1 of 35

Rutherford Model of the Atom

(The modern view of the atom was developed by Ernest Rutherford)

2 of 35

Ernest Rutherford (1871-1937)

Learned physics in J.J. Thomson’ lab.

Noticed that ‘alpha’ particles were sometime deflected by something in the air.

Gold-foil experiment

Rutherford

PAPER

Ernest Rutherford received the Nobel Prize in chemistry (1908) for his work with radioactivity.

Ernest Rutherford (1871-1937) was born in Nelson, New Zealand in 1871. He began work in J.J. Thompson’s laboratory in 1895. He later moved to McGill University in Montreal �where he became one of the leading figures in the field of radioactivity. From 1907 on he was professor at the University of Manchester where he worked with Geiger and Marsden. He was awarded the Nobelwhere he became one of the leading figures in the field of radioactivity. From 1907 on he was professor at the University of Manchester where he worked with Geiger and Marsden. He was awarded the Nobel Prize for Chemistry in 1908 for his work on radioactivity. In 1910, with co-workers Geiger and Marsden he discovered that alpha-particles could be deflected by thin metal foil. This work enabled him to propose a structure for the atom. Later on he proposed the existence of the proton and predicted the existence of the neutron. He died in 1937 and like J.J. Thompson is buried in Westminster Abbey. He was one of the most distinguished scientists of his century.�

Is the Nucleus Fundamental?

Because it appeared small, solid, and dense, scientists originally thought that the nucleus was fundamental. Later, they discovered that it was made of protons (p+), which are positively charged, and neutrons (n), which have no charge.

3 of 35

Rutherford ‘Scattering’

In 1909 Rutherford undertook a series of experiments
He fired α (alpha) particles at a very thin sample of gold foil
According to the Thomson model the α particles would only

be slightly deflected

Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source

particle

source

Lead collimator

Gold foil

α

θ

4 of 35

Rutherford’s Apparatus

beam of alpha particles

radioactive

substance

gold foil

circular ZnS - coated

fluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3^rd Edition, 1990, page 120

Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.

5 of 35

Rutherford’s Apparatus

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3^rd Edition, 1990, page 120

beam of alpha particles

radioactive

substance

fluorescent screen

circular - ZnS coated

gold foil

MODERN ALCHEMY

Ernest Rutherford (1871-1937) was the first person to bombard atoms artificially to produce transmutated elements. The physicist from New Zealand described atoms as having a central nucleus with electrons revolving around it. He showed that radium atoms emitted “rays” and were transformed into radon atoms. Nuclear reactions like this can be regarded as transmutations – one element changing into another, the process alchemists sought in vain to achieve by chemical means.

Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 35

Ernest Rutherford English physicist. (1910)

Wanted to see how big atoms are.

Used radioactivity, alpha particles - positively charged pieces given off by polonium atoms.

Shot them at a thin gold foil (~0.5 um thick) which can be made a few atoms thick.

When the alpha particles hit a florescent screen, it glows.

Approximately 1/20,000 bounced back at the alpha emitter source.

Rutherford said this was like shooting a 15" shell at tissue paper and the shell came back and hit you.

It was clearly, NOT what he thought should happen if Thomson's model of the atom was correct.

Ernest Rutherford received the 1908 Nobel prize in chemistry for his work at McGill University with radioactive substances.

6 of 35

Geiger-Muller Counter

Speaker gives

“click” for

each particle

Window

Particle

path

Argon atoms

Hans Geiger

7 of 35

Geiger Counter

e-

+

Metal tube

(negatively

charged)

Ionization of fill gas

takes place along

track of radiation

Ionizing

radiation

path

Window

Atoms or molecules

of fill gas

Central wire electrode

(positively charged)

Wilbraham, Staley, Matta, Waterman, Chemistry, 2002, page 857

Free e^- are attracted to

(+) electrode, completing

the circuit and generating a current. The Geiger counter then translates the current reading into a measure of radioactivity.

Speaker gives

“click” for

each particle

(+)

(-)

8 of 35

Lead block

Polonium

Gold Foil

Florescent

Screen

California WEB

9 of 35

What He Expected

The alpha particles to pass through without changing direction (very much)
Because
The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

California WEB

10 of 35

What he expected…

California WEB

11 of 35

What he expected…

12 of 35

Because

he thought the mass was evenly distributed in the atom.

-

13 of 35

Because, he thought the mass was evenly distributed in the atom

-

14 of 35

What he got…

richocheting

alpha particles

15 of 35

What he got…

richocheting

alpha particles

16 of 35

The Predicted Result:

expected

path

expected

marks on screen

mark on

screen

likely alpha

particle path

Observed Result:

17 of 35

Interpreting the �Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3^rd Edition, 1990, page 120

.

gold foil

deflected particle

undeflected

particles

.

beam of

alpha

particles

.

The observations:

(1) Most of the alpha particles pass through the foil un-deflected.

(2) Some alpha particles are deflected slightly as the penetrate the foil.

(3) A few (about 1 in 20,000) are greatly deflected.

(4) A similar small number do not penetrate the foil at all, but are reflected back toward the source.

Rutherford believed that when positively charged alpha particles passed near the positively charged nucleus, the resulting strong repulsion caused them to be deflected at extreme angles.

Rutherford's interpretation: If atoms of the foil have a massive, positively charged nucleus and light electrons outside the nucleus, one can explain how:

(1) an alpha particle passes through the atom un-deflected (a fate share by most of the alpha particles);

(2) an alpha particle is deflected slightly as it passes near an electron;

(3) an alpha particle is strongly deflected by passing close to the atomic nucleus; and

(4) an alpha particle bounces back as it approaches the nucleus head-on.

18 of 35

Density and the Atom

Since most of the particles went through, the atom was mostly empty.
Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.
Small volume and big mass = big density
This small dense positive area is the nucleus

California WEB

19 of 35

Rutherford Scattering (cont.)

Rutherford interpreted this result by suggesting that the α particles interacted with very small and heavy particles

Particle bounces off

of atom?

Particle attracts

to atom?

Particle goes through

atom?

Particle path is altered

as it passes through atom?

.

Case A

Case B

Case C

Case D

In the first case, one would assume the alpha particle (positively charged) struck another positively charged particle. Perhaps William Thomson (Lord Kelvin) was correct and the atom is like plum-pudding and is a positive ball with electrons embedded.

In the middle example, where the alpha particles pass straight through and are not deflected, it implies the atom is mostly empty space or the alpha particle is too penetrating to give any useful information about the composition of an atom.

The third example is NOT what is observed. For this to occur, the atom would have to be negatively charged and absorb all the positively charged alpha particles. At some point the atom would be “full” of alpha particles and then the atom would begin to bounce off of its surface alpha particles.

The last example also occurs. In the gold foil experiment, Rutherford observed case A and D (rarely) and mostly case B. This was explained by saying the atom was mostly empty space where electrons spin rapidly around a positively charged, massive (most of the mass of the atom) but tiny nucleus.

20 of 35

Table: hypothetical description of alpha particles

alpha rays don’t diffract

alpha rays deflect towards a negatively

charged plate and away from a positively

charged plate

alpha rays are deflected only slightly by

an electric field; a cathode ray passing

through the same field is deflected

strongly

... alpha radiation is a stream of particles

... alpha particles have a positive charge

... alpha particles either have much

lower charge or much greater mass

than electrons

observation

hypothesis

(based on properties of alpha radiation)

21 of 35

Explanation of Alpha-Scattering Results

Plum-pudding atom

+

-

Alpha particles

Nuclear atom

Nucleus

Thomson’s model

Rutherford’s model

22 of 35

Results of foil experiment if plum-pudding had been correct.

Electrons scattered

throughout

positive

charges

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

+

-

23 of 35

Interpreting the Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3^rd Edition, 1990, page 120

.

gold foil

deflected particle

undeflected

particles

.

beam of

alpha

particles

.

24 of 35

Rutherford’s�Gold-Leaf Experiment��Conclusions:��Atom is mostly empty space��Nucleus has (+) charge��Electrons float around nucleus

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3^rd Edition, 1990, page 120

“Rutherford’s Gold-Leaf Experiment”

Description

This slide illustrates Ernest Rutherford’s experiment with alpha particles and gold foil and his interpretation of the results.

Basic Concepts

When charged particles are directed at high speed toward a metal foil target, most pass through with little or no deflection, but some particles are deflected at large angles.

Solids are composed of atoms that are closely packed. The atoms themselves are mostly empty space.

All atoms contain a relatively small, massive, positively charged nucleus. The nucleus is surrounded by negatively charged electrons of low mass that occupy a relatively large volume.

Teaching Suggestions

Use this slide to describe and explain Rutherford’s experiment. Rutherford designed the apparatus shown in figure (A) to study the scattering of alpha particles by gold. Students may have difficult with the concepts in this experiment because they lack the necessary physics background. To help students understand how it was determined that the nucleus is relatively massive, use questions 3 and 4 to explain the concept of inertia.

Explain that the electrostatic force is directly proportional to the quantity of electric charge involved. A greater charge exerts a greater force. (Try comparing the electrostatic force to the foce of gravity, which is greater near a massive object like the sun, but smaller near an object of lesser mass, such as the moon.) The force exerted on an alpha particle by a concentrated nucleus would be much greater that the force exerted on an alpha particle by a single proton. Hence, larger deflections will result from a dense nucleus than from an atom with diffuse positive charges.

Point out that Rutherford used physics to calculate how small the nucleus would have to be produce the large-angle deflections observed. He calculated that the maximum possible size of the nucleus is about 1/10,000 the diameter of the atom. Rutherford concluded that the atom is mostly space.

Questions

If gold atoms were solid spheres stacked together with no space between them, what would you expect would happen to particles shot at them? Explain your reasoning.
When Ernest Rutherford performed the experiment shown in diagram (A) he observed that most of the alpha particles passed straight through the gold foil. He also noted that the gold foil did not appear to be affected. How can these two observations be explained?
Can you explain why Rutherford concluded that the mass of the f\gold nucleus must be much greater than the mass of an alpha particle? (Hint: Imagine one marble striking another marble at high speed. Compare this with a marble striking a bowling ball.)
Do you think that, in Rutherford’s experiment, the electrons in the gold atoms would deflect the alpha particles significantly? Why or why not? (Hint: The mass of an electron is extremely small.)
Rutherford experimented with many kinds of metal foil as the target. The results were always similar. Why was it important to do this?
A friend tries to convince you that gold atoms are solid because gold feels solid. Your friend also argues that, because the negatively charged electrons are attracted to the positively charged nucleus, the electrons should collapse into the nucleus. How would you respond?
As you know, like charges repel each other. Yet, Rutherford determined that the nucleus contains all of an atom’s positive charges. Invent a theory to explain how all the positive charges can be contained in such a small area without repelling each other. Be creative!

�

25 of 35

Rutherford’s Experiment

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

26 of 35

Actual Results of Gold-Leaf Experiment

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n ⁺

27 of 35

The Rutherford Atom

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n ⁺

-

28 of 35

This is the modern atom model.

Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks jiggle within the protons and neutrons.

This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space!

Website “The Particle Adventure”

29 of 35

Scale of the atom.

While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are; they are definitely smaller than 10^-18 meters, and they might literally be points, but we do not know.

It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles. (Oh, will this madness ever end?)

Website “The Particle Adventure”

30 of 35

Physicists have developed a theory called The Standard Model that explains what the world is and what holds it together. It is a simple and comprehensive theory that explains all the hundreds of particles and complex interactions with only:

6 quarks.

6 leptons. The best-known lepton is the electron.

Force carrier particles, like the photon. We will talk about these particles later.

All the known matter particles are composites of quarks and leptons, and they interact by exchanging force carrier particles.

The Standard Model is a good theory. Experiments have verified its predictions to incredible precision, and all the particles predicted by this theory have been found. But it does not explain everything. For example, gravity is not included in the Standard Model.

Website “The Particle Adventure”

31 of 35

Discovery of the electron

1807 Davy suggested that electrical forces held compound together.

1833 Faraday related atomic mass and the electricity needed to free an element during electrolysis experiments.

1891 Stoney proposed that electricity exists in units he called electrons.

1897 Thomson first quantitatively measured the properties of electrons.

32 of 35

Coulomb’s Law

Why don’t electrons collide while moving around the outside of atom?

Why can’t we add protons to nucleus?

When an ion forms:

cation…gain protons or lose electrons?

anion…lose protons or gain electrons?

Coulomb’s law

Both negative charges (repel each other)

Hard to hit small nucleus

(+) will repel (+)

33 of 35

Hit moth driving car – no change in car direction
Hit deer – car changes direction

Alpha particle

Large angle of deflection, must have hit massive object!

moth

deer

Gold Atom

34 of 35

Hit moth driving car – no change in car direction
Hit deer – car changes direction

Alpha particle

Large angle of deflection, must have hit massive object!

moth

deer

Gold Atom

35 of 35

Force

Definite proportions

H₂O 2 H @1 g/mol = 2 g

1 O @ 16 g/mol = 16 g

1:8 H:O

by mass

Coulomb’s law