Isolated O atom: 6 valence electrons QUESTION: Based on formal charges, which of the Lewis structures shown below is the most likely structure of HNO2? A. Structure I, B. Structure II, C. Structure III Isolated O atom: 6 valence electrons Left O in structure I: 5 valence electrons 2 in lone pair 1 from each bonding pair F.C. = 6– 5 = +1 +1 0 -1 +1 +1 -2 0 0 0 All of these structures follow the octet/duet rule. In general, most feasible structure: zero formal charges. H, N, and O have zero formal charge if: H: single bond; no lone pairs N: three bonding pairs; one lone pair O: two bonding pairs; two lone pairs SCRIPT: 3-8-2 Based on formal charges, which of the Lewis structures shown below is the most likely structure of CH4O? CLICK These are all valid Lewis structures… the hydrogen atoms follow the duet rule… And all the oxygen and nitrogen atoms follow the octet rule… The total number of electrons represented correspond to the correct total number of valence electrons. So how do we determine which structure best resembles reality? In general, the most feasible structure is one where all the atoms have zero formal charge. Let’s examine these structures. A hydrogen atom has a zero formal charge if it has only one single bond and it doesn’t have a lone pair. HIGHLIGHT single bond; no lone pair That is the case in all structures. How about nitrogen? A nitrogen atom has a zero formal charge if it has THREE bonding pairs and one lone pair. HIGHLIGHT three bonding pairs; one lone pair That’s not the case in structure2. HIGHLIGHT N in structures 2. In structure 2, the nitrogen atom has one bonding pair and three lone pairs. One… Two… CLICK Three… How about oxygen? An oxygen atom has a zero formal charge if has TWO bonding pairs and two lone pairs. HIGHLIGHT: two bonding pairs; two lone pairs We can see that this is the case in structure 2. Both oxygen atom sin structure have 3 bonding pairs and one lone pair. This is also the case for one of the oxygen atoms in structure one. HIGHLIGHT left O in structure 1 Therefore, structure 3 is the only structure shown here where all the atoms have a zero formal charge. The correct answer is C, structure 3. Here’s a quick review for calculating formal charges. An oxygen atom by itself, has six valence electrons. We can determine this by looking at the periodic table. Oxygen belongs to group six A. Atoms belonging to group six A have six valence electrons In structure 1, it appears to own five ---- two in the lone pair, and one from each of the three bonding pairs. Remember that for purposes of calculating formal charge, an atom is assigned ownership of half of the shared electrons. Therefore, the oxygen atom in structure 1 appears to have lost one electron. Its formal charge is +1. The formal charges for O, N and O in structure 1 are +1, 0, and –1, respectively. For structure 2, it’s +1, +1, and –2 CLICK For structure 3, it’s 0, 0, and 0 Structures with like charges on neighboring atoms are very unlikely. In structure 2, neighboring oxygen atoms both have a formal charge of +1. That’s not good. Structures with formal charges of +2 or –2 are very unlikely to resemble reality. Again, we see that in structure 2. The formal charges in structure 1 are OK. Actually, structures 1 and 3 are resonance structures. They have the same connectivity… only the electron distribution is different. But we would expect structure 3 to have a more significant contribution to the resonance hybrid. In other words, we expect reality to resemble the electron distribution in structure 3 more than in structure 1.

Video ID: 3-8-2 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08