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History - Crooks to Chadwick

Mr. Shields Regents Chemistry U01 L02

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History of the AtomHistory 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)William Crooks (1832 -1919)In the 1870’s Crooks experimented with CathodeRay Tubes (CRT’s)

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William Crooks (1832 -1919)William Crooks (1832 -1919)

Crooks discovered several things about CathodeRays:

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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Movie of cathode ray tube

source source

Magnetic field bending a cathode ray beam

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source

In 1897 he discovers the electron

In 1896 J.J. Thomson (1856 – 1940) began a series of experiments on Cathode Rays

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J.J. Thomson (1856 – 1940)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 particlesbut in fact had a SUB-STRUCTURE!

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J.J. Thomson (1856 – 1940)J.J. Thomson (1856 – 1940)In 1899 Thomson proposed a new model of the atom:

Atoms contain electrons

And since Atoms were knownto be electrically neutral…

Electrons had to beimbedded in an equallypositive medium to cancelthe effect of the electron

This came to be Known asthe “PLUM PUDDING” modelof the atom

electrons

Equally positive medium

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J.J. Thomson (1856 – 1940)J.J. Thomson (1856 – 1940)Here is what he wrote in 1899:

"I regard the atom as containing a large number of smaller bodieswhich I will call corpuscles, these corpuscles are equal to eachother.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpusclesbehave 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 thenegative 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 thepart of the atom left behind behaves like a positive ion with the unit positive charge anda mass large compared with that of the negative ion."

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ERNEST RUTHERFORD (1871 – 1937)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 thatEmanate from the atoms in the Radioactive source

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Deflection of a beam from uranium ore in an electric field

Evidence that atoms are more complexthan either the Dalton or Thompson models

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No charge0

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RUTHERFORD BACK RUTHERFORD BACK SCATTERING EXPERIMENT SCATTERING EXPERIMENT

(1909-1911)(1909-1911)

Rutherfords’s Question:

If an atom was a bunch of electronsimmersed 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 - 1910source

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Experimental Nos. & why the alpha particles were deflected

Most particles

1:8000 particles

1:20,000 particles

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Rutherford’s exp’t: animation

If Thomson’s Plum Pudding Model was correctall alpha particles should have passed Straightthrough the foil with no deflection.

(Why?)

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RUTHERFORD BACK RUTHERFORD BACK SCATTERING EXPERIMENTSCATTERING EXPERIMENT

1911 - Puzzle solved1911 - Puzzle solved

Thompson’s PLUM PUDDING MODEL had to beWRONG!

Rutherford concluded in 1911 that most of themass of the atom is concentrated in a very smallcore at the atom’scenter

Rutherford name thisCORE the NUCLEUS

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RUTHERFORD BACK RUTHERFORD BACK SCATTERING EXPERIMENTSCATTERING 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 BOHRNIELS BOHR1885-19621885-1962

1913 – Bohr, working for Thompson, suggestsRutherford’s model is probably more accurate thatThomson’s Plum pudding model

Thomson suggests he go work for Rutherford – So he did…

To solve the Newtonian problem of the electronfalling into The Nucleus Bohr suggests a hypothesis:

“There can only be discrete orbits in which theelectron can reside.”

1919

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 BOHRNIELS BOHRElectrons can not reside between these orbits, representingSpecific energy levels . To move from one orbit to the next they had to absorb orRelease very specific amounts of energy called “quantum“

This model worked exceptionally well for the Hydrogen atom

But … As we’ll see later thereWere problems with this atomicmodel

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HENRY MOSELEY (1887 – 1915)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 offundamental importance in 1913

His studies of x-rays indicated their frequency shiftedLinearly 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 fromanother wasnow understood!

It was theIncremental Change in postivecharge

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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 fora number of the lighter elements. By examination of the wave-length of thecharacteristic X rays emitted by twelve elements varying in atomic weightbetween 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 ithas long been known that they occupyan anomalous relative position in the periodic classification of the elementsaccording 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 Moseleywanting to study X-rays, but the energy and

enthusiasm of the younger man soon wore Rutherford down.

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Rutherford - protonsRutherford - protons & neutrons& neutrons

1920 – Rutherford bombarded Nitrogen atoms withalpha particles (helium nuclei)

Bombardment Produced a positively charged hydrogen nucleiHe called it the “Proton”

Suggested protons were the basic positive charges in theNucleus

Hydrogen had one proton, helium two etc.

Problem: Helium was considerably more than twice the massof Hydrogen

Rutherford therefore proposed the nucleus also containedan another uncharged particle with same mass as a proton(the Neutron)

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James Chadwick (1891 – 1974)James Chadwick (1891 – 1974)

1932 – Chadwick Performed Experiments that led to theActually 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 andStudied this new nuclear particle – The Neutron

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1/1836 mass of the Proton

1/200011

1/183611

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So what did we know?

Different conceptsof the atom over time

Haven’t talked aboutThis one yet!