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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

(7.42% x 6.015) + (92.58% x 7.016)

100

Natural lithium is:

7.42% 6Li (6.015 amu)

92.58% 7Li (7.016 amu)

Average atomic mass of lithium:

= 6.941 amu

3.1

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:

= 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

- 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.

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

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

DETECTOR

ION SOURCE

ANALYSER

- Step 1: IONIZATION
- Step 2: ACCELERATION
- ions are charged so can be ACCELERATED by an electric field

DETECTOR

ION SOURCE

ANALYSER

- Step 1: IONIZATION
- Step 2: ACCELERATION
- Step 3: DEFLECTION
- charged particles will be DEFLECTED by a magnetic or electric field

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? - 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

1+ ions

20Ne

22Ne

ABUNDANCE

Doubling the charge, halves the m/z value

Abundance stays the same

0 4 8 12 16 20 m/z values

HOW DOES IT WORK? - Deflection

20Ne

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

21Ne 0.26%

22Ne 8.82%

19 20 21 22 23

WHAT IS A MASS SPECTRUM?

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

TIPIn 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).

- 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

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