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SUBJECT-CHEMISTRY

CHAPTER-BASIC CONCEPTS OF CHEMISTRY

SUBTOPIC-DALTONS ATOMIC THEORY,ATOMS AND MOLECULES,ATOMIC
AND MOLECULAR MASS.

KEY NOTES:

 DALTON’S ATOMIC THEORY
Postulates:
 Matter consists of indivisible atoms.
 All the atoms of a given element have identical properties including identical mass.
Atoms of different elements differ in mass.
 Compounds are formed when atoms of different elements combine in a fixed ratio.
 Chemical reactions involve reorganization of atoms. These are neither created
nordestroyed in a chemical reaction.
 Dalton’s theory could explain the laws of chemical combination.
 Limitations of Dalton atomic theory

 It failed to explain law of gaseous combining volumes.
 It failed to explain why atoms of different elements have different masses,
sizes and valencies.
 It makes no distinction between the ultimate particles of an element or a
compound.
 An atom is the smallest particle of an element which can take part in a chemical
reaction. It may or may not be capable of independent existence.
 A molecule is the smallest particle of an element or a compound which is capable of

independent existence.
 Types of molecules
 Homoatomic-contains same types of atoms
Monoatomic-He, Ne, Ar etc.
Diatomic-H 2,O 2,N 2 etc.
Triatomic-O 3
 Heteroatomic-contain different types of atoms

Diatomic-CO, NO
Triaomic-CO 2, NO 2, H 2O etc.
Polyatomic-H 2SO 4, HNO 3, NH 4OH etc.
 Atomic mass-The atomic mass of an element is the number of times an atom of that
element is heavier than an atom of carbon taken as 12.
 One atomic mass unit is defined as a mass exactly equal to onetwelfththe mass of one carbon -
12 atom.