Atomic Mass is not a whole number because it takes into account all the isotopes of an element To calculate, multiply the % abundance of each isotope by the relative mass of each and add together 3.1

Average atomic mass of lithium: Natural lithium is: 7.42% 6Li (6.015 amu) 92.58% 7Li (7.016 amu) Average atomic mass of lithium: (7.42% x 6.015) + (92.58% x 7.016) 100 = 6.941 amu 3.1

Average atomic mass of nitrogen: EXAMPLE: There are 2 isotopes of nitrogen, one with an atomic mass of 14.0031 amu and one with a mass of 15.001 amu. What is the percent abundance of each? X% N (14.0031 amu (1-X)% N (15.001 amu) Average atomic mass of nitrogen: X(14.001) + (1-X)( 15.001) = 14.0067 amu 14.001X + 15.001 – 15.001X = 14.0067 -.9943 = -.9979X X = .996 or 99.6% 1-X = .004 or .4% 3.1

How do we know this? Dalton’s Atomic Theory had to be modified due to the discovery of the isotope. The mass spectrometer, built by Aston, a student of JJ Thompson, identified the existence of isotopes 3.1

How does a Mass Spectrometer work? articles are turned into positive ions, accelerated and then deflected by an electric or magnetic field. The resulting path of ions depends on their ‘mass to charge’ ratio (m/z). Particles with a large m/z value are deflected least Particles with a low m/z value are deflected most.

PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED A MASS SPECTROMETER DETECTOR ION SOURCE ANALYSER A mass spectrometer consists of ... an ion source, an analyser and a detector. PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED

HOW DOES IT WORK? Step 1: IONIZATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION gaseous atoms are bombarded by electrons from an electron gun and are IONIZED sufficient energy is given to form ions of 1+ charge

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION ions are charged so can be ACCELERATED by an electric field Some type of voltage

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION Step 3: DEFLECTION charged particles will be DEFLECTED by a magnetic or electric field Usually passed through an evacuated tube where they come in contact with electric or magnetic field The magnetic field causes the ions to be deflected by different amounts depending on their mass. The ions are then detected and shown in a graph called a mass spectrum.

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION Step 3: DEFLECTION Step 4: DETECTION by electric or photographic methods

HOW DOES IT WORK? IONIZATION ACCELERATION DEFLECTION DETECTION DETECTOR ION SOURCE ANALYSER IONIZATION ACCELERATION DEFLECTION DETECTION

HOW DOES IT WORK? - Deflection 20Ne 21Ne 22Ne HEAVIER ISOTOPES ARE DEFLECTED LESS the radius of the path depends on the value of the mass/charge ratio (m/z) ions of heavier isotopes have larger m/z values so follow a larger radius curve as most ions are 1+charged, the amount of separation depends on their mass

HOW DOES IT WORK? - Deflection 20Ne 0 4 8 12 16 20 m/z values ABUNDANCE 1+ ions 2+ ions 20Ne 22Ne Doubling the charge, halves the m/z value Abundance stays the same 21Ne 22Ne HEAVIER ISOTOPES ARE DEFLECTED LESS if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

WHAT IS A MASS SPECTRUM? 19 20 21 22 23 20Ne 90.92% 21Ne 0.26% 22Ne 8.82% MASS SPECTRUM OF NEON In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes ... 20Ne, 21Ne and 22Ne. positions of the peaks gives atomic mass peak intensity gives the relative abundance highest abundance is scaled to 100% and other values are adjusted accordingly

CALCULATING AVERAGE ATOMIC MASS Calculate the average relative atomic mass of neon using data on the previous page. Out of every 100 atoms... 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne Answer: 20.18 A TIP In calculations of this type... multiply each relative mass by its abundance add up the total of these values divide the result by the sum of the abundances; (100 in this case) * if the question is based on percentage abundance, divide by 100 but if it is based on heights of lines in a mass spectrum, add up the heights of the lines and then divide by that number (see later).

Closest Element: Zirconium (Zr, Z=40) IDENTIFY THE ELEMENT First, calculate average atomic mass Then find the element with the closest mass on periodic table Average Atomic Mass = 91.32 Closest Element: Zirconium (Zr, Z=40)

How to Read a Mass Spec Diagram beyond analyzing isotopes of elements HW: If you need a refresher to how the mass spectrometer works: http://www.bozemanscience.com/ap-chem-009-mass-spectrometry/ 2. Watch from about 21:38 to 43:16 to see how to analyze many different types of spec data http://www.youtube.com/watch?feature=player_detailpage&v=biH065_y1I4#t=1292