ATOMIC MASS UNIT

Nota:

•  masses of other atoms or molecules  measured relative to the mass of an atom of carbon-12. 

1. relative isotopic mass

   Nota:

   •  Most elements exist as two or more isotopes.   The relative isotopic mass of an isotope is defined as the ratio of the mass of one atom of the isotope to 1/12 of the mass of one atom of carbon-12 isotope   Relative isotopic mass on the carbon scale  = Mass of one atom of the isotope / Mass of one atom of carbon-12 X (12)
   •  For elements that have isotopes, the relative atomic mass is calculated by using the formula:  = m1x + m2y + m3z /100  Where m1, m2 and m3 = nucleon numbers of isotopes 1,2,3 respectively x, y and z = relative abundance of isotopes 1, 2, 3 respectively  
2. relative molecular mass

   Nota:

   •  Is the ratio of the average mass of one molecule of a substance to 1/12 of the mass of an atom of carbon-12 
   •  Relative molecular mass (Mr) of a substance on the carbon-12 scale = Average mass of one molecule of substance / Mass of one atom of carbon-12 X (12)  The relative molecular mass of a substance is calculated by adding up the relative atomic masses of all the atoms present in one molecule of the substance
3. relative atomic mass

   Nota:

   •  The relative atomic mass (Ar) of an element is defined as the ratio of the mass of one atom of the element to 1/12 of the mass of a carbon-12 atom. 
   •  Relative atomic mass (Ar) of an element on the carbon-12 scale = Mass of one atom of the element x 12 Mass of one atom of carbon-12 
   • *Atomic mass = atomic weight = average atomic mass = average atomic weight 

Nota:

• Percentage mass (%) = mass of element                                        --------------------    X  100 %                                  relative formula mass

Nota:

•  Modern technique that uses mass spectrometer to measure very accurately   The relative isotopic mass   The relative abundance of an isotope in a sample of the element   The relative atomic mass   The relative molecular mass    The structure of organic compounds 

1. mass spectrometer
   1. Stage 1: Vaporisation chamber

      Nota:

      • •Sample is being vaporised before injected into ionisation chamber
   2. Stage 2: Ionisation

      Nota:

      • •The atom is ionised by knocking one or more electrons off to give a positive ion.  •This include element that expected to form negative ions (i.e.chlorine) or never form ions at all (i.e.argon).  •Mass spectrometers always work with positive ions.
   3. Stage 3: Acceleration

      Nota:

      • The ions are accelerated so that they all have the same kinetic energy.
   4. Stage 4: Deflection

      Nota:

      • •The ions are then deflected by a magnetic field according to their masses.   •The lighter they are, the more they are deflected.  •The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage.  •The more the ion is charged, the more it gets deflected. 
   5. Stage 5: Detection

      Nota:

      • •The beam of ions passing through the machine is detected electrically 
2. Output

   Nota:

   •  Commonly the diagram are converted into a "stick diagram".   This shows the relative current produced by ions of varying mass/charge ratio.  the vertical axis is labeled as either "relative abundance" or "relative intensity“   the greater the current, the more abundant the ion. 
3. interpreting mass spectra
   1. molecular fragment

      Nota:

      •  For a compound, the mass spectrum will show the peaks of molecular ion and also the ion of molecular fragment.  The highest m/e value is usually the molecular ion (parent ion) and the mass of this ion is the relative molecular mass of the compound
   2. relative abundace of isotopes

      Nota:

      •  These are where element that have isotopes will shows multiple peak corresponding to each isotopes. It also show the intensities of the peak that correlate to the abundance  
4. determination of relative atomic mass

   Nota:

   •  If an element A has three isotopes X, Y and Z with m/e ratios of a,b and c and the heights of intensities of h1, h2 and h3 then Relative atomic mass(Ar) = ah1 + bh2 + ch3 h1+h2+h3 
   • I.E.  The isotopic composition of a certain element X is 80% 24{X}, 10% 25{X} and 10% 26{X}. The relative-atomic mass of X is ?  Answer: Ar = (80x24)+(10x25)+(10x26) 80+10+10 = 24.3