A simple procedure for writing Lewis structures is given in a previous article entitled “Lewis Structures and the Octet Rule”.
Several worked examples relevant to this procedure were given in previous posts please see the Sitemap - Table of Contents (Lewis Electron Dot Structures).
Another example for writing Lewis structures following the above procedure is given below:
Let us consider the case of nitrogen dioxide
NO2. NO2 is a
red-brown gas with a pungent and irritating odour. Let us draw the Lewis dot structures of NO2 molecule (resonance structures of nitrogen
dioxide):
It is one of the most prevalent oxides of nitrogen, NO is the other one. Both are toxic gases with NO2 being a highly reactive oxidant and corrosive. NO2 forms quickly from emissions from cars, power plants and off-road equipment. NO2 can also come from appliances inside homes that burn fuels such as gas, kerosene and wood. In general, the NOx gases are believed to worsen asthmatic conditions and bronchitis, react with the oxygen in the air to produce ground-level ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result is acid rain which can damage trees and crops, entire forest ecosystems, lakes.
Step 1: The central atom will be the N atom since it is the less electronegative. Connect the atoms with single bonds:
Step 2: Calculate the # of electrons in π 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 = (5 + 6 + 6 ) = 17
Therefore, P = 6n + 2 – V = 6 * 3 + 2 – 17 = 3 Thus, there are 2 π electrons in NO2 and a lone electron.
Therefore, there is a double bond and a lone electron that is not involved in multiple bonding and 1 double bond must be added to the structure of Step 1 and a lone electron.
Step 3 & 4: One double bond must therefore be placed to the structure in Step1. Therefore, the Lewis dot structures of nitrogen dioxide NO2 are as follows:
As it can be seen above the Lewis dot structures of nitrogen dioxide do not obey the octet rule. The N atom has 7 electrons around it (violation of the octet rule).
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