Transcript Topic 2.2 Atomic Structure The Mass Spectrometer

2.2 The Mass Spectrometer
Assessment Objectives
 2.2.1 Describe and explain the operation of a mass
spectrometer.
 2.2.2 Describe how the mass spectrometer may
be used to determine relative atomic mass using
the 12C scale.
 2.2.3 Calculate non-integer relative atomic masses
and abundance of isotopes from given data.
Reference
 Textbook on Moodle (Topic 2 Atomic Structure)
 Powerpoint presentation on Moodle
 Workbook
Key terms
 Mass Spectrometer
 Vapourise
 Ionisation
 Isotope
Starter
 Work out the number of protons, neutrons and
electrons:
23
Na
11
56
Fe
26
119
Sn
50
19
F
9
27
Al
13
Starter
 Write the symbols for the following elements.
 Hydrogen
 Lithium
 Sulphur
 Sodium
 Potasium
 Write the names of the following symbols.
 O
 He
 B
 Cl
Assessment Objective
 2.2.1 Describe and explain the operation of a mass
spectrometer.
The Mass Spectrometer
 An instrument that separates particles according
to their masses and records the relative
proportions of each.
 Powerpoint presentation of functioning of the mass spectrometer.
How it Works…………
1.
The material is VAPOURISED (turned into a gas) by heating (if
already a gas then used at low pressure).
2.
The gas is then turned from a neutral atoms to positive ions.
This is done by passing the gas through a stream of electrons
between two horizontal plates. These electrons collides with
electrons on the atoms and knocks them off.
X + e-  X+ + 2e-
The Particles become IONISED (charged)
3.
These charged particles are then ACCELERATED between two
vertical plates with high electrical potential. A narrow stream
of these particles are made by passing them through a hole at
the end plate (Note: this is done in a vacuum to stop particles
colliding with each other).
4. The ionised particles are the passed through a
magnetic field which alters their direction according
to their mass. The lighter the particle the more it will
be deflected.
5. By increasing the magnetic field strength the
particles of certain weights can be brought to FOCUS
of the detector and measured.
6. The detector measures the strength of the current
produced when these charged particles collide with
it. The more particles the higher the current.
Summary
 Vapourised sample.
 Ionisation with electron bombardment
 Positive ions accelerated be electrical field
 Ions deflected by a magnetic field
 Detector records ions with particular masses
 Vacumm prevents ions colliding.
Assessment Objectives
 2.2.2 Describe how the mass spectrometer may
be used to determine relative atomic mass using
the 12C scale.
“Weighing” atoms: The mass spectrometer
The more mass the atom has, the more nearly it travels
in a straight line.
Atoms are given
a + charge
+
+
+
+ +
++ + +
These charged
atoms (ions) are
accelerated past a
magnet
The deflected
ions are
detected on a
+
screen or film.
+ +
+ +
+
+
+
The magnet deflects this
ion. The lighter the ion
the more it is deflected
Relative Atomic Mass 1
 The deflection in the mass spectrometer varies
with the mass of the atom.
 However, this does not tell us the mass in
grams.
 What it tells us is the relative masses of atoms –
or relative atomic mass (RAM)
 The element carbon is the atom against which
the mass of all other atoms are compared.
Carbon is given a RAM value of 12.
Relative atomic mass = 12
C
Relative Atomic Mass 2
 The lightest atom is hydrogen. It has one
twelfth the mass of carbon and so has a RAM
of 1.
 Magnesium is twice as heavy as carbon. It
has a RAM of 24.
C
H H
HH H H
H H H
H HH
H H
HH H H
H H H
H HH
1 x 12
=
C
12 x 1
C C
C
C
C
C
C
C
Mg
Mg
Mg
Mg
24 x 1
=
12 x 2
Activity
Relative Atomic Mass
 The Table shows the mass of various atoms
relative to carbon.
 Calculate their relative atomic mass.
Element
Symbol
Times as heavy as carbon
R.A.M
Helium
He
one third
4
Beryllium
Be
three quarters
9
Molybdenum
Mo
eight
96
Krypton
Kr
seven
84
Oxygen
O
one and one third
16
Silver
Ag
nine
108
Calcium
Ca
three and one third
40
How does it Work???
The isotope with the highest abundant is
given a value of 100.
%
10
0
75
100
24Mg
= 100 x 100/127.2 = 78.6%
50
25Mg
= 100 x 12.8/127.2 = 10.0%
25
26Mg
= 100 x 14.4/127.2 = 11.3%
12.8
24
25
14.4
26
The relative atomic mass of magnesium =
m/Z (24 x 0.786) + (25 x 0.10) + (26 x 0.113) = 24.3
Try These………….
 The element nitrogen has two isotopes with
masses of 14 (99.5%) and 15 (0.5%). Calculate the
average atomic mass of nitrogen. (show your work)
 The element chlorine has two isotopes with
masses of 35 (75.5%) and 37 (24.5%). Calculate
the average atomic mass of chlorine. (show your
work)
 Germanium (atomic number 32) contains 20%
germanium-70, 27% germanium-71, 8% geramium72, 37% germanium-73 and 8% germanium-74.
Draw a graph of the mass spectrum that you
would expect germanium to produce.
If an atom of germanium-70 lost two electrons to
become a doubly charged ion, at what m/z would it
appear?
Assessment Objectives
 2.2.3 Calculate non-integer relative atomic masses
and abundance of isotopes from given data.
Plenary: Exam Question
 Silicon has three stable isotopes, 28Si, 29Si and
30Si. The heaviest isotope, 30Si, has a
percentage abundance of 3.1 %. Calculate the
percentage abundance of the lightest isotope to
one decimal place.
(Total 2 marks)
Plenary: Exam Question Answer
 2809 = 3.10 × 30 + 28x + 29(96.9 – x);
% 28Si = (93 + 2810.1 – 2809) = 94.1 %; 2
Award [2] for correct final answer.
Homework
 Learn the symbols for the following elements.
 Bromine (Br)
 Chromium (Cr)
 Manganese (Mn)
 Iron (Fe)
 Cobalt (Co)
 Nickel (Ni)
 Copper (Cu)
 Zinc (Zn)