Steric number of C in the structure is 3  sp2 hybrid QUESTION: In the structure below, the sigma bond between which two atoms is due to an sp2-to-sp2 overlap? A. C and O, B. C and N, C. N and H In the structure shown here, the sigma bond between which two atoms is due to an sp2-to-sp3 overlap? A. C and O, B. C and N, C. N and H PAUSE CLICK In valence bond theory, a sigma bond refers a high electron density between two atoms due the overlap of their valence orbitals. The only orbital used for bonding by hydrogen is an s orbital. Therefore, choice C can’t be correct. CROSS OUT C Let’s examine choice A. The carbon atom has a steric number of 3. There are three regions of high electron density around this atom. one... HIGHLIGHT single bond two... HIGHLIGHT double bond three... HIGHLIGHT other single bond CLICK According to valence bond theory, when the steric number is three, then three of the atom’s valence orbitals are hybridized. The hybrid orbitals are called sp2 orbitals. This means that the valence s and two of the valence p orbitals HIGHLIGHT s, p, p, in “s, p, p, p” are replaced by three sp2 orbitals and a p orbital HIGHLIGHT sp2, sp2, sp2, in “sp2, sp2, sp2, p” In this structure, carbon uses one of the three sp2 hybrids to form a sigma bond with oxygen. The other two are used to form sigma bonds with the nitrogen atoms. How about oxygen. There are also three regions of high electron density around oxygen. Its steric number is 3. So, an sp2 hybrid describes this lone pair... HIGHLIGHT one lone pair of O ... this lone pair HIGHLIGHT the other lone pair ... and the sigma bond that it forms with carbon HIGHLIGHT one of the two lines in the double bond. Therefore, the carbon-to-oxygen sigma bond involves an overlap of the carbon’s sp2 orbital with the oxygen’s sp2 overlap. The correct answer is A. CLICK CONTINUED ON NEXT SLIDE Steric number of C in the structure is 3  sp2 hybrid Replace: s, p, p, p  sp2, sp2, sp2, p Steric number of O in the structure is 3  sp2 hybrid Therefore: C=O bond is due to sp2-sp2 overlap (sigma bond) and p-p overlap (pi bond)

sigma bond Here’s a simple illustration of the idea of bonding due to overlapping atomic orbitals. Imagine this is the sp2 orbital of carbon CLICK and that this is the sp2 orbital of oxygen When these two overlap what we get is called a sigma bond. Now imagine this is the p orbital of carbon CLICK and that this is the p orbital of oxygen CLICK When these two overlap what we get is called a pi bond The double bond between carbon and oxygen is said to involve a sigma bond, which involves an sp2-to-sp2 overlap and and a pi bond, which involves a p-to-p overlap. One difference between a sigma bond and a pi bond is that a pi bond has a node at the internuclear axis. The internuclear axis refers to the line from the nucleus of one atom to the other. WHen we say that there is a a node here, we mean that the probability of finding the electrons described by the bond is zero here. For sigma bonds, the internuclear axis is not a node. CLICK CONTINUED ON NEXT SLIDE pi bond

Steric number of N in the structure is 4  sp3 hybrid QUESTION: In the structure below, the sigma bond between which two atoms is due to an sp2-to-sp2 overlap? A. C and O, B. C and N, C. N and H What about the Carbon-to-nitrogen bond? CLICK BOth nitrogen atoms in this structure have a steric number of 4. For example, this nitrogen atom has four regions of high electron density around it. One... HIGHLIGHT single bond to C Two... HIGHLIGHT single bond to H Three... HIGHLIHGT single bond to other H Four... HIGHLIGHT lone pair. When the steric number is 4, the hybridization is sp3. CLICK Since the hybridization of carbon is sp2. CLICK The carbon-to-nitrogen bond involves an sp2-to-sp3 overlap. CLICK PAUSE END RECORDING Steric number of N in the structure is 4  sp3 hybrid Steric number of C in the structure is 3  sp2 hybrid Therefore: C-N bond involves an sp2-to-sp3 overlap.

Video ID: 3-14-3 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08