Bond angles depends on steric number of the atom at the vertex. QUESTION: For the molecule represented by the Lewis structure below, the N-C-N bond angle is closest to... A. 180o, B. 109.5o, C. 120o Bond angle is defined by specifying three atoms. Middle atom is vertex. Bond angles depends on steric number of the atom at the vertex. Steric number = number of electron domains around the atom. For the molecule represented by the Lewis structure shown here, the N-C-N bond angle is closest to... A. 180o, B. 109.5o, C. 120o PAUSE CLICK A bond angle is defined by specifying three atoms. The middle atom is called the vertex of the angle. Imagine a line from the first atom to the vertex, and a line from the vertex to the second atom. For this question, our vertex is the carbon atom. CALLOUT “vertex” pointing to C in N-C-N It may seem obvious based on the Lewis structure that N-C-N the angle should be 180 degrees. In fact, it is not. A Lewis structure does not necessarily show the correct bond angles. A Lewis structure only needs to show how the atoms are connected and how the valence electrons are shared. Whenever you’re asked for the shape, you should answer based on what the actual molecule would look like. CLICK Bond angles depend on the steric number of the atom at the vertex. CLICK Steric number refers to the number of regions of high electron density around the atom. Each region is called an electron domain. In the case of carbon in the structure shown, the steric number is 3. Each single bond counts as one electron domain. Here we have two single bonds. HIGHLIGHT two C-N A double bond counts as just one domain. So, the carbon-to-oxygen double bond HIGHLIGHT C=0 also counts as one domain. CLICK CONTINUED ON NEXT SLIDE

If steric number=3: electron domains are 120o apart QUESTION: For the molecule represented by the Lewis structure below, the N-C-N bond angle is closest to... A. 180o, B. 109.5o, C. 120o Steric number of C is 3. VSEPR: electron domains are oriented as far away from each other as possible. If steric number=3: electron domains are 120o apart The vertex is called a “trigonal planar” center. CLICK So the steric number for the carbon atom in the structure shown here is 3. CLICK Valence shell electron pair repulsion theory, VSEPR, says that regions of high electron density are oriented as far away from each other as possible. CLICK If there are 3 regions of high electron density around an atom, then these regions would be farthest from each other by being oriented 120 degrees apart. Therefore, we expect the NCN angle to be close to 120. Actually, it is slightly less than 120 because the double bond repels the single bonds more strongly. The correct answer is C. Since the three groups of electrons sticking out of the carbon atom are all on the same plane and are pointing towards the corners of a triangle, the carbon atom in this case is called a trigonal planar center. Here’s a better way of showing the orientation of the electron domains around carbon. CLICK THe N-C-N bond angle refers to the angle formed by this line CLICK and this line PAUSE CLICK PAUSE END RECORDING

Video ID: 3-11-3 © 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