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Molecule - Concept of Ion

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Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl– ). Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion.

(i) Cation: It is positively charged ion and is formed by the loss of electron from an atom e.g. H+, Na+, Ca2+, Al3+, NH4+ etc.
(ii) Anion: It is negatively charged ion and is formed by the gain of electrons by an atom, e.g. Cl–, O2-, C–, F–, CO32- PO43- etc.

Some ionic compounds :

Ionic Compound

Constituting Elements

Ratio by Mass

Calcium oxide

Calcium and oxygen

5:2

Magnesium sulphide

Magnesium and sulphur

3:4

Sodium chloride

Sodium and chlorine

23:35.5

S
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