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The Chemist’s element vs. the Physicist’s element. Chemist’s element. Physicist’s element. Periodic table entry for `average’ atom of C. Nuclear notation for specific isotope of C. Atomic # = p. Mass #= n + p. charge. 6 C 12.01. 12 O C 6. Atomic # = p.

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Presentation Transcript
slide1

The Chemist’s element vs. the Physicist’s element

Chemist’s element

Physicist’s element

Periodic table entry for `average’ atom of C

Nuclear notation for specific isotope of C

Atomic # = p

Mass #= n + p

charge

6

C

12.01

12 O

C

6

Atomic # = p

Average atomic mass (not mass number ??)

slide2

Why the chemist’s C lists 12.01 and not 12

Imagine `fishing’ out 100 atoms of Carbon from a sample of graphite (pure carbon). What would you catch ?

# p # n mass # caught out of 100 sampled C atoms

6

6

12

99

6

7

13

1

Both kinds isotopes of C act exactly the same, chemically so chemists just average the masses

99*12 + 1*13

100

Average mass of each C=

= 12.01

slide3

U-do it exercise with mercury: Compute the average mass of Hg from abundance data

~ abundances of the 6 main isotopes of Hg

slide4

DALTON WAS WRONG (A LITTLE)

  • “All atoms of a given element weigh the same”

…he didn’t know about isotopes and neutrons

…but he can be forgiven…in 1805 his equipment was little better than kitchen ware….

Dalton’s measured mass

Correct average mass

12

14

16

23

12.01

14.01

15.99

22.99

C

N

O

Na

slide5

1912 : Ernst Rutherford is the `Man’…and presides over the`Golden Age of Experimental Physics’…THE ATOM APPEARS CONQUERED

vintage Rutherford:

“All science is either Physics or stamp collecting.”

His students find neutrons, build first mass spectrometers, establish source ofisotopes,measureatomic charge, mass & dimensions

slide6

1912: Rutherford ‘s atomic model rules

Electrons (-) out here

+

Protons (+) and neutrons squeezed in here

slide7

But there are 2BIG Problems with Rutherford’s model

1)Why don’t the p+and e- attract and come together ??? (or…why isn’t Earth the size of a golf ball?)

???

slide8

Rutherford atom’s problems (continued)

2)Why doesn’t the sun show all colors (e.g. show white light) when telescopes record spectrum?

Diffraction grating divides up light colors

???

Why only few reallystrong lines

slide9

AN EVEN BIGGERthird PROBLEM FOR RUTHERFORD’S LAB

3) The photoelectric effect problem

and the trouble with the theory of light

Help!!!!

slide10

Language of classical light theory (see also: p 58-60)

A

C

  • A= amplitude
  • = wavelength (meters)
  • c= speed of light = 3*108 meters/second

“nu”

=f = frequency = # full waves passing a point in a second (cycles/second)

slide11

Equations of classical light theory (see also:

p 58-60)

A

C

f*=c

Wave Energy ~ A2

=f = frequency

= # full waves passing a point in a second

slide12

Units for wave equation

frequency * wavelength= speed of light

f *  =c

1

seconds(s)

*

meters(m)

=3.0*108m

s

Hertz (Hz)

Cycles/second (cps)

slide14

Photoelectric effect: the textbook version

E=Energy of ejected electron from metal

See also p. 62 of text

Threshold frequency for emission varies with metals (f1=1 for metal 1, f2=2 for metal 2 etc.)

slide15

The usual textbook explanations of the photoelectric effect blur the really bizarre, underlying meaning of the experiment.

The Mickey Mouse alternative:

slide16

According to `common’ sense, which wave capsizes Mickey and the gang ?

High amplitude (A)

Low frequency (f)

Low amplitude (A)

high frequency (f)

Let Mickey and friends be electrons in a metal

slide17

What actually happens in the photoelectric effect experiment

High amplitude (A)

Low frequency (f)

Low amplitude (A)

high frequency (f)

Let Mickey and friends be electrons in metal