CBSE Class 9 Chemistry Atoms and Molecules
Basic Units of Matter
A molecule is the smallest particle of matter which can exist independently.
The molecule of an element contains two or more similar atoms while that of a compound contain two or more different types of atoms chemically combined together.
Laws of Chemical Combination
Law of definite proportions states that a given chemical compound always contains the same proportion of its constituent elements by mass.
Dalton's Atomic Theory states that chemical reactions involve re-organisation of atoms but most of his postulates no longer holds true.
Atomic Mass and Molecular Mass
Atomicity is the number of atoms constituting a molecule. Valency is the combining capacity of elements.
The chemical formula of a compound shows the exact number of constituent atoms.
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