Valency & Chemical Formulas | ICSE Class 9 Chemistry

Valency and Chemical Formulas

Complete ICSE Class 9 Guide | chemca.in

1. Electronic Concept of Valency

According to the modern electronic theory, valency is determined by the number of electrons in the outermost shell (valence shell).

Octet Rule: Atoms tend to lose, gain, or share electrons to achieve a stable configuration of 8 electrons in their valence shell (or 2 for Helium).

Electropositive Valency: Metals lose electrons to become positive ions (Cations). E.g., $Na \rightarrow Na^+ + e^-$.
Electronegative Valency: Non-metals gain electrons to become negative ions (Anions). E.g., $Cl + e^- \rightarrow Cl^-$.

2. Variable Valency

Certain elements exhibit more than one valency due to the involvement of electrons from the penultimate shell (shell before the outermost).

Element	Lower Valency (-ous)	Higher Valency (-ic)
Iron ($Fe$)	$+2$ (Ferrous)	$+3$ (Ferric)
Copper ($Cu$)	$+1$ (Cuprous)	$+2$ (Cupric)
Mercury ($Hg$)	$+1$ (Mercurous)	$+2$ (Mercuric)
Lead ($Pb$)	$+2$ (Plumbous)	$+4$ (Plumbic)
Tin ($Sn$)	$+2$ (Stannous)	$+4$ (Stannic)

3. Comprehensive List of Radicals

Positive Radicals (Cations)

Monovalent ($+1$)	Divalent ($+2$)	Trivalent ($+3$)
Ammonium ($NH_4^+$)	Calcium ($Ca^{2+}$)	Aluminum ($Al^{3+}$)
Potassium ($K^+$)	Magnesium ($Mg^{2+}$)	Ferric ($Fe^{3+}$)
Sodium ($Na^+$)	Zinc ($Zn^{2+}$)	Chromium ($Cr^{3+}$)

Negative Radicals (Anions)

Monovalent ($-1$)	Divalent ($-2$)	Trivalent ($-3$)
Chloride ($Cl^-$)	Sulphate ($SO_4^{2-}$)	Phosphate ($PO_4^{3-}$)
Nitrate ($NO_3^-$)	Carbonate ($CO_3^{2-}$)	Nitride ($N^{3-}$)
Hydroxide ($OH^-$)	Oxide ($O^{2-}$)	Phosphide ($P^{3-}$)
Bicarbonate ($HCO_3^-$)	Dichromate ($Cr_2O_7^{2-}$)	-

4. Step-by-Step Formula Writing

To write the formula for Magnesium Phosphate:

Identify Symbols: Magnesium is $Mg$, Phosphate is $PO_4$.
Assign Valencies: $Mg$ is $2+$, $PO_4$ is $3-$.
Criss-Cross: The 3 goes to $Mg$ and 2 goes to $PO_4$.
Result: $Mg_3(PO_4)_2$.