```Topic 1.5 Atomic structure and the periodic table
1: Mass spectroscopy
a. recall the definitions of relative atomic mass, relative isotopic mass and relative
molecular mass and understand that they are measured relative to 1/12th the mass of a
12C atom
b. demonstrate an understanding of the basic principles of a mass spectrometer and
interpret data from a mass spectrometer to:
i. Determine isotopic mass of an element
ii. measure the relative deduce the isotopic composition of a sample of an
element, eg polonium
iii. deduce the relative atom molecular mass of a compound
c. describe some uses of mass spectrometers, eg in radioactive dating, in space research,
in sport to detect use of anabolic steroids, in the pharmaceutical industry to provide an
identifier for compounds synthesised for possible identification as drugs
Connector – Define the following:
•
•
•
Atomic number.
Mass number.
Isotopes
Atomic number & Mass number
Mass Number
23
Atomic Number
11
Na
Mass Number is the number of protons and neutrons
Atomic Number is the number of protons only
Isotopes Atoms that have the same number of protons but
different numbers of neutrons are called isotopes.
Copy and complete the following table
Atom
Number of
protons
Number of
neutrons
Number of
electrons
191
Ir
77
77
114
77
96
42
54
42
Mo
42
BIG picture
• What skills will you be developing this lesson?
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ICT
Numeracy
Literacy
Team work
Self management
Creative thinking
Independent enquiry
Participation
Reflection
• How is this lesson relevant to every day life?
(WRL/CIT)
What is relative atomic mass?
• Relative atomic mass:
is the average mass of the atoms of an element
relative to 1/12th the mass of C=12 atom.
• Relative isotopic mass:
is the mass of one atom of an isotope relative to
1/12th the mass of C=12 atom.
• Relative molecular mass:
is the sum of all the relative atomic masses of
the constituent atoms.
Question
•
The species X, Y and Z are atoms or ions.
species
atomic number
X
16
Y
16
Z
16
mass number
net charge
32
0
35
0
32
–2
(i)
State the number of electrons, protons and neutrons in X
(ii)
What is the relationship between X and Y?
(iii) Identify X, Y and Z
• Mass of an atom cannot be measured directly as it is far
too small.
• A mass spectrometer is the instrument used to
determine the mass of an atom.
Edexcel animation
Mass Spectrometry
• Mass Spectrometry is the generation, separation and detection of
gas phase ions according to their relative mass as a function of
charge (m/e)
1. A small quantity of sample is injected and vaporized under high
vacuum. Particles in gaseous state are free to move in the
machine.
2. The sample is then ionized by being bombarded with high energy
electrons.
A valence electron is “knocked” off and a positive ion (cation) is
formed.
e.g.
Fe (g) + Energy
gaseous atom
 Fe+ (g) + egaseous ion
3. The charged ions are now accelerated by an Electric field.
4. Ions pass through a velocity selector, which makes sure that all
ions are traveling at the same speed.
5. Ions enter a uniform Magnetic field where they are deflected.
The amount of deflection of ions depends on the mass of the ion
and charge on it.
(Heavier ions are deflected less than lighter ions, and ions with
smaller positive charge are deflected less than ions with bigger
positive charge).
6. Finally the positive ions are detected.
The detector is basically a counter, that produces a peak with a
height proportional to the number of ions that strike it at
different magnetic fields. This data is sent to a computer
interface for graphical analysis of the mass spectrum
Process of mass spectrometry
Process of mass spectrometry
Parts of the mass spectrometer
Mass spectra of monatomic elements
Mass spectra of diatomic elements
The main steps inside a mass spectrometer - summarised
vaporized
ionized
accelerated
deflected
detected
A mass spectrum
Each peak represents a positive
ion that has been detected.
The position on the x-axis gives
the relative atomic mass.
The relative heights of the peaks
show the relative abundance of
each isotope.
Relative atomic mass
• Read page 53, 56-59 of Edexcel book
To describe some uses of mass spectrometers, e.g. in radioactive
dating, in space research, in sport to detect the use of anabolic
steroids, in the pharmaceutical industry to provide and identifier for
compounds synthesised for possible identification as drugs
Why does sodium have a whole number atomic mass,
and chlorine doesn’t?
There is only one type of sodium atom, but chlorine atoms exists as two isotopes.
Calculating relative atomic mass from mass spectra
Suppose an element E had two isotopes E1 and E2
then relative atomic mass of E is given by:
Ar (E) = (mass x abundance of E1) + (mass x abundance of E2)
100
Write the expression to calculate the relative atomic mass of element Z
that has three isotopes Z1, Z2 and Z3.
What is the relative atomic mass of this element?
Ar (Na) = 23
(or m/e value)
What is the relative atomic mass of this element?
Ar (Cl) = (35 x 75%) + (37 x 25%)/100
= 35.5
(or m/e value)
What is the relative atomic mass of this element?
55.91
(or m/e value)
Explain this mass spectrum
Isotopes of chlorine
molecules of chlorine with
different isotopes
(or m/e value)
• For a particular sample of sulphur atoms the following
isotopic composition was recorded.
Isotope
Percentage
composition
32S
95.00
33S
0.76
34S
4.24
• Calculate the relative atomic mass of this sample of
= [95.0 × 32 + 0.76 × 33 + 4.24 × 34] / 100
= 32.0924
= 32.09
So 31 + 37x = 142
x = (142 – 31)/37 = 3
Questions
(a)
(b)
(c)
(i) Write the equation for the reaction of lithium with water.
(ii) Describe what you would expect to see during the reaction.
State the number of protons, neutrons and electrons in a lithium
ion.
The mass spectrum of lithium shows two peaks. Their mass/charge
ratios and percentage abundance are shown below.
Mass/charge
6.02
7.02
% Abundance
7.39
92.61
three significant figures.
(d) Describe a test that you would do to distinguish between solid
lithium chloride and solid sodium chloride. Clearly state what you
would do and what you would see with both substances.
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