Formulae of Ions

Learn all of the correct formula for the main Cations and Anions (1+, 2+, 3+, 1- and 2-). Useful for studying ionic equations and for writing formula in general.

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Formulae of Ions [KEYWORDS]

ION: An atom or group of atoms in which the total number of elctrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. When an atom loses or gains an electron it becomes an ion.


CATION: A positively charged ion. Most cations are metal ions, e.g. Na+. 


ANION: A negatively charged ion. Anions usually contain a full outer orbital of electrons. Non-metals form anions but complex ions can also be anions.

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+ The Common Cations +

      1+                    2+                         3+

H+         Hydrogen               Mg2     Magnesium           Al3+        Aluminium    

Li+        Lithium                   Fe2      Iron (ll)                   Fe3     Iron (lll)    

Na+       Sodium                  Ba2      Barium     

K+         Potassium             Cu2      Copper (ll)    

Cu+       Copper (l)             Ca2+        Calcium   

Ag+       Silver                     Pb2+        Lead

NH4+    Ammonium              Zn2+        Zinc

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- The Common Anions -

            1-                               2-    

OH-           Hydroxide                                O2-            Oxide           

HCO3-       Hydrogencarbonate                CO32-        Carbonate              

Cl-             Chlorine                                 SO42-         Sulphate

Br            Bromine                                  S2-             Sulphide 

I-               Iodide           

NO3-         Nitrate      

                          [Notable:  HNO3-    Nitric Acid]             

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Working out the Formula

Working out the formula for common compounds is easy, It's all about balancing!

If we take a compound such as Iron(ll) nitrate;

  • We know Iron (ll) ion has a charge of 2+ and nitrate ion has a charge of 1-.
  • So to balance this up with have to include an additional nitrate ion to match Iron(ll)'s charge of 2. 
  • This means the compound is written as Fe (NO3)2 the reason we include brackets is that we need 2 nitrates and if we wrote the compound without brackets it would look like 32 and people might get mixed up.(NB: The numbers should be written in subscript)

 Lets do another, Aluminium hydrogencarbonate;

  • Aluminium has a charge of 3 + and hydrogencarbonate has a charge of 1-.
  • So we need 3 hydrogencarbonate ions to balance this up.
  • The compound is written as Al (HCO3)3         

Can you do these? barium hydroxide, lithium sulphate, calcium nitrate

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