### 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.

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 $$2$$.

$2H+{\mathit{SO}}_{4}^{2-}\to {H}_{2}{\mathit{SO}}_{4}$

One chlorine atom couple with one ammonium ion  to form ammonium chloride.

${\mathit{NH}}_{4}^{+}+\mathit{Cl}\to {\mathit{NH}}_{4}\mathit{Cl}$

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 ${\mathit{SO}}_{4}^{2-}$ $$2$$ $$HNO_3$$ Nitrate ${\mathit{NO}}_{3}^{-}$ $$1$$ $$H_2CO_3$$ Carbonate ${\mathit{CO}}_{3}^{2-}$ $$2$$ $$H_3PO_4$$ Phosphate ${\mathit{PO}}_{4}^{3-}$ $$3$$ $$H_2O$$ Oxide ${O}^{2-}$ $$2$$ $$H_2S$$ Sulphide ${S}^{2-}$ $$2$$ NaOH Hydroxide $$OH^-$$ $$1$$ $$LaBO_3$$ Borate ${\mathit{BO}}_{3}^{3-}$ $$3$$

Some cations' valencies:

 Compound Name of the anion Formula Valency NaCl Sodium $$Na^+$$ $$1$$ KCl Potassium $$K^+$$ $$1$$ $$NH_4Cl$$ Ammonium ${\mathit{NH}}_{4}^{+}$ $$1$$ $$MgCl_2$$ Magesium ${\mathit{Mg}}^{2+}$ $$2$$ $$CaCl_2$$ Calcium ${\mathit{Ca}}^{2+}$ $$2$$ $$AlCl_3$$ Aluminium ${\mathit{Al}}^{3+}$ $$3$$ $$FeCl_3$$ Iron ${\mathit{Fe}}^{3+}$ $$3$$