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Language of Chemistry - Writing Chemical Formulae

Grade 7ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A symbol is the shorthand representation of an element. For example, HH represents Hydrogen and FeFe represents Iron (Ferrum).

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Valency is defined as the combining capacity of an atom of an element or a radical. It is the number of electrons lost, gained, or shared by an atom to achieve a stable electronic configuration.

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Radicals are groups of atoms of different elements that behave as a single unit and carry a charge. Common examples include Ammonium (NH4+NH_4^+), Hydroxide (OH−OH^-), and Sulphate (SO42−SO_4^{2-}).

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The Criss-cross method is used to write chemical formulae. This involves writing the symbols of the elements/radicals with their valencies, and then swapping the valency numbers to use as subscripts for the opposite symbol.

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Variable Valency: Some elements exhibit more than one valency. For instance, Iron shows valency 22 in Ferrous compounds (Fe2+Fe^{2+}) and valency 33 in Ferric compounds (Fe3+Fe^{3+}).

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If the subscripts in a formula have a common factor, they are divided by that factor to get the simplest whole-number ratio. For example, Calcium Oxide is written as CaOCaO instead of Ca2O2Ca_2O_2.

📐Formulae

Magnesium Chloride→Mg2++Cl1−→MgCl2\text{Magnesium Chloride} \rightarrow Mg^{2+} + Cl^{1-} \rightarrow MgCl_2

Aluminium Oxide→Al3++O2−→Al2O3\text{Aluminium Oxide} \rightarrow Al^{3+} + O^{2-} \rightarrow Al_2O_3

Sodium Carbonate→Na1++CO32−→Na2CO3\text{Sodium Carbonate} \rightarrow Na^{1+} + CO_3^{2-} \rightarrow Na_2CO_3

Calcium Hydroxide→Ca2++OH1−→Ca(OH)2\text{Calcium Hydroxide} \rightarrow Ca^{2+} + OH^{1-} \rightarrow Ca(OH)_2

Ferric Sulphate→Fe3++SO42−→Fe2(SO4)3\text{Ferric Sulphate} \rightarrow Fe^{3+} + SO_4^{2-} \rightarrow Fe_2(SO_4)_3

💡Examples

Problem 1:

Write the chemical formula for Calcium Nitrate.

Solution:

Ca(NO3)2Ca(NO_3)_2

Explanation:

Calcium (CaCa) has a valency of 22 and the Nitrate radical (NO3NO_3) has a valency of 11. By swapping these valencies, 11 goes to CaCa and 22 goes to NO3NO_3. Since NO3NO_3 is a polyatomic radical, it is placed in brackets, resulting in Ca(NO3)2Ca(NO_3)_2.

Problem 2:

Determine the formula for Aluminium Phosphate.

Solution:

AlPO4AlPO_4

Explanation:

Aluminium (AlAl) has a valency of 33 and Phosphate (PO4PO_4) has a valency of 33. Swapping them gives Al3(PO4)3Al_3(PO_4)_3. Since both subscripts are 33, they are divided by the common factor to give the simplest ratio: AlPO4AlPO_4.

Problem 3:

Find the formula for Potassium Sulphide.

Solution:

K2SK_2S

Explanation:

Potassium (KK) is a monovalent metal (valency 11) and Sulphur (SS) is a divalent non-metal (valency 22). Criss-crossing the valencies gives the formula K2SK_2S.