ELECTRICAL FIELD, MAGNETIC FIELD, ION DETECTOR, MODERN SPECTROGRAPGS, CALCULATIONS INVOLVED DETERMINING THE RELATIVE ATOMIC MASS OF AN ELEMENT FROM THE GIVEN RELATIVE ABUNDANCE OF ITS ISOTOPES, OR FROM ITS MASS SPECTRUM:

 

(iii)     ELECTRICAL FIELD

            A potential difference of 500 – 2000V is applied between the plates to accelerate these positive ions.  These ions are accelerated due to repulsion by high voltage positively charged electrodes.

 

(IV)     MAGNETIC FIELD:

            After acceleration the positive ions are sent to the magnetic field which is perpendicular to the path of positive ions. Due to this positive ions move in a semi-circular path. It is observed that lighter ions are deflected more and  heavy ions travel slowly in a semi-circular path hence, separation occurs according to m/e values.

 

(v)      ION DETECTOR:

            A suitable detector is used to collect the deflected positive ions. The charge on each ions produces a tiny electric current at the detector. The more ions there are of the same mass, the higher the current and higher the peak generated by such isotope.

 

            The same experiment is performed with carbon – 12. The current straingths produced by other particles are compared with the current strengths of carbon-12. In this way exact mass number of the isotopes is measured.

 

(VI)     MODERN SPECTROGRAPGS:

            Now a days, in modern spectrometres, each ions strikes a detector, ions current is amplified and is fed to the recorder. The recorder makes a graph showing the relative abundance of each isotope. In the graph, the abundance is on the verticle exis and mass-to-charge ratio (M/e) is on horizontal axis. As the ions almost always carry a single positive charge, hence, m/e ratio is equal to atomic mass of that isotope. Some modern mass spectrometers can determine isotopic  mass up to four or five decimal places. Spectrograph obtained is also called mass spectrum.

 

CALCULATIONS INVOLVED DETERMINING THE RELATIVE ATOMIC MASS OF AN ELEMENT FROM THE GIVEN RELATIVE ABUNDANCE OF ITS ISOTOPES, OR FROM ITS MASS SPECTRUM:

 

PROBLEM:

The mass spectrum of neon is given below. Show how the datacan be used to determine the relative atomic mass of neon.

 

                                                                     

SOLUTION:

            Relative atomic mass (Ar) of Neon  = (20x10) + (22x1) / 11

                                                                        = 20.18

 

PROBLEM:

            Relative abundance of ³⁵Cl is 75% and of ³⁷Cl is 25%. What is the relative atomic mass of chlorine?

 

SOLUTION:

            Relative atomic mass (Ar) of chlorine = (35x75) + (37x25) / 100

                                                                             = 35.5