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Atomic Structureand Relative Masses

1.1 The Atomic Nature of Matter

1.2 The Experimental Evidence of Atomic Structure

1.3 Sub-atomic Particles

1.4 Atomic Number, Mass Number and Isotopes

1.5 Mass Spectrometer

1.6 Relative Isotopic, Atomic and Molecular Masses

1

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1.1

The AtomicNature of Matter

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What is atom?

1.1 The atomic nature of matter (SB p.2)

The Greek philosopher Democritus(~460 B.C. 370 B.C.)

Atomos = indivisible

Atomism()

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Iron

Continuousdivision

Continuousdivision

These are

iron atoms!!



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Daltons atomic theory

1803 AD John Dalton


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Main points of Daltons atomic theory

1. All elements are made up of atoms.

2. Atoms cannot be created, divided into

smaller particles, nor destroyed in thechemical process.

A chemical reaction simply changes the

way atoms are grouped together.


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Main points of Daltons atomic theory

5. When atoms of different elements combine to

form a compound, they do so in a simple wholenumber ratio to each other.

3. Atoms of the same element are identical. Theyhave the same mass and chemical properties.

4. Atoms of different elements are different.They have different masses and chemicalproperties.

Check Point 1-1


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The ExperimentalEvidence of

Atomic Structure

1.2

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1.2 The experimental evidence of atomic structure (SB p.3)

Steps to Thomsons Atomic Model

1876 Goldstein

Discovery of cathode rays from

discharge tube experiment.

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Discovery of Cathode Rays

A beam of rays came out from thecathode and hit the anode

Goldstein called the beamcathode rays


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Steps to Thomsons Atomic Model

1876 Goldstein

Discovery of cathode rays from

discharge tube experiment.

1895 Crookes

Cathode rays are negatively chargedparticles which travelled in straightline. electrons


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Deflected inthe electric

field

Deflected inthe magnetic

field


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The beam was composed ofnegativelycharged fast-moving particles.


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Measurement of the m/e ratio of electron

J J Thomson (1856-1940)

1897


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Measure themass tocharge ratio(m/e) ofthe particles produced Independent of the

nature of the gasinsidethe discharge tube

The particleswere constituents

of all atoms!!

Thomson called the

particleselectrons.


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Thomsons atomic model

Atom


An atom was a positivelycharged sphere of lowdensity+ +

+

+ +

+

The positively chargedsphere is balancedelectrically by negatively

charged electronsElectron

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How are the particles distributed inan atom?

++

+

+ +

+

Positive

charge

Most of the mass of theatom was carried by theelectrons (>1000 e-)

Electron


An atom was a positivelycharged sphere of lowdensity with negativelycharged electronsembedded in it like aplum pudding

1 h l d f ( B )

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++

+

+ +

+

Positive

charge

Like a raisin bun ()

Electron


1 2 Th i l id f i ( B 3)

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Experimental evidence : -

Powerful projectiles such as -particles passes

straight through a thin gold foil.Analogy : -

-particle vs a thin gold foil

15-inch canon ball vs a piece of paper


1 2 Th i l id f i (SB 3)

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Steps to Rutherfords Atomic Model

Nobel laureates, Physics, 1903

Becquerel Marie CuriePierre Curie


1 2 Th i t l id f t i t t (SB 3)

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1896 Becquerel

1st discovery of radioactive substance.

(an uranium salt)



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1898 Pierre & Marie Curie

Radioactive polonium and radium wereisolated

1g from 500 Kgpitchblende



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The Curie Family

Pierre & Marie Curie

Nobel laureate, Physics, 1903

Marie CurieNobel laureate, Chemistry, 1911

Federic Joliet & Irene Joliet-Curie

Nobel laureate, Chemistry, 1935



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1899 Rutherford

(Nobel laureate, Physics, 1908)

Discovery of and radiations.

radiation He2+

radiation e


1 2 Th xp im nt l id n f t mi st t (SB p 3)

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Rutherfords scattering experiment


1 2 The experimental evidence of atomic structure (SB p 3)

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A thin gold foil was bombarded with a beam offast-moving -particles (+ve charged)

Observation:

most -particles

passed through thefoil withoutdeflection

very few -particleswere scattered orrebounded back


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It was quite the most incredible event that

has ever happened to me in my life.It was almost as incredible as if you fired a15-inch shell at a piece of tissue paper and it

came back and hit you.


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Interpretation of the experimental results

Nucleus is positively charged because itrepels the positively charged alpha particles.



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Nucleus occupies a very small space (10-12

ofsize of atom) because very few particlesare deflected.



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The radius of an atom is about 20,000 timesthat of the nucleus. Thus, if we imagine alarge football stadium as being the wholeatom, then the nucleus would be about thesize of a peanut.



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Nucleus is relatively massive and highlycharged because of the large deflection.



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Presence of protons in nucleus

Number of positive charges in each nucleus canbe calculated from experimental results


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Steps to Chadwicks Atomic Model

Ne20

10 Ne22

10

1919 F. W. Aston

(Nobel laureate, Chemistry, 1922)


Isotopes of Neon were discovered usingmass spectrometry


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1920 Rutherford

Postulated the presence of neutrons inthe nucleus



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James Chadwick

(Nobel laureate, Physics, 1935)



Discovery of the neutron


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Chadwicks Experiments



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Be9

4He

4

2C

12

6n

1

0++

Interpretation : -


1.2 The experimental evidence of atomic structure (SB p 3)

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Chadwicks atomic model

ProtonElectron

Neutron Check Point 1-2


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Sub-atomicParticles

1.3

1.3 Sub-atomic particles (SB p.6)

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Sub-atomic particles

1.3 Sub atomic particles (SB p.6)

3 kinds of sub-atomic particles:

Protons

Neutrons Electrons

Inside the condensed

nucleusMoving around thenucleus


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A carbon-12 atom

. Sub atom c part cles (SB p.6)


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Characteristics of sub-atomic particlesSub-atomic

particle

Proton Neutron Electron

Symbol p or n or e- or

Location inatom

Nucleus Nucleus Surrounding thenucleus

Actual charge(C)

1.6 10-9 0 1.6 x 10-9

Relative charge +1 0 -1

Actual mass (g) 1.7 10-24 1.7 10-24 9.1 10-28

Approximaterelative mass(a.m.u.)

1 1 0

H11

n10

e0-1

m p ( p )


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1 a.m.u.= 1/12 of the mass of a C-12 atom

mass of a C-12 atom 6p + 6n

mass of p mass of n 1 a.m.u.

One C-12 atom has 6 p, 6n and 6e

mass of p mass of n

mass of e can be ignored

mass of a C-12 atom 6p + 6n 12p 12n

p ( p )


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Express the masses of the following isotopesin a.m.u..

C146C136

C126

12

p ( p )

~13 ~14

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Atomic Number,Mass Number

and Isotopes

1.4

1.4 Atomic number, mass number and isotopes (SB p.7)

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Atomic numberp ( p )

Theatomic number (Z)of an element isthenumber of protonscontained in thenucleus of the atom.

Atomic

number =

Number of

protons

Number of

electrons=

Atoms are electrically neutral


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Mass number

p p

Themass number (A)of an atom is thesumof the number of protons and neutronsinthe nucleus.

Massnumber

=Number of

protonsNumber ofneutrons

+

Number of neutrons = Mass number Atomic number


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Isotopes

Isotopes are atoms of the same element withthe same number of protonsbutdifferent numbers of neutrons. Or

Isotopes are atoms of the same element withthe same atomic number butdifferent mass numbers

p p


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XA

Z

Notation for an isotope

Symbol ofthe

element

Mass number

Atomic number


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Atomicnumber

Massnumber

Numberof

protons

No. ofelectron

s

No. ofneutrons

Notation

5 10

8 8 9

28 14 14

10 12

78 4466 30

5 5 5

8 17

14 14

22 10 10

34 34 3430 30 36

B10

5

O17

8

Si2814

Ne22

10

Se

78

34

Zn66

30

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A boron isotope has a relative mass of ~10 a.m.u.

Give the isotopic notation.

B10

5


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Discovery of isotopes by mass spectrometry

What is the difference in mass betweenthe two isotopes of hydrogen ?

H1

1 H

2

1

1 a.m.u.

= 1.7

10-24

g= 0.0000000000000000000000017 g

No balance is accurate enoughto distinguish this difference


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What is the relative abundances ofthe two isotopes of hydrogen ?

H1

1H21

99.8% 0.02%

Both tasks can be accomplished with amass spectrometer !!

What is the difference in mass betweenthe two isotopes of hydrogen ?

1.5 Mass spectrometer (SB p.10)

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Mass spectrometer

Extremelyaccurate

Resolution :

1024 g


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Mass spectrometer

Highly preciseResults ofmeasurement are

reproducible


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Mass spectrometer

Highlysensitive

Sample size :as small as 1 g


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Mass spectrometer

Highlyefficient

Analysis can beaccomplished in acouple of minutes.


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The sample (element or compound) isvaporized

+


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Positive ions are produced from the vapour

X(g) + e X+(g) + 2e

+


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X(g) + e X+(g) + 2e

+

Atom Simple ionMolecule Molecular/polyatomic ion


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+ve ions accelerated by a known andfixedelectric field

+


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+ve ions are then deflected by a known andvariablemagnetic field

+


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The ions are detected

+


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The mass spectrumis traced out by therecorder

+


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Mass spectrum of Rb:

x-axis :-

For singly charged ions, e = 1

m/e = m

= isotopic mass (relative to C-12)

mass number (whole number)

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Relative isotopic mass

The relative isotopic mass of a particularisotope of an element is the relative mass of

one atom of that isotope on the12

C = 12.0000scale.


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Mass spectrum of Rb:

Y-axis :-

Relative abundance,

Ion intensity, or

Detector current

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Relative atomic mass

The relative atomic mass of an element isthe weighted average of the relative

isotopic masses of the natural isotopes onthe 12C = 12.0000 scale.

Q 11.9 Relative isotopic, atomic and molecular masses (SB p.22)

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Q.172.12%

27.88%

Relative atomic mass of Rb

= 85 72.12% + 87 27.88%

= 85.56

The mass spectrum of lead is given below Given that the

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The mass spectrum of lead is given below. Given that therelative atomic mass of lead is 207.242, calculate therelative abundance of the peak at m/e of 208.

x47.7

x208

x47.7

22.6207

x47.7

23.6206

x47.7

1.5204207.242

Let x be the relative abundance of the peak at m/e of 208

x = 52.3

1.9 Relative isotopic, atomic and molecular masses (SB p.22)

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Q.2(a)

Relative atomic mass of Pb)

10

5.2(208)()

10

2.2(207)()

10

2.4(206)()

10

0.2(204)(

= 207.2Q.2(b) 103 2206Pb

104 2208

Pb


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Q.3(a)(i)/(ii)

Rn222

Rn220

The lighter ions(220Rn+)

with a smaller m/e ratioare defected more


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the strength of the magnetic field or the strength of the electric field wouldbring the ions from Y onto the detector.

In practice, the strength of the electricfield is fixed while that of the magneticfield is increased gradually to bring ions of

increasing m/e ratios onto the detector.

3.(b)


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Rn2+ would be deflected more than theions at X and Y. (Rn2+ has a smaller m/e)

3.(c)

If magnetic field strength and electricfield strength are fixed,

m/e deflection


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m/eRelative

abundance

Ionic

species14 4.0

16 0.820 0.3

28 100

29 0.76

2Ar40

N]N[1414

N]N[ 1514

Ne20 ,

21414 N]N[

21616

O]O[

N14 ,

O16

,


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m/eRelative

abundance

Ionic

species32 23

33 0.0234 0.09

40 2.0

44 0.10

O]O[ 1616

O]O[1716

O]O[ 1717

Ar40

O]CO[ 161216


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Relative molecular mass

Therelative molecular massis the relativemass of a molecule on the carbon-12 scale.

Relative molecular mass can be determinedby mass spectrometer directly.

M t f Cl1.5 Mass spectrometer (SB p.10)

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Mass spectrum of Cl2:

The peaks with higher m/e ratiocorrespond to molecular ions

Fragmentation of molecules always

occurs during the ionization process.

Cl2(g) Cl(g) + Cl(g)

M t f Cl


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Mass spectrum of Cl2:

The scale has been enlargedfor these two peaks.

C l t th f ll i t bl


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Complete the following table

m/eratio Correspondingion

35

37

70

72

74

35Cl+

37Cl+

[35Cl-35Cl]+

[35Cl-37Cl]+

[37Cl-37Cl]+

Wh i h l i i f Cl?


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What is the relative atomic mass of Cl?

The relative abundances ofCl-35and Cl-37 are75.77and24.23respectively

100

24.2337

100

75.7735RAM

= 35.48

Wh i h l i l l f Cl ?


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What is the relative molecular mass of Cl2 ?

Method 170.9635.482RMM

Method 2

48

374

48

1872

48

2770RMM

= 71


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What is the RMM of CH3Cl?

1654052

165251

16512350RMM

= 50.50

Molecularions


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m/e Corresponding ion

35

37

50

51

52

35Cl+

37Cl+

[12C1H335Cl]+

[12C1H337Cl]+[13

C1

H335

Cl]+

,

[12

C2

H1

H235

Cl]+

The mass spectrum of dichloromethane is given below.C l l h l i l l f di hl h

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Calculate the relative molecular mass of dichloromethane.174.5

0.890

174.5

2.589

174.5

1388

174.5

2.287

174.5

5986

174.5

385

174.5

9484RMM

= 85.128

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The END


Back

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(a) What does the word atom literally mean?

(b)Which point of Daltons atomic theory is based on the lawof conservation of mass proposed by Lavoisier in 1774which states that matter is neither created nor destroyed

in the course of a chemical reaction?

(c) Which point of Daltons atomic theory is based on the lawof constant proportion proposed by Proust in 1799 whichstates that all pure samples of the same chemicalcompound contain the same elements combined togetherin the same proportions by mass?

(a) Indivisible

(b) Atoms can neither becreated nor destroyed.

(c) Atoms of different elements combine to form a

compound. The numbers of various atoms combined

bear a simple whole number ratio to each other.

Back

Answer

1.2 The Experimental evidence of atomic structure (SB p.4)

Back

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(a) Atoms were found to be divisible. What names weregiven to the particles found inside the atoms?

(b) Give the most important point of the followingexperiments:

(i) E. Goldsteins gas discharge tube experiment;

(ii)J. J. Thomsons cathode ray tube experiment;

(iii) E. Rutherfords gold foil scattering experiment.

(a) Electron, proton

and neutron

(b) (i) Discovery of cathode rays

(ii) Discovery of electrons

(iii) Discovery of nucleus in atoms

Back

Answer


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The identity of an element is determined by thenumber of which sub-atomic particle?

Back

The identity of an element is determined by

the number of protons in its atomic nucleus.

Answer

1.3 Sub-atomic Particles (SB p.7)

Back

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(a) Which part of the atom accounts for almost all the mass ofthat atom?

(b) The mass of which sub-atomic particle is often assumedto be zero?

(a) Nucleus

(b) Electron

Back

Answer


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Are there any sub-atomic particles other than protons,neutrons and electrons?

Back

Other than the three common types of sub-

atomic particles (proton, neutron and

electron), there are also some sub-atomic

particles called positron (anti-electron) and

quark.

Answer


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If bromine has two isotopes, 79Br and 81Br, how many

physically distinguishable combinations of Br atoms arethere in Br2?

Back

79

Br

79

Br79Br81Br

81Br

81Br

They have different molecularmasses and thus have different

density


Back

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Write the symbol for the atom that has an atomic number of11 and a mass number of 23. How many protons, neutronsand electrons does this atom have?

, 11 protons, 12 neutrons, 11 electrons.Na

23

11

Answer


Back

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Label the different parts of the mass spectrometer.

A Vaporization chamber

B Ionization chamber

C

Accelerating electric field

D Deflecting magnetic field

E Ion detector

Answer


Back

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The mass spectrum of neon is given below. Determine the

relative atomic mass of neon.

Relative atomic mass of neon

=

= 20.18

)2.112.0114(

)2.1122()2.021()11420(

Answer


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(a) The mass spectrum of lead is given below. Given that therelative atomic mass of lead is 207.242, calculate therelative abundance of the peak at m/e of 208.

Let x be the relative abundance

of the peak at m/e of 208.(204 1.5 + 206 23.6 + 207 22.6 + 208x) (1.5 + 23.6 + 22.6+ x) = 207.242

x = 52.3

The relative abundance of the

peak at m/e of 208 is 52.3.

Answer

Back1.6 Relative isotopic, atomic and molecular masses (SB p.14)

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(b) The mass spectrum of dichloromethane is given below.Calculate the relative molecular mass of dichloromethane.

The relative molecular mass of

dichloromethane

= (84 94 + 85 3.0 + 86 59 +87 2.2 + 88 13 + 89 2.5 +90 0.8) (94 + 3.0 + 59 + 2.2 +13 + 2.5 + 0.8)

= 85.128

The relative molecular mass of

dichloromethane is 85.128.

A

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