Polyatomic ions with Group-15 central atoms
All of the ions in this tool contain a central group-15 element (N or P):
\(\mathrm{NO_2^-}\), \(\mathrm{NO_3^-}\), \(\mathrm{NH_4^+}\),
\(\mathrm{PO_4^{3-}}\), \(\mathrm{HPO_4^{2-}}\), and \(\mathrm{H_2PO_4^-}\).
The nitrogen or phosphorus atom is surrounded by several ligands (O or H),
and the overall charge is spread out by resonance or by the electronegativity
of oxygen.
For any polyatomic ion you can first count total valence electrons,
then place bonds and lone pairs, and finally check formal charges:
\[
FC = N_\text{valence} - \bigl(N_\text{nonbonding}
+ \tfrac{1}{2}N_\text{bonding}\bigr)
\]
Good Lewis structures minimize formal charges and keep negative charge on
the more electronegative atoms (usually oxygen).
Nitrite and nitrate: resonance and bond order
For \(\mathrm{NO_2^-}\) (nitrite) and \(\mathrm{NO_3^-}\) (nitrate),
nitrogen is the central atom and oxygen atoms are equivalent by resonance:
-
\(\mathrm{NO_2^-}\): total valence electrons
\[
N_e = 5(\mathrm{N}) + 2 \cdot 6(\mathrm{O}) + 1
= 18
\]
The Lewis structure has one \(\mathrm{N=O}\), one \(\mathrm{N-O^-}\),
and a lone pair on N. Two resonance forms give an average
N–O bond order of about \(1.5\).
-
\(\mathrm{NO_3^-}\): total valence electrons
\[
N_e = 5(\mathrm{N}) + 3 \cdot 6(\mathrm{O}) + 1
= 24
\]
The Lewis structure has three equivalent resonance forms with one
\(\mathrm{N=O}\) and two \(\mathrm{N-O^-}\) in each form.
All three N–O bonds are equivalent with average bond order
\(\approx 1.33\), and the ion is trigonal planar.
In both ions the central nitrogen carries a positive formal charge in the
common resonance description, while the negative charge is mainly on the
oxygens.
Ammonium: a tetrahedral cation
\(\mathrm{NH_4^+}\) (ammonium) is formed when \(\mathrm{NH_3}\) accepts a
proton:
\[
\mathrm{NH_3 + H^+ \rightarrow NH_4^+}
\]
Nitrogen has four equivalent \(\mathrm{N-H}\) bonds and no lone pairs,
giving a tetrahedral geometry (\(\mathrm{AX_4}\)). The central nitrogen
has a formal charge of \(+1\), which matches the overall charge of the
ion.
Phosphate family: tetrahedral oxyanions of phosphorus
\(\mathrm{PO_4^{3-}}\), \(\mathrm{HPO_4^{2-}}\), and
\(\mathrm{H_2PO_4^-}\) are all tetrahedral around phosphorus. They are
related by stepwise deprotonation of phosphoric acid:
\[
\mathrm{H_3PO_4 \rightleftharpoons H_2PO_4^- \rightleftharpoons
HPO_4^{2-} \rightleftharpoons PO_4^{3-}}
\]
-
\(\mathrm{PO_4^{3-}}\): four P–O bonds; resonance delocalizes the
three negative charges mainly over the oxygens.
-
\(\mathrm{HPO_4^{2-}}\): one \(\mathrm{P-O-H}\) group and three other
P–O connections; two negative charges remain on oxygen atoms.
-
\(\mathrm{H_2PO_4^-}\): two \(\mathrm{P-O-H}\) groups; one negative
charge is delocalized over the remaining P–O framework.
In all three ions, the electron-pair geometry at phosphorus is tetrahedral
(\(\mathrm{AX_4}\)), and resonance helps spread the negative charge over
several oxygen atoms, stabilizing the anions.
