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SAI International School
                                                         Class-IX

                                                 Subject- Chemistry
                                            Topic-Atoms and Molecules
                                SubTopic-Atomic and Relative atomic masses

                                                    LESSON NOTE
               Recapitulation:


               Atoms:  These  are  the  smallest  particle  of  matter  which  may  or  may  not  have  an
                    independent existence and take part in chemical reactions.

               Molecules: These are the smallest particle of matter which can exist independently.

                   Size of an atom:  The atoms are extremely small particles and their size is
                   measured in terms of atomic radius. Atomic radius is measured in nanometres.





                         Atomic radii of hydrogen atom = 1 × 10 –10  m.
                   Atomic mass: Since the atoms are extremely small particles their masses are
                   extremely small numbers.
                                                                                  th
                     Therefore, the masses of atoms are compared with 1/12  of the mass of one
                       atom of C-12 which is taken as a standard and it is known as Relative Atomic
                       Mass. The relative atomic masses of all elements have been established with
                       reverence to an atom of carbon-12. It is measured in unit called amu  (atomic
                       mass unit) which is also known as unified mass(u). 1/12 of the mass of one atom
                       of carbon is termed as relative atomic mass. It does not possess a unit.
                     Mathematically,
                       Mass of one atom of an element = atomic mass X (1/12th) of the mass of one
                       atom of carbon.
                       Or
                       Atomic mass = mass of one atom of an element / (1/12th) of the mass of one
                       atom of carbon.
                       Atomic masses of some common elements:

               ELEMENT                                        ATOMIC MASS (in amu or u)


               Hydrogen                                       1

               Carbon                                         12


               Nitrogen                                       14






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