ATOMIC MASS UNIT

ATOMIC MASS UNIT

1. STANDARD RELATIVE MASS

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

2. PERCENTAGE MASS

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

3. MASS SPECTROMETRY

   •  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 
   • mass spectrometer
     • Stage 1: Vaporisation chamber
       • •Sample is being vaporised before injected into ionisation chamber
     • Stage 2: Ionisation
       • •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.
     • Stage 3: Acceleration
       • The ions are accelerated so that they all have the same kinetic energy.
     • Stage 4: Deflection
       • •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. 
     • Stage 5: Detection
       • •The beam of ions passing through the machine is detected electrically 
   • Output
     •  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. 
   • interpreting mass spectra
     • molecular fragment
       •  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
     • relative abundace of isotopes
       •  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  
   • determination of relative atomic mass
     •  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