The Project Physics Course: Unit 5 Transparencies: Models of Atom

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The Project

Physics

Course UNIT

Transparencies

5

Models

of the

Published by HOLT,

Atom

RINEHART and WINSTON,

Inc.

New

York, Toronto

Project Physics

Overhead Projection Transparencies Unit 5

T35

Periodic Table

T36

Photoelectric Experiment

T37

Photoelectric Equation

T38

Alpha Scattering

T39

Energy Levels

— Bohr Theory

m f-

Periodic Table

T35

Periodic Table

This transparency will be useful

in

discussions centered around the classification of the elements.

Various overlays highlight chemical families and other pertinent groupings. Overlay

A

Overlay B

The modern long form of the Periodic Table. The number below each chemical symbol represents the element's atomic number. The Roman numerals identify the Groups or Families. The numbers to the left of Group I identify the Periods. This includes the 62 elements which Mendeleef included

in his

grouping was, of course, unlike the present long form. Rather "short" form of the Table. Remove

Overlay

C

The Alkali Metal Family (Group

Overlay

D

The Halogen Family (Group

it

1872 classification. His

resembled the modern

this overlay.

I).

VII).

Overlay E

The Noble Gas Family (Group

Overlay F

The elements known

O).

as the Transition Elements.

Remove Overlays

C, D. E. and F.

Overlay

G

The Natural Radioactive Elements. Those which undergo J-decay only are lighter. Technetium (Tc, Atomic Number 43) is a synthetic element.

Overlay

H

The Transuranium Elements. These have

all

been synthesized

in

tinted

the laboratory.

T-35

Periodic Table of the Elements

1

H 1

T5

O

'^

4

TRK Periodic Table of the Elements

1

B

m 1

TT3a5

Periodic Table of the Elements

1

im Periodic Table of the Elements

1

Photoelectric Experiment

Photoelectric Experiment

T36

the mechanism of the photoelectric effect This transparency can be used to help students visualize and the method of measuring the stopping voltage. microA schematic diagram is presented of the photoelectric tube connected to a Overlay A above the voltmeter) ammeter, voltmeter, and a power supply (the empty rectangle left. Note that the the on is rod curved emitter is on the right and the collecting

The

circuit

Overlay B

is

open between the emitter and

collector.

the voltmeter power supply provides a positive bias on the collecting rod, commg depicted are frequency mdicates this positive potential. Photons of a particular photoelectrons whose paths are illustrated by in to the emitter. These photons eject circuit as they accelerate toward the arrows. The negative photoelectrons complete the micro-ammeter. Remove this overlay positive collector and register a current on the

When

the

DC

and introduce overlay C. Overlay

C

reversing the terminals on the The bias on the collecting rod is now made negative by photons eject photoelecpower supply. The voltmeter indicates this reversal. Now as As a by the negative field surrounding the collecting rod.

trons thev are slightlv repelled get to the collector. The resulting reducresult only the more energetic photoelectrons the ammeter. Remove this overlay and in current is shown as a lower reading on tion

introduce overlay D.

Overlay

D

An

increased voltage

is

applied to the collector.

When

it is

sufficiently high

it

"stopping ^oltage photoelectrons (note zero reading on ammeter). This photoelectrons. the of used as a measure of the maximum kinetic energy

all

will

stop

can be

T-36

m

W --^i

Current

Collector Voltage

i

T-36 A B

+ Current

Collector Voltage

T'36 A

C

T^BS

Current

j^ CO

"^

Photoelectric Equation

T37

Photoelectric Equation

This transparency

will

be useful

voltage (see T36). With

Overlay

A

A

it

in

you can

analyzing data obtained from experiments dealing with stopping arrive at the Einstein photoelectric equation.

plot of typical experimental data

vacuum phototube

on the voltage required between the terminals of a Remove this overlay and in-

to reduce the tube current to zero.

troduce overlay B.

Overlay B

Such data imply that the maximum

kinetic energy of the emitted photoelectrons

proportional to the frequency of the incident photons.

Remove

this

is

overlay and

introduce overlay C.

Overlay

C

This shows Einstein's interpretation of the data

in

terms of a "work function" W, an

amount of energy that must be supplied to the electrons before they can escape the The energy supplied to the electron by the photon is hf, so the emission energy

surface.

KE ^ax = hf-

W.

T-37

Experimental Data on "Stopping Voltages"

>

©

3 O

©

O

©

on

>

© no emission

light

frequency

(f

137 B

o

Inference about Kinetic Energy of Photoelectrons

it

o E

O o

E

no emission

light

frequency

(f)

T-37

Einstein's Interpretation of

Data

o o

maximum

KE of

photoelectron

no emission

—

.*—

energy required to escape surface

light

frequency

(f)

Alpha Scattering

oo

T38

Alpha Scattering

This transparency contrasting the

Overlay

A

is

useful in discussing Rutherford's alpha particle scattering experiment

and

in

Thomson and Rutherford models.

A

diagrammatic sketch of the Rutherford scattering experiment. A magnifying glass (not shown) could be moved around the ZnS screen to detect flashes produced by the alpha particles as they strike the screen.

Overlay B

Shows

the expected results of the scattering experiment under the assumption that

matter

is

composed of Thomson atoms. Note and add overlay C.

that there

is

very

little

deflection.

Remove

this overlay

Overlay

C

These are the

results

which Rutherford and

large deflections required a completely

Remove Overlay

D

overlays

A

and

C and

alpha particles encounter

E

co-workers actually observed. The the structure of the atom.

introduce overlay D.

Two representations of the Thomson model are shown. The left side depicts large (lA diameter) spheres of positive electrification with negative electrons imbedded in them "Uke raisins in a muffin". The right side shows a "potential hill" which positive will

Overlay

his

new explanation of

be caused by this

The paths of alpha

like

marbles rolhng up a slope. Relatisely small deflections

hill.

particles

would be only

slightly deviated as they pass

through

Thomson atoms. Overlay

F

Thomson model will account for experiments. The new model contains a very

The Rutherford modification on flections actually

observed

in

the

nucleus with negative electrons surrounding particles

and

a

it.

the large de-

small positive

This allows a close approach by alpha

consequent large deflection. The "potential

hill"

model

is

also adapted

to a very steep slope (greatly broadened in the diagram) causing sharp deflections the

closer the particles

approach the center. Remove overlays

overlay G.

Overlay

G

Alpha

particle deflections

now match

observations.

D

and E and introduce

T-38

-

•^>-i,"

^Sm.^:'--:

Gold Foil

Zinc Sulfide

Screen

Alpha Particle Source

ir» A B

Results expected

with

Thomson Model

.^yti-:'- -p;--!^

;

'

Y-!*-!--!

•*^^ 5^?:.

Zinc Sulfide

Screen **--<-.i»V,:j..'«v. ;;

Alpha Particle Source

T'38

A c

Results obtained

by Rutherford

<^o\d^ !•-

Zinc Sulfide

Screen

Alpha Particle Source

^

T-38

\

+

Thomson Model

TraasB

D E

^-scattering

I

Thomson Model

T-38

+

Rutherford Model

tT^W F (

^-scattering

+

Rutherford Model

Energy Levels

as

— Bohr Theory

T39

Energy Levels

This transparency gen.

It

will

be useful

in relating

includes a general treatment of the

— Bohr Theory

Bohr theory of energy levels to the spectrum of hydroL\man. Balmer and Paschen Series with a more detailed

the

coverage of the Balmer Series.

Overlay

A

On

the

left

hydrogen are drawn to scale (since the radii should = 0.5 A li-). An energy level diagram is mcluded and a space for spectral Imes across the bottom. Introduce overlays B,

the

Bohr

orbits for

increase according to the expression Rn

on the

right,

C and D Overlay B

Representations of electron quantum "falls" from higher energy levels to the ground slate.

Overlay

C

in order.

The

resulting emission of the

The Balmer

Series

is

Lyman

Series

is

produced by excited electrons

shown

falling

in

the spectrum

window.

back to the second energy

level.

Overlay

D

level.

Overlay E

produced by excited electrons falling back to the third energy Remove overlays B, C and D. Add the remaining overlays in order.

The Paschen The Ha

Series

line in the

is

Balmer Series is produced by an excited electron falling from the Note that the scale of the spectral line representation

third energy level to the second.

has been changed.

Overlay F

The

H.j line in the

Balmer Series

is

produced by an excited electron

falling

from the

fourth energy level to the second.

Overlay

G

The

H-, line in the

Balmer Series

is

produced by an excited electron

falling

from the

fifth

energy level to the second.

Overlay

H

The

H-, line in the

Balmer Series

is

produced by an excited electron

falling

from the

sixth energy level to the second.

Overlay

I

The

limit of the

higher than

/i

=

Balmer Series is approached as excited electrons from energy 6 fall back to energy level 2.

levels

T-39

T=ai

f49

r

Lyman

Series

1

r

I

F

Balmer Series

infrared

ultraviolet

limits of visible range

ft

B

c I-

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i'y^ ;>'«';

m -V.

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