Simple Procedure for writing Lewis Structures – Lewis Structures for NO2+ and HCN | Chemistry Net

Simple Procedure for writing Lewis Structures – Lewis Structures for NO2+ and HCN

Simple Procedure for writing Lewis Structures - Lewis Structures for NO2+ and HCN

Lewis Electron Dot Structures of Hydrogen Cyanide (HCN) and Nitronium Ion (NO2+)

A simple procedure for drawing Lewis dot structures was given in a previous post entitled Lewis Structures and the Octet Rule”.

Several worked examples relevant to this procedure were given in previous posts please see the index page Lewis Structures & the Octet Rule - Theory & Examples . Let us examine the case of nitronium ion NO2+and hydrogen cyanide HCN.

Let us consider the case of NO2+. The Lewis dot structures of NO2+ with formal charges are as follows:

Step 1: The central atom will be the N atom since it is the less electronegative.  Connect the N with the O atoms with single bonds:


Step 2: Calculate the # of electrons in p bonds (pi bonds, multiple bonds) using formula (1) in the article entitled Lewis Structures and the Octet Rule. 

Where n in this case is 3 since NO2+ consists of three atoms. Where V = (6 + 5 + 6) - 1 = 16   Therefore, P = 6n + 2 - V = 6 * 3 + 2 - 16 = 4

Therefore,there are 4 electrons (pi electrons) in NO2+   and 2 double bonds must be added to the structure of Step 1 or 1 triple bond.

 

Step 3 & 4: Two double bonds between N and O are added to the structure in step 1. Alternatively, 1 triple bond is added between N and O. Unshared electron pairs are added so that there is an octet of electrons around each atom. All the equivalent resonance structures of NO2+ are drawn by delocalizing electron pairs. Therefore, the Lewis structures for NO2+ are as follows:

Figure 1: Lewis structures for the NO2+. The first Resonance form is of lower energy than the remaining two because of less charge separation. It will therefore make the largest contribution to the ground state.

 

Let us consider the case of  hydrogen cyanide, HCN and draw the Lewis structures of HCN molecule:

Step 1: The central atom will be the C atom since it is the less electronegative (H is a terminal atom so it cannot be a central atom). Connect the atoms with single bonds.

 


Step 2: Calculate the # of electrons in p bonds (pi bonds, multiple bonds) using formula (1) in the article entitled "Lewis Structures and the Octet Rule". 

Where n in this case is 2 since HCN consists of three atoms but one of them is a H atom. Where V = (1 + 4 + 5) = 10   Therefore, P = 6n + 2 - V = 6 * 2 + 2 - 10 = 4      Thus, there are 4 pi electrons (pi electrons) in HCN and therefore 2 double bonds must be added to the structure of Step 1 or 1 triple bond.

Step 3 & 4: Since only two electrons can be accommodated by a H atom double bonds cannot be added to the structure of Step 1. Alternatively, 1 triple bond is possible between C and N atoms. Unshared electron pair (lone pair) is added so that there is an octet of electrons around each atom. Therefore, the Lewis structures of HCN molecule are as follows:

Figure 2: Lewis structures for HCN
 


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