to calculate, multiply the % abundance of each isotope by the relative mass of each and add together...
TRANSCRIPT
To calculate, multiply the % abundance of each isotope by the relative mass of each and add together
Atomic Mass is not a whole number
3.1
because it takes into account all the isotopes of an element
Natural lithium is:
7.42% 6Li (6.015 amu)
92.58% 7Li (7.016 amu)
(7.42% x 6.015) + (92.58% x 7.016)100 = 6.941 amu
3.1
Average atomic mass of lithium:
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)
X(14.001) + (1-X)( 15.001) = 14.0067 amu
3.1
Average atomic mass of nitrogen:
14.001X + 15.001 – 15.001X = 14.0067
-.9943 = -.9979X
X = .996 or 99.6% 1-X = .004 or .4%
How do we know this?
3.1
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
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.
A mass spectrometer consists of ... an ion source, an analyser and a detector.
ION SOURCE
ANALYSER
DETECTOR
A MASS SPECTROMETER
PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND
DEFLECTED
HOW DOES IT WORK?
ION SOURCE
ANALYSER
DETECTOR
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?
ION SOURCE
ANALYSER
DETECTOR
Step 1: IONIZATION
Step 2: ACCELERATION• ions are charged so can be ACCELERATED by an electric field
HOW DOES IT WORK?
ION SOURCE
ANALYSER
DETECTOR
Step 1: IONIZATION
Step 2: ACCELERATION
Step 3: DEFLECTION•charged particles will be DEFLECTED by a magnetic or electric field
HOW DOES IT WORK?
ION SOURCE
ANALYSER
DETECTOR
Step 1: IONIZATION
Step 2: ACCELERATION
Step 3: DEFLECTION
Step 4: DETECTION•by electric or photographic methods
HOW DOES IT WORK?
ION SOURCE
ANALYSER
DETECTOR
IONIZATION
ACCELERATION
DEFLECTION
DETECTION
HOW DOES IT WORK? - Deflection
• 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
20Ne21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED
LESS
HOW DOES IT WORK? - Deflection
• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved
20Ne21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED
LESS
0 4 8 12 16 20 m/z values
ABU
ND
AN
CE
1+ ions2+ ions
20Ne
22Ne
Doubling the charge, halves the m/z value
Abundance stays the same
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
MASS SPECTRUM OF NEON
19 20 21 22 23
20Ne 90.92%
21Ne 0.26%
22Ne 8.82%
WHAT IS A MASS SPECTRUM?
Calculate the average relative atomic mass of neon using data on the previous page.
CALCULATING AVERAGE ATOMIC MASS
Out of every 100 atoms... 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne
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).
A Answer: 20.18
IDENTIFY THE ELEMENT
• First, calculate average atomic mass
• Then find the element with the closest mass on periodic table
Average Atomic Mass = 91.32Closest Element: Zirconium (Zr, Z=40)
HW: 1. 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
How to Read a Mass Spec Diagram beyond analyzing isotopes of elements