Lewis Structure (6): Nitrite ion

Chemistry Video by Janet Gray Coonce, MS

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Review these notes after watching the video:

This video demonstrates how to draw Lewis dot structures for polyatomic cations and polyatomic anions.  A polyatomic ion is a charged molecule composed of more than one atom held together by covalent bonds.  Anions are negatively charged and will migrate toward the anode (positive electrode) in electrolysis. Cations are positively charged and will migrate toward the cathode (negative electrode) in electrolysis.

NO₂^– is the formula of the nitrite ion.  It is negatively charged and can therefore more specifically be referred to as the nitrite anion.  This is not to be confused with NO₃^– which is the nitrate anion.

Nitrogen must be in the middle of our Lewis dot structure with an oxygen on either side.  We then add dots for the valence electrons. Nitrogen is in group Va so we know there are 5 valence electrons.  Nitrogen has 1 pair of electrons and 3 lone electrons available for bonding.  Oxygen is in group VIa so there are 6 valence electrons.  Oxygen has 2 pair of electrons and 2 lone electrons available for bonding.  At this stage of drawing the Lewis dot structure, none of the atoms have a complete octet of electrons.  All of them have only 7 and each has a lone pair. 

O = 7             N = 7               O = 7  valence shell electrons at this stage of our diagram.

The nitrite anion NO₂^– gets its negative charge from an electron which is participating in an ionic bond with its cation.  How can we show this donated electron in our Lewis structure?  This electron is added the most electro-negative atom.  The most electronegative atoms are on the right of the periodic table.  Oxygen is therefore more electronegative than nitrogen.  The extra electron could go on either one of the oxygen molecules.  In the next drawing the extra electron was added to the oxygen on the right. 

   -e

  O = 7               N = 7          O = 8         valence shell electrons at this stage in our drawing.  An additional electron was added to the oxygen on the right (donated from its cation) which will complete that oxygen’s octet, but it will  give the molecule a negative charge.  Now both oxygen and nitrogen have a lone electron available for covalent bonding.

Here a line is drawn between the lone electrons indicating that they are now sharing another electron in a covalent bond.

This is the same drawing re-drawn for neatness and also to show that the 3 bonds will try to get as far apart from each other as possible so the molecule is not linear but bent in a trigonal configuration. 

It should be obvious that there are two possible configurations, the left and right as illustrated here are mirror images.  There is a double arrow between them and brackets around them.  This movement of the electron between these two configurations is referred to as resonance.  This is the Lewis dot structure notation to designate the two resonant structures for this nitrite anion.

Transcription by James C. Gray MD FACOG