15 Important Polyatomic Ions Examples You Need To Know

“Poly” meaning many, which means these are charged ions that contain more than one atoms. They can be a singly charged molecular ion or multi-charged polyatomic ion. A polyatomic ion might have a central atom that is less electronegative than the other atoms attached to it like PO43- or that is more electronegative from the rest of the atoms like NH4+ or it might have entirely similar or same atoms like I3 .

  1. Carbonate, CO32-
  2. Phosphate, PO43-
  3. Chlorate, ClO3 
  4. Nitrate, NO3
  5. Sulphate, SO42-
  6. Bromate, BrO3
  7. Borate, BO33-
  8. Perchlorate, ClO4
  9. Cyanide, CN
  10. Hydroxide, OH
  11. Acetate, CH3COO
  12. Chromate, CrO4
  13. Dichromate, Cr2O7
  14. Azide, N3
  15. Ammonium cation, NH4+  

1. Carbonate, CO32- :

It is an oxoanion of carbon that forms stable carbonate salts with alkali and alkaline earth metals like Na2CO3 , CaCO3 , MgCO3 . It is used for treatment of hard water containing Ca2+ and Mg2+ bicarbonates.

Hybridisation and Shape of CO32- : It is a sp2 hybridized molecule with trigonal planar geometry.

polyatomic ions examples

2. Phosphate, PO43- :

It is an oxoanion of phosphorous. It is related to orthophosphoric acid (H3PO4) as its conjugate base. This ion is highly important for biological functions like nucleic acid synthesis, ATP production.

Hybridisation and Shape of PO43- : It is a sp3 hybridised molecule with tetrahedral geometry.

3. Chlorate, ClO3  :

It is an oxyanion of Chlorine in +5 oxidation state that acts as a strong oxidizing agent, conversion of biomolecules etc.

Hybridisation and Shape of ClO3 : It adopts a trigonal pyramidal structure to avoid repulsion between the electrons residing on Cl and negative charge on O atoms.

4. Nitrate, NO3 :

It is an oxoanion of nitrogen. It can also be viewed as the conjugate anion of Nitric acid. It is widely used in fertilisers due to their high ionic stability.

Hybridisation and Shape of NO3–  : It is a sp2 hybridised molecule with trigonal planar geometry.

5. Sulphate, SO42- :

It is the oxyanion of S, that forms a very important conjugate anion of widely used sulphuric acid. It is also found in soil samples.

Hybridisation and Shape of SO42-  : It is a sp hybridized molecule with tetrahedral geometry.

6. Perchlorate, ClO4 :

It is the conjugate anion of perchloric acid, an oxyanion of Cl. They have medicinal importance.

Hybridisation and Shape of ClO4: It is a sp3 hybridized molecule with tetrahedral geometry.

7. Borate, BO33- :

These are oxoanions of boron and plays a vital role in synthetic chemistry and industrial applications. E.g. Removing stain, batteries. They exists mainly in polymeric forms with shapes varying from trigonal planar to tetrahedral.

Hybridisation and Shape of BO33- :  The simplest borate unit is shown with a trigonal planar structure. 

8. Bromate, BrO3 :

It is the conjugate anoion of bromic acid that is similar to chlorate ion in its shape and hybridization.

Hybridisation and Shape of BrO3 : It adopts a trigonal pyramidal structure to avoid repulsion between the electrons residing on Br and negative charge on O atoms.

9. Cyanide, CN :

It is considered the acme of human poisons. It interferes with ferric heme of the enzyme cytochrome c oxidase thereby causing mitochondrial respiration difficult, most commonly known as cyanide poisoning. Monitoring cyanide levels in water and food samples is very crucial, therefore, it is important to know the structure, bonding connectivity of cyanide ion.

Hybridisation and Shape of CN- : It is a sp hybridized pseudohalogen ion with linear shape.

10. Hydroxide, OH :

These are ions that are released when dissolved in H2O. Basicity magnitude of Bronsted bases is determined by the amount of hydroxide ions released in solution. They do not exist freely but are surrounded by other H2O molecules.

Hybridisation and Shape of OH : Hydroxide ion consists of an oxygen atom   that is covalently bonded to hydrogen atom in a linear fashion with sp hybridization.

11. Acetate, CH3COO  :

It is the most common ion in organic chemistry. The most common is the conjugate base of acetic acid, ammonium acetate etc. They are found in organic matter, biomolecules present in our body.

Hybridisation and Shape of CH3COO : The C of CH3 is sp3 hybridized and the C of COO is sp2 hybridized.

12. Chromate, CrO4 :

Cr is known to exhibit various oxidation states hence enabling them to form different types of oxyanions, one of them is chromate in +6 oxidation state. As a result, they act as an oxidizing reactions in many organic reactions. It also be viewed as an oxyanion of chromium. It is yellow in colour.

Hybridisation and Shape of CrO4: It is a sp3 hybridized molecule with tetrahedral geometry.

13. Dichromate, Cr2O7:

It contains metal-oxygen bridging. It is used for protection of metals from corrosion by chrome plating technique. It is in +6 oxidation state with 4 double bonds and the lone and bond pairs are arranged in such a manner to have minimum repulsion among themselves.

Hybridisation and Shape of Cr2O7 : It can be viewed as two tetdrahedral units of chromate joined together to form dichromate.

14. Azide, N3:

It is used for generation of nitrenes, in manufacturing of fertilisers, diazomethane etc.   

Hybridisation and Shape of N3:  It is a linear molecule with sp2 and sp hybridization.

15. Ammonium cation, NH4+  :

Polyatomic cations are not usually observed unless the case of extra stability arises. Here, the ammonium cation achieves stable and filled electronic configuration, i.e., follows octet rule.

Hybridisation and Shape of NH4+   : It is a sp3 hybridized molecule with tetrahedral geometry.

In other words, these are molecular ions that contains either excess protons or excess electrons in their nucleus or outermost shell. A metal plus a polyatomic ion yields an ionic compound.

It is extremely important to know the total charge on these ions to construct the molecular formula of a compound.  

How are Polyatomic ions formed ?

They are formed when a neutral molecule loses or gain one or more electrons. Most polyatomic ions are acidic in nature that loses a proton to give a polyatomic anion with fewer polyatomic cation as an exception like NH4+ .

Note :

  • Polyatomic cations are rarely formed or found because the atoms that form covalent bonds in these ions lie to the right side of the periodic table which has varying higher electronegativities tending to form anionic species.
  • Hybridisation of a molecule = ( Valence electrons of the central atom + Number of monovalent atoms attached to the central atom + Negative charge on the molecule – Positive charge on the molecule )/2

Conclusion :

In the above article, we have covered 15 most important ions : CO32- , PO43- , ClO3 , CH3COO , NH4+ , ClO4 , BrO3 , NO3, BO33- , OH , N3, Cr2O7 , CrO4 CN , SO42- .

Nandita Biswas

Hello. I am doing my Masters in Chemistry with specialisation in organic and physical chemistry. Also, I have done two projects in chemistry- One dealing with colorimetric estimation and determination of ions in solutions. Other in Solvatochromism study of fluorophores and their uses in the field of chemistry alongside their stacking properties on emission. I have also specialised in Resin chemicals.

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