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Atomic Structure - Chemical Formulae

Grade 7ICSE

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

🔑Concepts

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A chemical formula is a symbolic representation of one molecule of an element or a compound. It indicates the types of atoms present and their respective numbers, such as H2OH_2O representing two atoms of Hydrogen and one atom of Oxygen.

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Valency is defined as the combining capacity of an atom or a radical. For example, Hydrogen has a valency of 11, while Oxygen has a valency of 22.

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Variable Valency: Certain elements exhibit more than one valency. For instance, Iron (FeFe) can have a valency of 22 (Ferrous, Fe2+Fe^{2+}) or 33 (Ferric, Fe3+Fe^{3+}).

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Radicals (Polyatomic Ions) are groups of atoms of different elements that behave as a single unit and carry a charge, such as the Ammonium radical (NH4+NH_4^+) or the Phosphate radical (PO43−PO_4^{3-}).

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The Criss-Cross Method is used to write chemical formulae by writing the symbols of the elements/radicals, placing their valencies at the top, and then swapping the valencies to become subscripts.

📐Formulae

AxBy→AyBxA^x B^y \rightarrow A_y B_x

Mg2++Cl1−→MgCl2Mg^{2+} + Cl^{1-} \rightarrow MgCl_2

Al3++O2−→Al2O3Al^{3+} + O^{2-} \rightarrow Al_2O_3

Ca2++CO32−→CaCO3Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3

NH41++SO42−→(NH4)2SO4NH_4^{1+} + SO_4^{2-} \rightarrow (NH_4)_2SO_4

💡Examples

Problem 1:

Write the chemical formula for Magnesium Nitride.

Solution:

Mg3N2Mg_3N_2

Explanation:

The symbol for Magnesium is MgMg with a valency of 22. The symbol for Nitrogen (Nitride) is NN with a valency of 33. By using the criss-cross method, the valency of NN (33) becomes the subscript for MgMg, and the valency of MgMg (22) becomes the subscript for NN, resulting in Mg3N2Mg_3N_2.

Problem 2:

Determine the formula for Calcium Hydroxide.

Solution:

Ca(OH)2Ca(OH)_2

Explanation:

Calcium (CaCa) has a valency of 22. The Hydroxide radical (OHOH) has a valency of 11. When criss-crossing, the valency 22 is applied to the entire OHOH group, necessitating the use of brackets: Ca(OH)2Ca(OH)_2.

Problem 3:

What is the chemical formula for Ferric Oxide?

Solution:

Fe2O3Fe_2O_3

Explanation:

The term 'Ferric' indicates Iron with its higher valency of 33 (Fe3+Fe^{3+}). Oxygen (Oxide) has a valency of 22. Swapping these values gives the formula Fe2O3Fe_2O_3.