1. 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

2. 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
= amu
3.1

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

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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.

10. 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

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

12. 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

13. HOW DOES IT WORK? - Deflection
20Ne
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

14. WHAT IS A MASS SPECTRUM?
20Ne %
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 Ne, 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

15. CALCULATING AVERAGE ATOMIC MASS
Calculate the average relative atomic mass of neon using data on the previous page.
Out of every 100 atoms are 20Ne , are 21Ne and 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).

16. 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)

17. How to Read a Mass Spec Diagram beyond analyzing isotopes of elements
HW:
If you need a refresher to how the mass spectrometer works:
2. Watch from about 21:38 to 43:16 to see how to analyze many different types of spec data