autumn lecture 1 (isotopes mass spec)
TRANSCRIPT
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Lecture 1: Atoms, Isotopes and Mass Spectrometry(Chemistry in Context ch. 1.1-1.3 and 5.3-5.5)
Structure of atoms-protons, neutrons and electrons
-atomic number and mass number
Isotopes
-relative isotopic abundance and relative atomic masses
The Mass Spectrometer
-vapourisation-ionisation-acceleration-deflection-detection
Interpreting Mass Spectra of Elements-calculating RAM from the mass spectrum
-predicting an elements mass spectra
from its isotopic abundance
Introducing Spectra & Terminology Recap
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19thCentury atomic masses determined accurately for known
elements
John Dalton
Daltons Atomic Theory (1803)
each element has characteristic
atomsAtoms of different
elements differ.l Compounds are
formed when atoms of different
elements combine.each
compound has the same relativenumber of atoms.Reactions are
reorganisationsof atoms.
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What is an atom? (1.1)
-smallest particle of an element
that retains the elements properties
-positively charged nucleus surrounded
by orbiting electron(s)
-protons are positively charged-electrons are negatively charged
-neutrons are neutral
+
++
hydrogen
atom
helium
atom
proton
electron
neutron
+
atoms of copper
in Cu metal lattice
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atomic mass unit(a.m.u. or Dalton) defined as 1.66 x 10-24g
-the proton and neutron both weigh about 1 a.m.u. ot 1 Dalton (1 Da)
both types of nucleon are about 1850 times heavier than an electron
orbiting
electrons
protons
neutrons
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Atomic Number (Z)
Moseley (1913) fired electron beam (cathode ray) at elements:
emitted X-ray wavelength depended on atomic number.
(since no. of protons in nucleus of atom = no. of electrons in neutral atom)
He found way to count electrons in an atom
+
++
proton
electron
neutron
+
hydrogen
atomhelium
atom
Z = 1
A = 1
Z = 2
A = 4
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+
++
proton
electron
neutron
+
hydrogen
atomhelium
atom
Z = 1
A = 1
Z = 2
A = 4
Atomic Number (Z)
Moseley (1913) fired electron beam (cathode ray) at elements:
emitted X-ray wavelength depended on atomic number.
(since no. of protons in nucleus of atom = no. of electrons in neutral atom)
He found way to count electrons in an atom
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Isotopes (5.4)
-all Li atoms have 3 protons. But mass of Li atoms varies. Why?
isotopesare atoms of the same element with differing mass
(isotopes contain differing numbers of neutrons)
BECAUSE: lithium exists naturally as three isotopes:
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Isotopes (5.4)
isotopesare atoms of the same element with differing mass
isotopes contain differing numbers of neutrons neutral particles
8 particles
in nucleus
3 protons
in nucleus
number of
neutrons=
8- 3 = 5
(heaviest isotope)(lightest isotope)
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XAZMass Number
Atomic NumberElement Symbol
Atomic number(Z) = number of protonsin nucleus
Mass number(A) = number of protons+ number of neutrons
= Z + number of neutrons
Atomic Structure Recap (5.4)
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XAZMass Number
Atomic NumberElement Symbol
Atomic number(Z) = number of protonsin nucleus
Mass number(A) = number of protons+ number of neutrons
= Z + number of neutrons
A
Z
3He2
3
2
Atomic Structure Recap (5.4)
4He2
4
2
+
++
+
-helium exists naturally as two isotopes.
(heavier isotope is far more abundant,
whereas helium-3 is rare)
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Isotopes recap
Q: How many protons, neutrons and electrons are there in the
following atoms/ions? Identify the missing isotope.
P31
15 Sb122 3-
93 protons
147 neutrons
91 electrons
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Isotopes recap
Q: How many protons, neutrons and electrons are there in the
following atoms/ions? Identify the missing isotope.
P31
15 Sb122 3- Np
240
932+
93 protons
147 neutrons
91 electrons
51 protons
71 neutrons
54 electrons
15 protons
16 neutrons15 electrons
(phosphorus)
(antimony) (neptunium)
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-most elements are occur as
a mixture of isotopes e.g. Li ++ +
7
Li
+++
6
Li
(fluorine is an exception all of its nuclei have 10 neutrons)
(isotope abundance, %)
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Weighing Atoms with Mass Spectrometry (1.2)
-first, element is vapourisedand then ionised
(i.e. bombarded by electrons to give negative charge)-then acceleratedtowards electromagnet that deflectsions
before it is finally detectedat a certain position
amount of
deflection
indicates
mass-to-charge
(m/z) ratio
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Cation and Anion formation
positive ions are called cations
negative ions are called anions
cationformation
anionformation
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Atomic Ions and Molecular Ions
-atoms can donate or accept electrons to form atomic ions
(atoms become ions when they accept or donate electron(s))
Al Al3+ + 3e-
Na Na+ + e-
F + e- F-
O + 2e- O2-
n.b. stable ions to obtain a octetof electrons
similarly, a molecule can also become a charged molecular ionif
it gains or loses electrons e.g. NH4+, NO2
, HOOetc.
positive ions are called cations
negative ions are called anions
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Mass Spectra Reveal Isotopes (5.5)
ionisation:
Ne + e Ne+ + e + efast, bombarding
electron
-lighter ions are deflected morethan heavier ones
-mass spectrum of neon shows
three isotopes from singlycharged Ne+cations
removed
electron
rebounding
electron
-peak size indicates relative amount (abundance) of each isotope
20Ne is neons most common isotope
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-because there are threeisotopes of neon:
Ne2010Ne
21
10
Ne2210
90.5%
(most common
isotope)
0.3%
(rarest
isotope)
9.2%
isotopesare atoms of the same element with differing mass
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Predicting Mass Spectra from Known Elements (5.5)
-we can predict the appearance of a mass spectrum of we know an
elements natural isotopic composition e.g. Kr
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84 86828078
-we can predict the appearance of a mass spectrum of we know an
elements natural isotopic composition e.g. Kr
84Kr should show the tallest peak
78Kr & 80Kr should show small peaks
79Kr, 81Kr and 85Kr should show no peak
Predicting Mass Spectra from Known Elements (5.5)
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84 86828078
-we can predict the appearance of a mass spectrum of we know an
elements natural isotopic composition e.g. Kr
84Kr should show the tallest peak
78Kr & 80Kr should show small peaks
79Kr, 81Kr and 85Kr should show no peak
So why is atomic mass of Kr
given as 83.798 g/mol?
Predicting Mass Spectra from Known Elements (5.5)
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relative atomic mass(atomic weight) of an element is a weighted
average of isotope masses of that element
e.g. chlorineconsists of 75% chlorine-35 and 25% chlorine-37.
its R.A.M. is about 35.5 g/mol (average value)
Q: Cuis found in two isotopes. 65Cu (mass 64.93 Da)
occurs with 30.85% abundance the rest is 63Cu (62.93 Da).
What is coppers R.A.M. (to 4.s.f.)?
A:
Relative Atomic Mass (1.3 & 5.5)
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Q: Cuis found in two isotopes. 65Cu (mass 64.93 Da)
occurs with 30.85% abundance the rest is 63Cu (62.93 Da).
What is coppers R.A.M. (to 4.s.f.)?
A: Relative atomic mass is a weighted average of mass
numbers. (64.93 0.3085) + (62.93 0.6915)
= 63.55Da(4 s.f.)
relative atomic mass(atomic weight) of an element is a weighted
average of isotope masses of that element
e.g. chlorineconsists of 75% chlorine-35 and 25% chlorine-37.
its R.A.M. is about 35.5 g/mol (average value)
Relative Atomic Mass (1.3 & 5.5)
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(continuous spectrum)e.g. light bulb, sunlight
-to find out how electrons are arranged in
atoms we can study the spectra of the elements
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long wavelength ()low energy (E)
low frequency (f)
short wavelength ()high energy (E)
high frequency (f)
Wavelength, Energy and Frequency Reminder
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-atomic spectra prove that electrons only adopt certain energy levels
E = hc = h
i.e. larger the energydrop, the higher the energyof the emitted photon
(the higher its frequency and shorter its wavelength)
long
wavelengthshort
wavelength
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The Spectrum of Atomic Hydrogen
-model (theory) of structure of atom based on
negative electrons orbiting a compact, positive
nucleus electrostatic attraction
-spectrum of atomic hydrogen:
energy is lost by emission of light
problem why does spectrum of atomic hydrogen showemissions at certain wavelengths only?
Why not continuous range of wavelengths like a rainbow?
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(line spectrum e.g. sparks, excited gases)
neon signs use similar glowing electrified noble gas
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Learning Resources
Recommended Textbooks:
Chemistry- PalgraveFoundations Series
(by R. Lewis, W. Evans)
Chemistry
(by R. Chang)
Chemistry in Context
(by Graham Hill& John Holman)
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Learning Resources
Moodle
used for posting of:
-work tasks
-amended deadlines
-extra learning materials
-forthcoming quiz dates
try to login twice every day
Chem Factsheets
-week-by-week summaries of
lecture material (useful for
revision, posted on Moodle)
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Next Lectures:
2 Electron Configurations (ch. 6)
3 Molecular Structure & Bonding (ch. 8)
4 Intermolecular Forces (ch. 9)
5 Volumetric Analysis
You must read ahead (before lecture!)
Wh i l?
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