation is relatively straightforward.