Autumn Lecture 1 (Isotopes Mass Spec)

7/27/2019 Autumn Lecture 1 (Isotopes Mass Spec)

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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)


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%



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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



84Kr should show the tallest peak

78Kr & 80Kr should show small peaks

79Kr, 81Kr and 85Kr should show no peak



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84 86828078



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?



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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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Autumn Lecture 1 (Isotopes Mass Spec)