ATOMS, MOLECULES AND STOICHIOMETRY

 

ATOMS, MOLECULES AND STOICHIOMETRY

GCE Advance subsidiary Level CHEMISTRY 9701

(AS LEVEL CHEMISTRY 9701)

 

RELATIVE ATOMIC MASS:

Atomic mass of an isotopes of an element compound with 1/12^th part of an isotopes of carbon-12 is called relative atomic mass or relative isotopic mass.

 

            The relative atomic mass can be calculated by multiplying the relative isotopic mass of each isotopes by its isotopic abundance divided by total number of atomics. e.g; Chlorine have two isotopes of relative atomic mass 35 and 37 with relative abundance 75% and 25% respectively. The relative atomic of mass of chlorine is;

(35x75) + (37 x 25)

Ar of chlorine =                                                                                            

100

= 35.5

 

RELATIVE MOLECULAR MASS:

The term relative molecular mass, (Mr) is only referred  to a substance with a molecular structure i.e.;  the covalent molecule. Relative molecular mass is the molecular mass of a covalent compound compared with 1/12^th part of an isotopes of carbon-12. It can be calculated by adding all the Ar of atoms present in one molecules of a compound. e.g;.

            Mr of Br₂ = 80 x 2      

                             = 160        

Mr of H₂O = 1 x 2 + 6

       = 18

 

RELATIVE FORMULA MASS: javeedsamejo786

An ionic compound like sodium chloride gas a giant structure of ions which is represented by a formula. Hence, relative formula mass of an ionic compound is the formula mass of an ionic compound compared with 1/12^th part of an isotope of carbon-12. It is calculated by adding all the relative ionic masses present in the formula of an iconic compound. e.g;

 

Sodium chloride     NaCl

                                 23 + 35.5

          = 58.5  

ISOTOPES:

            The atoms of the same element which have same atomic number but different atomic masses are called isotopes of each other and this phenomenon is called isotopy. i.e.;

 

Hydrogen       ¹H₁                  ²H₁                  ³H₁

 

Chlorine         ³⁵Cl17             ³⁷Cl₁₇

 

Oxygen           ¹⁶O₈                 ¹⁷O₈                 ¹⁸O₈

 

PROPERTIES OF ISOTOPES:

(1)              They have different physical properties.

(2)              They have same chemical properties.

(3)              They have same atomic number i.e.; having same number of protons in their nucleus.

(4)              Their position is nit mentioned  in the periodic table.