Understanding the Charge of NO3-: A Closer Look at Molecular Formal Charges
Understanding the Charge of NO3-: A Closer Look at Molecular Formal Charges
A common question in chemistry is why the nitrate ion, NO3-, carries a charge of -1 instead of -9. The answer lies in the distribution of electrons among its atoms and the concept of formal charges.
Composition and Valence Electrons
The nitrate ion consists primarily of one nitrogen atom (N) and three oxygen atoms (O). To break down the charge:
Each nitrogen atom has 5 valence electrons. Each oxygen atom has 6 valence electrons, which contributes to a total of 18 valence electrons for the three oxygens. Thus, the nitrate ion has 5 (from N) 18 (from O) 23 valence electrons in total.Formal Charges
Formal charges are used to determine the charge of individual atoms within a molecule and are important in understanding the stability of molecular structures.
Calculation of Formal Charges
The formal charge (FC) of an atom is calculated using the formula:
$$FC V - N - frac{1}{2}B$$
Where:
V is the number of valence electrons on the free atom. N is the number of non-bonding electrons. B is the number of bonding electrons.Calculation Breakdown for Nitrogen in NO3-
For nitrogen:
Zero non-bonding electrons. Eight bonding electrons: 4 from the double bond and 2 from each single bond.The formal charge on nitrogen is calculated as:
$$FC_{N} 5 - 0 - frac{1}{2} times 8 5 - 4 1$$
Calculation Breakdown for Single-Bonded Oxygen in NO3-
For each single-bonded oxygen:
Six valence electrons (no non-bonding electrons). Three bonding electrons (2 from the bond with nitrogen and 1 from the lone pair).The formal charge on each single-bonded oxygen is calculated as:
$$FC_{O} 6 - 6 - frac{1}{2} times 2 6 - 7 -1$$
Calculation Breakdown for Double-Bonded Oxygen in NO3-
For the double-bonded oxygen:
Six valence electrons, minus two non-bonding electrons. Six bonding electrons (4 from the double bond and 2 from the single bond).The formal charge on the double-bonded oxygen is calculated as:
$$FC_{O} 6 - 4 - frac{1}{2} times 4 6 - 6 0$$
Total Charge Calculation
Adding the formal charges gives:
1 (N) (-1) (O x 2) 0 (O) -1
This results in a net charge of -1 for the nitrate ion.
Conclusion
The nitrate ion has a charge of -1, not -9, because the formal charges of the atoms within it balance out, resulting in a net charge of -1. The misconception about the charge adding up to -9 likely arises from a misunderstanding of how formal charges work in molecular structures.