The dot formula for nitrate ion (NO3–) can be represented using Lewis structures. In NO3–, nitrogen (N) is the central atom surrounded by three oxygen (O) atoms. The overall charge of -1 is accounted for in the electron counting.
To construct the dot formula, we start by counting the valence electrons: nitrogen has 5, and each oxygen has 6, giving us a total of 5 + (3 × 6) + 1 (for the extra negative charge) = 24 valence electrons. We place nitrogen in the center and connect it to the three oxygen atoms with single bonds. Each of the outer oxygen atoms is assigned 6 electrons to fill their octets, but since nitrogen can expand its octet, one of the oxygen atoms can form a double bond with nitrogen, making the distribution of electrons optimal.
The most stable structure will have one double bond with one of the oxygen atoms and single bonds with the other two, and in resonance, the double bond can be with any of the three oxygen atoms. The dot formula looks like this:
O
\
N = O
/
O–
The molecular shape of the nitrate ion is trigonal planar, as it has three regions of electron density (the three N-O bonds) around the nitrogen atom, which results in bond angles of approximately 120 degrees.
Regarding polarity, the molecule is polar. Although the nitrogen-oxygen bonds may have differing electronegativities and cause dipole moments, the symmetrical arrangement of the electron domains means that the dipole moments of the N-O bonds cancel each other out in terms of the overall molecular shape. However, because one of the bonds is a double bond and the other two are single bonds, there is still an overall dipole, resulting in a net dipole moment.