QUESTION: Which of the Lewis structures shown below for azide ion (N3-) has a nitrogen atom with a formal charge of +2? A. Structure I only, B. Structure II only C. Both structures, C. neither structure SCRIPT: 3-7-1 Which of the Lewis structures shown below for azide ion (N3-) has a nitrogen atom with a formal charge of +2? A. Structure I only, B. Structure II only C. Both structures, C. neither structure CLICK CONTINUED ON NEXT SLIDE

Formal charge = fictitious charge assigned to an atom in a Lewis structure for a molecule or polyatomic ion F.C. = F - A F = # valence electrons in a Free atom A = Apparent # valence electrons “owned” by atom in structure = number of electrons in lone pairs + (1/2) of electrons shared with other atoms = 2L + B Left N appears to own: 1 lone pair x 2 = 2 3 bonding pairs = 3 A = 2+3 = 5 F = 5 (group VA) F.C. = 5-5 = 0 SCRIPT: HIGHLIGHT Formal charge Formal charge is a fictitious charge assigned to an atom in a Lewis structure for a molecule or polyatomic ion. We can calculate it using the formula HIGHLIGHT F.C. = F-A In this formula, F is the number of valence electrons in a free atom… that is… the number of valence electrons that the atom has if it were alone. We can determine this by looking up the element in the periodic table. For example, a nitrogen atom belongs to group VA. Therefore, alone by itself, a nitrogen atom has five valence electrons. A is the APPARENT number of valence electrons owned by the atom in the structure. For purposes of calculating formal charge, we assume that an atom owns all of the electrons in its lone pairs HIGHLIGHT number of electrons in lone pairs And half of the electrons that it is sharing with other atoms. In other words, if an atom has L lone pairs HIGHLIGHT L and B bonding pairs HIGHLIGHT B Then the number of valence electrons that it appears to own is 2L plus B Let’s illustrate this for the Nitrogen atom on the left in structure one. CLICK This atom has one lone pair. HIGHLIGHT lone pair on N It owns both electrons in the lone pair. 2L equals 2. HIGHLIGHT 1 lone pair x 2 = 2 This atom also has three bonding pairs. HIGHLIGHT triple bond in the structure That’s six electrons, but it owns only have of those --- the other half belongs to the nitrogen atom in the middle. Half of six is 3. B equals 3 HIGHLIGHT 3 bonding pairs Therefore, the total number of electrons that the nitrogen atom on the left appears to own is 5. 2+3 equals 5. HIGHLIGHT A=2+3=5 As mentioned earlier, a nitrogen atom that is all alone by itself has five valence electrons. We determine this By locating nitrogen in the periodic table. It belongs to group five A. Therefore, F equals 5. Therefore the formal charge is 5 minus 5, or zero. CLICK It appears that the nitrogen atom has neither gained nor lost valence electrons. CLICK CONTINUED ON NEXT SLIDE

F = 5 (group VA) F.C. = F-A = 5-4 = +1 Formal charge = fictitious charge assigned to an atom in a Lewis structure for a molecule or polyatomic ion F.C. = F - A F = # valence electrons in a Free atom A = Apparent # valence electrons “owned” by atom in structure = number of electrons in lone pairs + (1/2) of electrons shared with other atoms = 2L + B Middle N appears to own: 0 lone pair x 2 = 0 4 bonding pairs = 4 A = 0+4 = 4 F = 5 (group VA) F.C. = F-A = 5-4 = +1 SCRIPT: Now, let’s determine the formal charge of the nitrogen atom in the middle. HIGHLIGHT Middle N It has no lone pair. So, L equals 0, and 2L equals 0. HIGHLIGHT 0 lone pair x 2 = 0 It has four bonding pairs. HIGHLIGHT 4 bonding pairs = 4 A triple bond with the nitrogen on the left. HIGHLIGHT triple bond And a single bond with the nitrogen on the right HIGHLIGHT single bond So, in this case, the nitrogen atom appears to own a total of four valence electrons. HIGHLIGHT: A=0+4 = 4 Once again, we subtract this from the number of valence electrons in the free atom HIGHLIGHT F=5 Five minus 4 equals +1. HIGHLIGHT: F.C. = F-A = 5-4 = +1 CLICK Therefore, the formal charge of the middle nitrogen is +1. We could say that this nitrogen atom appears to have lost one valence electron. If it were a free atom it would have 5, … but here, it appears to have only 4. CLICK CONTINUED ON NEXT SLIDE +1

F = 5 (group VA) F.C. = F-A = 5-7 = -2 Formal charge = fictitious charge assigned to an atom in a Lewis structure for a molecule or polyatomic ion F.C. = F - A F = # valence electrons in a Free atom A = Apparent # valence electrons “owned” by atom in structure = number of electrons in lone pairs + (1/2) of electrons shared with other atoms = 2L + B Right N appears to own: 3 lone pairs x 2 = 6 1 bonding pair = 1 A = 6+1 = 7 F = 5 (group VA) F.C. = F-A = 5-7 = -2 SCRIPT: Let’s now look at then nitrogen atom on the right. It appears to own… 6 valence electrons in the 3 lone pairs…. HIGHLIGHT 3 lone pairs x 2 = 6 HIGHLIGHT 3 lone pairs in the structure and one electron in the bonding pair… HIGHLIGHT single bond between middle and third N For a total of seven. HIGHLIGHT A=6+1 = 7 Once again, we subtract this from the number of valence electrons that a free, isolated nitrogen would have HIGHLIGHT F=5 And we get a formal charge of –2. HIGHLIGHT F.C.=F-A=5-7=-2 CLICK The formal charge of the nitrogen atom on the right is –2. CLICK CONTINUED ON NEXT SLIDE +1 -2

Structure II. Left N: L=2, B=2 A = 2L + B = 6 F.C.=5-6 = -1 Middle N: F.C. = 5-4 = +1 Right N, just like left N: F.C. = -1 SCRIPT: What about structure 2. The nitrogen on the left has 2 lone pairs and 2 bonding pairs. HIGHLIGHT L=2 and lone pairs with same color (yellow) HIGHLIGHT B=2 and double bond with same color (green) Its formal charge is –1. CLICK The nitrogen atom in the middle has no lone pair and four bonding pairs… HIGHLIGHT double bonds in the structure Its formal charge is +1 And finally, the nitrogen on the right is just like the nitrogen on the left. It has 2 lone pairs and 2 bonding pairs. Its formal charge is –1. PAUSE Going back to the question… CONTINUED ON NEXT SLIDE +1 +1 -1 -2 -1

QUESTION: Which of the Lewis structures shown below for azide ion (N3-) has a nitrogen atom with a formal charge of +2? A. Structure I only, B. Structure II only C. Both structures, C. neither structure SCRIPT: The structure that has a nitrogen atom with a formal charge of –2 is Structure one only. CLICK PAUSE 2 seconds END RECORDING +1 +1 -1 -2 -1

Video ID: 3-7-1 © 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