1.1 ATOMS AND MOLECULES
MATTER
CONTENTS
Define relative atomic mass and relative molecular mass based on the C-12 scale
Analyze mass spectra in terms of isotopic abundances and calculate Ar
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1.1 ATOMS AND MOLECULES
MATTER
CONTENTS
Define relative atomic mass and relative molecular mass based on the C-12 scale
Analyze mass spectra in terms of isotopic abundances and calculate Ar
Navigation button
Definition of relative atomic mass and molecular mass
based on the C-12
Define relative atomic mass and relative molecular mass based on the C-12 scale
1.1 ATOMS AND MOLECULES
MATTER
CONTENTS
Define relative atomic mass and relative molecular mass based on the C-12 scale
Analyze mass spectra in terms of isotopic abundances and calculate Ar
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Animation of mass spectrometer and how to
calculate the atomic mass from its isotopic abundances
Analyze mass spectra in terms of isotopic abundances and calculate Ar
Relative atomic mass, Ar of an element:
Ar = average mass of one atom of the element
mass of one atom C-121
12x
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:1) Define relative atomic mass and relative molecular mass based on C-12 scale
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Relative molecular mass, Mr of a molecular substance:
Mr = average mass of one molecule of the substance
mass of one atom C-121
12x
The mass of an atom depends on the number of electrons, protons and neutrons it contains. But atoms are extremely small particles. Clearly we cannot weigh a single atom, but it is possible to determine the mass of one atom relative to another experimentally.
By international agreement, atomic mass unit is define as a mass of the atom exactly equal to one-twelfth the mass of one carbon-12 atom.
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
19 20 21 22 23
20Ne 90.92%
21Ne 0.26%
22Ne 8.82%
This is the diagram of mass spectra obtained from mass spectrometer with the element Neon with three different isotopes at its own abundances.
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
Relative abundance
m/e
63
8.1 9.1
Relative abundance
Isotopes of Mg
24 25 26
The height is proportional to the amount of isotope present
m/e ratio or nucleon no. or relative atomic mass
Typical mass spectrum of Magnesium
At this stage, you need to know that the mass spectrum diagram consist of four important element. Let’s takes mass spectrum diagram of Magnesium as an example.
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
Since relative abundance can be presented in 2 ways, ratio & percentage, make sure you identify them before do the next step.
Relative ratio
8.0
3.5
85 87 m/e
Mass spectrum in ratio
Percentage %
70
30
85 87 m/e
Mass spectrum in percentage
Sum of abundances =
100
i.e. 70 + 30 = 100
Sum of abundances is not 100
i.e. 8.0 + 3.5 = 11.5
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
Why mass spectrum formula is very important?This formula is useful to calculate the average atomic mass of element!!This is general mass spectrum formula could be used and you have to memorize it!
Ar = (mi x Qi) Qi
Relative atomic mass
Isotopes masses
Relative abundances
Sum of relative abundances
Example
ExercisePast year
Question
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
Relative abundance
63
9.1 8.1
24
Calculate the relative molecular mass from the above mass spectrum.
Example 1From this diagram, we can deduce that:
1) The element has 3 isotopes: m1, m2 & m3 with value 24, 25 & 26 respectively
2) Type of abundance: Qi = Q1 + Q2 + Q3; 63 + 9.1 + 8.1 = 80.2. Not equal to 100. Thus, ratio type.
3) Write the equation: Ar = (mi x Qi)
(Qi)
Ar = (m1 x Q1) + (m2 x Q2) + (m3 x Q3)
Q1 + Q2 + Q3
4) Substitute the data into the equation to get the answer.
5) Ar = ( x ) + ( x ) + ( x )
m1 m2m3 Q3
Q1
Q1
Q2
Q2
+ + Q3
25 26 m/e24 25 26
63
9.1 8.1
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
Relative abundance
63
9.1 8.1
24
Calculate the relative molecular mass from the above mass spectrum.
Example 1
From this diagram, we can deduce that:
1) The element has 3 isotopes: m1, m2 & m3 with value 24, 25 & 26 respectively
2) Type of abundance: Qi = Q1 + Q2 + Q3; 63 + 9.1 + 8.1 = 80.2. Not equal to 100. Thus, ratio type.
3) Write the equation: Ar = (mi x Qi)
(Qi)
Ar = (m1 x Q1) + (m2 x Q2) + (m3 x Q3)
Q1 + Q2 + Q3
4) Substitute the data into the equation to get the answer.
5) Ar = ( x ) + ( x ) + ( x )
= 24
24 25 26 8.1
Q1
63
Q2
9.1
+ + Q3
25 26 m/e
9.1 8.1
63
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
The ratio of relative abundance of naturally occurring of copper is as follow:
Based on the carbon-12 scale, the relative atomic mass of 63Cu = 62.9396 and 65Cu = 64.9278. Calculate the Ar of copper.
Example 2
From question, we can deduce that:
1) Copper has 2 isotopes: 63Cu and 65Cu with value 62.9396 and 64.9278 respectively
2) means
3) Write the equation: Ar Cu = (mi x Qi)
Qi
Ar Cu = (m1 x Q1) + (m2 x Q2)
Q1 + Q2
4) Substitute the data into the equation
Ar Cu = (62.9396 x 2.333) + (64.9278 x 1)
2.333 + 1
Ar Cu = 63.5
63Cu
65Cu= 2.333
63Cu
65Cu= 2.333
63Cu
65Cu=
2.333
1
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
1. Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is 39.1.
(K-39 = 95%, K-41 = 5%)
2. Relative abundance of Rb is given below:
(a) What is the percentage abundance of each of the isotopes?
(Rb-85 = 72%, Rb-87 = 28%)
(b) Calculate the relative atomic mass of Rubidium. (Ar Rb = 85.56)
Exercises
85 87 m/e
18
7
solution
solution
TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
SOLUTION
STEP1: Write the equation
Ar K = (mi x Qi)
Qi
= (m1 x Q1) + (m2 x Q2)
Q1 + Q2
STEP2: List the data given
Ar K = 39.1, m1 = 39, m2 =41,
Q1 =?, Q2 = ?
STEP3: Substitute the data into the equation
Ar K = (m1 x Q1) + (m2 x Q2)
Q1 + Q2
39.1 = (39 x Q1) + (41 x (100 – Q1))
100
Q1 = 95%; Q2 = 100 – 95 = 5%
Abundance of K-39 = 95% and abundance of K-41 = 5%
K only has 2 isotopes; K-39 &
K-41
Assume that the abundance of potassium in percentage. Hence, Q1 + Q2
= 100 & Q2 = 100 – Q1
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TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
SOLUTION
(a) STEP1: List down the abundances of isotopes Rb
Abundance of Rb-85 = 18
Abundance of Rb-87 =7
STEP2: Convert it to percentage
Abundance of Rb-85 in %: 18 / (18 + 7) x 100% = 72%
Abundance of Rb-87 in %: 100 – 72 = 28%
(b) STEP1: Write the equation
Ar Rb = (m1 x Q1) + (m2 x Q2)
Q1 + Q2
STEP2: List down the data given
m1 = 85, Q1 = 72, m2 = 87, Q2 = 28, Ar Rb =?
STEP3: Substitute the data into the equation
Ar Rb = (85 x 72 ) + (87 x 28)
100
= 85.56
Rb has 2 isotopes; Rb-85 &
Rb-87
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TOPIC 1: MATTER SUB TOPIC: Atoms and Molecules
Objective:At the end of the lesson, you should be able to:2) Analyze mass spectra in terms of isotopic abundances and calculate A r
1. (i) Define isotope.
(ii) A sample of chlorine consists of 76% 35Cl and 24% 37Cl. Calculate the relative atomic mass of chlorine if the masses of isotopes are 34.96 amu and 36.97 amu respectively. (Mac 01)
2. Mass spectrometer is used to determine the relative atomic mass. State the main function for the component or part in the mass spectrometer:
(a) Vacuum pump
(b) Vapourisation chamber
(c) Electric field
(d) Magnetic field. (Jan 00)
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