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Chemistry - The Language of Chemistry (Symbols, Valency, Chemical Formulae, Equations)

Grade 9ICSE

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

🔑Concepts

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A symbol is a short-hand notation of an element. It represents one atom of the element, for example, HH for Hydrogen and NaNa for Sodium (from its Latin name Natrium).

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Valency is the combining capacity of an atom of an element or of a radical. It is determined by the number of electrons lost, gained, or shared during chemical combination. For example, Hydrogen has a valency of 11, while Oxygen has a valency of 22.

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Variable Valency occurs when certain elements exhibit more than one combining capacity. For example, Iron (FeFe) can have a valency of 22 (Ferrous, Fe2+Fe^{2+}) or 33 (Ferric, Fe3+Fe^{3+}).

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A radical is an atom or a group of atoms of the same or different elements that behaves as a single unit with a positive or negative charge. Examples include the Ammonium radical (NH4+NH_4^+) and the Sulphate radical (SO42−SO_4^{2-}).

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The Chemical Formula represents the symbolic expression of a molecule of a substance. It indicates the number of atoms of each element present, such as H2OH_2O for water and H2SO4H_2SO_4 for sulphuric acid.

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A Chemical Equation is the shorthand representation of a chemical change using symbols and formulae of the substances involved. For example: 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O.

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The Law of Conservation of Mass states that matter can neither be created nor destroyed. Thus, a chemical equation must be balanced so that the number of atoms of each element is the same on both the reactant and product sides.

📐Formulae

Valency of element=Number of atoms of HydrogenNumber of atoms of the element combining with Hydrogen\text{Valency of element} = \frac{\text{Number of atoms of Hydrogen}}{\text{Number of atoms of the element combining with Hydrogen}}

Ax+By→AyBx (Criss-cross Method)A^x + B^y \rightarrow A_y B_x \text{ (Criss-cross Method)}

2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO

Zn+2HCl→ZnCl2+H2↑Zn + 2HCl \rightarrow ZnCl_2 + H_2 \uparrow

CaCO3+2HCl→CaCl2+H2O+CO2↑CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \uparrow

BaCl2+H2SO4→BaSO4↓+2HClBaCl_2 + H_2SO_4 \rightarrow BaSO_4 \downarrow + 2HCl

💡Examples

Problem 1:

Write the chemical formula for Aluminium Sulphate.

Solution:

Al2(SO4)3Al_2(SO_4)_3

Explanation:

Aluminium (AlAl) has a valency of 3+3+. The Sulphate radical (SO4SO_4) has a valency of 2−2-. Using the criss-cross method, the valency of Aluminium becomes the subscript for Sulphate, and the valency of Sulphate becomes the subscript for Aluminium: Al3+SO42−→Al2(SO4)3Al^{3+} SO_4^{2-} \rightarrow Al_2(SO_4)_3.

Problem 2:

Balance the following chemical equation: Fe+H2O→Fe3O4+H2Fe + H_2O \rightarrow Fe_3O_4 + H_2.

Solution:

3Fe+4H2O→Fe3O4+4H23Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2

Explanation:

To balance the Oxygen atoms, we place a coefficient of 44 before H2OH_2O on the reactant side. This gives us 88 Hydrogen atoms, so we place a 44 before H2H_2 on the product side. Finally, to balance 33 Iron atoms, we place a 33 before FeFe on the reactant side.

Problem 3:

Identify the basic and acidic radicals in the compound Calcium Carbonate (CaCO3CaCO_3).

Solution:

Basic Radical: Ca2+Ca^{2+}; Acidic Radical: CO32−CO_3^{2-}

Explanation:

In an inorganic salt, the electropositive part (usually a metal ion) is the basic radical, and the electronegative part (usually a non-metal or group of atoms) is the acidic radical.