In 1909, an undergraduate, Ernest Marsden, was being trained by Dr. Hans Geiger. To quote Rutherford (in a lecture he gave much later):

In 1909, an undergraduate, Ernest Marsden, was being trained by Dr. Hans Geiger. To quote Rutherford (in a lecture he gave much later):

"I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?" I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying, "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

“Electrons are distributed uniformly within a continuous distribution of spherical positive charges, like plums in a Plum Pudding”

J.J. Thompson discovered electrons in 1897

Thompson’s Model: a sphere of positive charge embedded

with electrons

In 1898 Thompson proposed a model for the atom

10-10m

The positively charged matter accounts for the

neutrality of the atom, and for most of its mass

Rutherford decided to fire

alpha particles

at thin pieces of gold foil

in order

to investigate

Atomic Structure

particles are Helium nuclei and have a double positive

charge, He2+

What Rutherford Expected

The alpha particle projectiles were very fast, high energy, positively charged He nuclei. They should be only slightly deflected by gold atoms .

Rutherford expected all of the particles to be deflected just a bit as they passed through the

plum pudding, like a bullet being fired through a snow ball

particles, He2+

alpha source in lead box

lead collimator

light flash

(scintillation)

vacuum

thin Au foil

Rutherford’s Apparatus• Most particles passed straight through the gold foil• A small percentage were deviated through a small angle• 1 in 8000 were back scattered

zinc sulphide screen

microscope

Rutherford’s actual apparatus

Contrary to expectations, Rutherford found that a significantly large fraction (~1/8000) of the alpha particles “bounced back” in the same direction in which they came…The calculation, based on the plum-pudding model, was that fewer than

1/10 000 000 000 should do this !!!!!!

In Rutherford’s words…“It was quite the most incredible event that ever happened to me in my life. It was as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you.”

GOSH !GOSH !

+

+

++

+

+ + +

+

Atoms in gold foil most alpha particle pass

straight through

+ +

++

++ +

+

+

ALPHA PARTICLES ONLY FEEL A REPULSIVE FORCE WHEN THEY ARE CLOSE TO THE

NUCLEUS

THE (ONLY) INTERPRETATION

The atom must have a solid core capable of imparting large electric forces onto an

incoming charged particle.

From the angles of scatter, Rutherford deduced that an Inverse Square Law was operating

+ nucleus

Rutherford concludes in 1911:

1) Scattering of the alphas must be due to encounters with charged …atomic particles that are much smaller than the atom.

2) There must be some positive electric charges inside the atom to ….compensate for the negative charges of the electrons.

3) There must be something in the atom much heavier than an …electron to account for the mass of the atom.

4) Atoms must be mostly empty space.

5) Thus, an atom has a small central core (the nucleus) which …contains most of the mass of the atom, and carries a positive …electric charge. This charge attracts the negative electrons and …keeps them in orbit

Rutherford was able to calculate that the radius of the nucleus was about 1 x 10-15 m

The radius of an atom is about 1 x 10-10 m, so the atom’s radius is 100,000 times greater than the

radius of the nucleus.

1:101

000100

3434

153

3

3

3

3

nucleus

atom

nucleus

atom

nucleus

atom

r

r

r

r

V

V

Hence the nucleus is 1015 times denser than the atom itself, as virtually all of the atom’s mass resides within the nucleus.

If an atom is the size of of a stadium, then the nucleus would be represented by a fly on the centre spot.

Or if the Sun represents the nucleus, the edge of the atom would be 30 times farther away than the outermost planet, Pluto.