Transcript William Crooks (1832

History - Crooks to Chadwick
Mr. Shields
Regents Chemistry
U01 L02
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History of the Atom
In the period between the mid 1800’s to early 1900 ‘s our
understanding of Atomic Structure
Increased Dramatically
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William Crooks (1832 -1919)
In the 1870’s Crooks experimented with Cathode
Ray Tubes (CRT’s)
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William Crooks (1832 -1919)
Crooks discovered several things about Cathode
Rays:
1. Cathode Rays could spin a small windmill placed in the
path of the Ray moving down the tube
- This Suggested a Cathode Ray was a stream of
particles
2. Cathode Rays could also bent in magnetic fields
- North pole deflected it one way
- South pole deflected it the other way
Thus…
Cathode rays must have a charge
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Magnetic field bending a cathode ray beam
source
source
Movie of cathode ray tube
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In 1896 J.J. Thomson (1856 – 1940) began
a series of experiments on Cathode Rays
In 1897 he discovers the electron
source
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J.J. Thomson (1856 – 1940)
Thomson Concluded that …
Cathode rays are negatively charged particles
almost 2000x lighter than the hydrogen atom
He called these particles ELECTRONS
AND THE CONSEQUENCE?
Atoms are NOT indivisible marble like particles
but in fact had a SUB-STRUCTURE!
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J.J. Thomson (1856 – 1940)
In 1899 Thomson proposed a new model of the atom:
Atoms contain electrons
electrons
And since Atoms were known
to be electrically neutral…
Electrons had to be
imbedded in an equally
positive medium to cancel
the effect of the electron
This came to be Known as
the “PLUM PUDDING” model
of the atom
Equally positive
medium
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J.J. Thomson (1856 – 1940)
Here is what he wrote in 1899:
"I regard the atom as containing a large number of smaller bodies
which I will call corpuscles, these corpuscles are equal to each
other.... In the normal atom, this assemblage of corpuscles forms a
system which is electrically neutral. Though the individual corpuscles
behave like negative ions, yet when they are assembled in a neutral
atom the negative effect is balanced by something which causes the
space through which the corpuscles are spread to act as if it had a
charge of positive electricity equal in amount to the sum of the
negative charges of the corpuscles.... The detached
corpuscles behave like negative ions, each
carrying a constant negative charge which we
shall call for brevity the unit charge; while the
part of the atom left behind behaves like a
positive ion with the unit positive charge and
a mass large compared with that of the
negative ion."
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ERNEST RUTHERFORD (1871 – 1937)
Canadian - began studies of Radioactivity in the early 1900’s
1902 – Rutherford studied effect of an magnetic & electric
fields on the invisible radioactive beam released from a
sample of uranium ore
He discovers three particles that
Emanate from the atoms in the
Radioactive source
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Deflection of a beam from uranium ore in an electric field
+2
0 No charge
-1
Evidence that atoms are more complex
than either the Dalton or Thompson models
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RUTHERFORD BACK
SCATTERING EXPERIMENT
(1909-1911)
Rutherfords’s Question:
If an atom was a bunch of electrons
immersed in a positively charged Medium (Plum
pudding model) how were the electrons arranged??
Rutherford’s devised an experiment to look at how
ALPHA particles (Helium Nuclei) interacted with a
thin sheet of Gold foil (i.e. only a few stacked
atoms thick)
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Rutherford’s Experimental Results - 1910
source
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Experimental Nos. & why the alpha particles were deflected
Most particles
1:20,000 particles
1:8000 particles
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If Thomson’s Plum Pudding Model was correct
all alpha particles should have passed Straight
through the foil with no deflection.
(Why?)
Rutherford’s exp’t: animation
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RUTHERFORD BACK
SCATTERING EXPERIMENT
1911 - Puzzle solved
Thompson’s PLUM PUDDING MODEL had to be
WRONG!
Rutherford concluded in 1911 that most of the
mass of the atom is concentrated in a very small
core at the atom’s
center
Rutherford name this
CORE the NUCLEUS
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RUTHERFORD BACK
SCATTERING EXPERIMENT
Indicated electrons occupies mostly empty space
around the nucleus
These electrons could be
anywhere in this space.
- e- do not reside in
specific orbits. But…
The problem was that Newtonian mechanics predicted the
electron should eventually fall into the nucleus !
Obviously that would be a problem!
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NIELS BOHR
1885-1962
1913 – Bohr, working for Thompson, suggests
Rutherford’s model is probably more accurate that
Thomson’s Plum pudding model
Thomson suggests he go work for Rutherford –
So he did…
To solve the Newtonian problem of the electron
falling into The Nucleus Bohr suggests a hypothesis:
“There can only be discrete orbits in which the
electron can reside.”
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The model Bohr proposed set up electron energy levels
In which the e- resided. These were designated n=1, 2, 3
etc.
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NIELS BOHR
Electrons can not reside between these orbits, representing
Specific energy levels .
To move from one orbit to the next they had to absorb or
Release very specific amounts of energy called “quantum“
This model worked exceptionally well
for the Hydrogen atom
But … As we’ll see later there
Were problems with this atomic
model
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HENRY MOSELEY (1887 – 1915)
Up to 1913 No one knew what made atoms of one element
Different from atoms of another element!
Mosley, a Student of Rutherford’s, made a discovery of
fundamental importance in 1913
His studies of x-rays indicated their frequency shifted
Linearly from one element to the next according to the
equation:
(Frequency)1/2 = N
And N was an integer that shifted with each element
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Moseley called N the “ATOMIC NUMBER”.
He said N must represent the positive charge of the
nucleus.
What made
one Element
different from
another was
now understood!
It was the
Incremental
Change in postive
charge
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Rutherford (in 1914) described Moseley's discovery thus:
"Recently Moseley has supplied very valuable evidence that this rule
[atomic numbers changing by one from element to element] also holds for
a number of the lighter elements. By examination of the wave-length of the
characteristic X rays emitted by twelve elements varying in atomic weight
between calcium (40) and zinc (65.4), he has shown that the variation of
wave-length can be simply explained by supposing that the charge on the
nucleus increases from element to
element by exactly one unit. This holds
true for cobalt and nickel, although it
has long been known that they occupy
an anomalous relative position in the
periodic classification of the elements
according to atomic weights."
By the way, Moseley was part of Rutherford's
research group -- having arrived in Manchester just
weeks before Rutherford published his great nucleus
paper -- when he started his atomic number work.
Rutherford was not all that excited by Moseley
wanting to study X-rays, but the energy and
enthusiasm of the younger man soon wore Rutherford down.
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Rutherford - protons & neutrons
1920 – Rutherford bombarded Nitrogen atoms with
alpha particles (helium nuclei)
Bombardment Produced a positively charged hydrogen nuclei
He called it the “Proton”
Suggested protons were the basic positive charges in the
Nucleus
Hydrogen had one proton, helium two etc.
Problem: Helium was considerably more than twice the mass
of Hydrogen
Rutherford therefore proposed the nucleus also contained
an another uncharged particle with same mass as a proton
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(the Neutron)
James Chadwick (1891 – 1974)
1932 – Chadwick Performed Experiments that led to the
Actually Discovery of the Neutron
The neutron changes atomic mass but not the element
- leads to the understanding of Isotopes
Imagine…
It was only about 75 years ago that Chadwick found and
Studied this new nuclear particle – The Neutron
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1/1836 mass of the Proton
1/2000
1/1836
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11
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So what did we know?
Different concepts
of the atom over time
Haven’t talked about
This one yet!
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