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Lewis Structures: Nitrate Anion

November 30, 2012

Video by Janet Gray Coonce, MS

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How does one draw the Lewis dot structure for the nitrate anion?  The formula of the nitrate anion is NO3

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The first step is to identify the central atom, in this case nitrogen, and write the symbol for nitrogen, N.  The 3 oxygen atoms, symbol O, are distributed around the central atom, N.  Now look at the periodic table to determine the number of valence electrons.  Nitrogen is in Group VA and has 5 valence electronsOxygen is in Group 6 and has 6 valence electronsOxygen is more electronegative since it has more valence electrons and is to the right of nitrogen in the periodic table.  In this illustration the valence electrons are distributed around each of the atoms.  The octet rule predicts that the electrons will want to form 4 pairs around each atom.  Notice that oxygen has 2 electrons available to form a pair and nitrogen has 3 lone electrons.

Since this is an anion and has a negative charge, an extra electron must be added to one of the more electronegative oxygen atoms.  This extra electron is indicated at the arrow in the drawing above.  The minus sign indicates that this electron accounts for the negative charge of the anion.  This electron was donated by the associated cation which has a positive charge.

Now it’s time to “connect the dots.”

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Our working diagram is on the left.  In order to satisfy the octet rule on each atom, nitrogen formed a double bond with one of the oxygens, it donated an electron to another oxygen and the 3rd oxygen gained an electron from its associated cation.  The octet rule is fulfilled for each atom and the overall net charge on the molecule is negative 1.

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There are 3 possible Lewis structures for the nitrate anion.  These are referred to as resonant structures and reflect the dynamic relationship of the electron sharing in the overall molecule.

Transcription by James C. Gray, MD FACOG

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