GOA Class 9 Chemistry Atoms and Molecules
Basic Units of Matter
Atoms of most of the elements are reactive and do not exist in Free State.
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 conservation of mass states that matter can neither be created nor destroyed.
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
Scientists use carbon-12 scale (atomic mass unit) to compare masses of different atoms and molecules of elements and compounds.
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.Mole Concept
Mole is the counting unit for atoms, molecules, ions.
One mole of any substance contains Avogadro's number of particles.
Molar mass is numerically equal to gram atomic mass/gram molecular or gram formula mass.