diff --git a/src/pymatgen/core/ion.py b/src/pymatgen/core/ion.py
index 16c479dcf29..03055da8095 100644
--- a/src/pymatgen/core/ion.py
+++ b/src/pymatgen/core/ion.py
@@ -7,6 +7,7 @@
 from typing import TYPE_CHECKING
 
 from monty.json import MSONable
+
 from pymatgen.core.composition import Composition, reduce_formula
 from pymatgen.util.string import Stringify, charge_string, formula_double_format
 
@@ -177,7 +178,6 @@ def get_reduced_formula_and_factor(
         # they should be displayed as "OH" rather than "HO".
         if self.composition.get("H") == self.composition.get("O"):
             formula = formula.replace("HO", "OH")
-
         if nH2O > 0:
             formula += f".{nH2O}H2O"
 
@@ -190,12 +190,12 @@ def get_reduced_formula_and_factor(
             formula = "CH3COOH"
             factor /= 2
         # phosphoric acid system
-        elif formula == "PH3O4(aq)":
-            formula = "H3PO4(aq)"
-        elif formula == "PHO4[-2]":
-            formula = "HPO4[-2]"
-        elif formula == "P(HO2)2[-1]":
-            formula = "H2PO4[-1]"
+        elif formula == "PH3O4":
+            formula = "H3PO4"
+        elif formula == "PHO4":
+            formula = "HPO4"
+        elif formula == "P(HO2)2":
+            formula = "H2PO4"
         # acetate
         elif formula == "H3(CO)2":
             formula = "CH3COO"
@@ -215,30 +215,27 @@ def get_reduced_formula_and_factor(
             formula = "O3"
             factor /= 3
         # ammonia
-        elif formula == "H4N[+1]":
-            formula = "NH4[+1]"
-        elif formula == "H3N(aq)":
-            formula = "NH3(aq)"
+        elif formula == "H4N":
+            formula = "NH4"
+        elif formula == "H3N":
+            formula = "NH3"
         # methane
-        elif formula == "H4C(aq)":
-            formula = "CH4(aq)"
+        elif formula == "H4C":
+            formula = "CH4"
         # thiocyanate
-        elif formula == "CSN[-1]":
-            formula = "SCN[-1]"
+        elif formula == "CSN":
+            formula = "SCN"
         # triiodide, nitride, an phosphide
-        elif formula == "I[-0.33333333]":
-            formula = "I3[-1]"
-        elif formula == "N[-0.33333333]":
-            formula = "N3[-1]"
-        elif formula == "P[-0.33333333]":
-            formula = "P3[-1]"
+        elif formula in ["I", "N", "P"] and self.charge == -1:
+            formula += "3"
+            factor /= 3
         # formate # codespell:ignore
-        elif formula == "HCOO[-1]":
-            formula = "HCO2[-1]"
+        elif formula == "HCOO":
+            formula = "HCO2"
         # oxalate
-        elif formula == "CO2[-1]":
-            formula = "C2O4[-2]"
-
+        elif formula == "CO2":
+            formula = "C2O4"
+            factor *= 2
         # diatomic gases
         elif formula in {"O", "N", "F", "Cl", "H"} and factor % 2 == 0:
             formula += "2"
diff --git a/tests/core/test_ion.py b/tests/core/test_ion.py
index e60a01c4fc0..a3b4003a7fb 100644
--- a/tests/core/test_ion.py
+++ b/tests/core/test_ion.py
@@ -4,6 +4,7 @@
 from unittest import TestCase
 
 import pytest
+
 from pymatgen.core import Composition, Element
 from pymatgen.core.ion import Ion
 
@@ -46,6 +47,7 @@ def test_charge_from_formula(self):
         assert Ion.from_formula("SO42-").charge == -1
         assert Ion.from_formula("SO4--").charge == -2
         assert Ion.from_formula("SO4[--]").charge == -2
+        assert Ion.from_formula("N3-").charge == -1
 
         assert Ion.from_formula("Na[+-+]").charge == 1
 
@@ -68,17 +70,16 @@ def test_special_formulas(self):
             ("CH3OH", "CH3OH(aq)"),
             ("H4CO", "CH3OH(aq)"),
             ("CH4", "CH4(aq)"),
-            ("NH4", "NH4[+1]"),
+            ("NH4+", "NH4[+1]"),
             ("NH3", "NH3(aq)"),
             ("N3-", "N3[-1]"),
-            ("HCOO-", "HCOO[-1]"),
+            ("HCOO-", "HCO2[-1]"),
             ("C2H6O", "C2H5OH(aq)"),
             ("C3H8O", "C3H7OH(aq)"),
             ("C4H10O", "C4H9OH(aq)"),
             ("Fe(OH)4+", "Fe(OH)4[+1]"),
             ("Zr(OH)4", "Zr(OH)4(aq)"),
         ]
-
         for tup in special_formulas:
             assert Ion.from_formula(tup[0]).reduced_formula == tup[1]