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SAI International School
Class-IX

Subject- Chemistry
Topic-Atoms and Molecules
Topic-Writing chemical formulae of the compounds

LESSON NOTE

Recaptulation:
Ions
The charged particles (atoms) are called ions, they charge or negative charge on it:
-
Negatively charged ion is called anion (Cl )
+
Positively charge ion is called cation (Na ).
Polyatomic ions: These are the ions which contain atoms of more than one element
which carry a single charge and behave as a single unit in chemical combinations.
2-
+
Example: NH 4 ,SO 4
• Valency
The combining capacity of an element is known as its valency:
Writing formula of compounds:
Compounds
• When two or more elements chemically combine in a fixed ratio by mass, the
obtained product is known as a compound.
• Compounds are substances consisting of two or more different types of elements
in a fixed ratio of its atoms.
RULE I: Cross multiply the valencies of the elements to form the formula of the
respective compound.

In the above formulas Rule I is followed i.e. cross multiply the valencies of the elements
to form the formula of the respective compound.

RULE II: If a compound consists of both metallic as well non-metallic elements then
name or symbol of the metal is considered first.

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