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Atomic Structure - Radicals

Grade 7ICSE

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

🔑Concepts

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A radical is an atom or a group of atoms of different elements that behaves as a single unit with a positive or negative charge. These are also known as polyatomic ions.

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Electropositive (Basic) Radicals: These are radicals that carry a positive charge. The most common example in Grade 7 is the Ammonium radical, denoted as NH4+NH_4^+.

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Electronegative (Acidic) Radicals: These are radicals that carry a negative charge. Examples include the Hydroxide radical (OH−OH^-), Nitrate radical (NO3−NO_3^-), and Carbonate radical (CO32−CO_3^{2-}).

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The valency of a radical is equal to the number of charges it carries. For example, the Phosphate radical PO43−PO_4^{3-} has a valency of 33.

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When writing chemical formulae involving radicals, if more than one unit of the radical is required, the radical is placed inside a bracket and the number is written as a subscript, such as in Mg(OH)2Mg(OH)_2.

📐Formulae

NH4+ (Ammonium)NH_4^+ \text{ (Ammonium)}

OH− (Hydroxide)OH^- \text{ (Hydroxide)}

NO3− (Nitrate)NO_3^- \text{ (Nitrate)}

CO32− (Carbonate)CO_3^{2-} \text{ (Carbonate)}

SO42− (Sulphate)SO_4^{2-} \text{ (Sulphate)}

PO43− (Phosphate)PO_4^{3-} \text{ (Phosphate)}

HCO3− (Bicarbonate or Hydrogen Carbonate)HCO_3^- \text{ (Bicarbonate or Hydrogen Carbonate)}

💡Examples

Problem 1:

Identify the radical and its valency in the compound Calcium Carbonate (CaCO3CaCO_3).

Solution:

Radical: Carbonate (CO32−CO_3^{2-}); Valency: 22.

Explanation:

In CaCO3CaCO_3, the calcium atom is bonded to the carbonate group. The carbonate group CO3CO_3 acts as a single unit with a charge of −2-2, hence its valency is 22.

Problem 2:

Determine the chemical formula for Ammonium Sulphate using the criss-cross method.

Solution:

(NH4)2SO4(NH_4)_2SO_4

Explanation:

The Ammonium radical is NH4+NH_4^+ (valency 11) and the Sulphate radical is SO42−SO_4^{2-} (valency 22). By crossing the valencies, the 22 from sulphate goes to the ammonium radical and the 11 from ammonium goes to the sulphate radical, resulting in (NH4)2SO4(NH_4)_2SO_4.

Problem 3:

What is the difference between a simple ion and a radical like SO42−SO_4^{2-}?

Solution:

A simple ion consists of a single charged atom (e.g., Cl−Cl^-), while a radical (polyatomic ion) consists of a group of atoms of different elements acting as one unit (e.g., SO42−SO_4^{2-}).

Explanation:

The Sulphate radical contains one atom of Sulphur (SS) and four atoms of Oxygen (OO), but they collectively carry a charge of −2-2 and stay together during most chemical reactions.