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Theory:

Monovalent, divalent, trivalent ions:
  • During a chemical reaction, an atom can gain or lose more than one electron. If the number of charges over an ion or radical is \(1\), \(2\), \(3\), and \(4\), it is monovalent, divalent, trivalent, and tetravalent, respectively.
  • The valencies of the ions can vary depending on the charges they carry.
Valency of anions (negative radicals) and cations (positive radicals):
 
An anion or cation's valency represents the number of hydrogen atoms or any other monovalent atoms (\(Na\), \(K\), \(Cl\)) that combine with them to give a suitable compound.
Example:
Two hydrogen atoms couple with one sulphate ion to form sulfuric acid.
 
The valency of sulphate ion is \(2\).
 
2H+SO42H2SO4
 
One chlorine atom couple with one ammonium ion  to form ammonium chloride.
 
NH4++ClNH4Cl
 
The valency of ammonium ion is \(1\).
Some anions' valencies:
  
Compound
Name of the
anion
Formula
Valency
HCl
Chloride
\(Cl^-\)
\(1\)
\(H_2SO_4\)
Sulphate
SO42
\(2\)
\(HNO_3\)
Nitrate
NO3
\(1\)
\(H_2CO_3\)
Carbonate
CO32
\(2\)
\(H_3PO_4\)
Phosphate
PO43
\(3\)
\(H_2O\)
Oxide
O2
\(2\)
\(H_2S\)
Sulphide
S2
\(2\)
NaOH
Hydroxide
\(OH^-\)
\(1\)
\(LaBO_3\)
Borate
BO33
\(3\)
 
Some cations' valencies:
 
Compound
Name of the
anion
Formula
Valency
NaCl
Sodium
\(Na^+\)
\(1\)
KCl
Potassium
\(K^+\)
\(1\)
\(NH_4Cl\)
Ammonium
NH4+
\(1\)
\(MgCl_2\)
Magesium
Mg2+
\(2\)
\(CaCl_2\)
Calcium
Ca2+
\(2\)
\(AlCl_3\)
Aluminium
Al3+
\(3\)
\(FeCl_3\)
Iron
Fe3+
\(3\)