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3.13.1 Early History of Atomic TheoriesThe history of atomic theories is full of success and failure stories for hundreds ofchemists. In textbooks such as this one, only the success of a few is documented. However,the success of these chemists was often facilitated by both the success and failure ofmany others.

Recall that by the use of deductive logic the Greeks (for example, Democritus) inabout 300 B.C. hypothesized that matter cut into smaller and smaller pieces would even-tually reach what they called the atom — literally meaning indivisible. This idea wasreintroduced over two thousand years later by an English chemist/schoolteacher namedJohn Dalton in 1805. He re-created the modern theory of atoms to explain three impor-tant scientific laws — the laws of definite composition, multiple proportions, and con-servation of mass. The success of Dalton’s theory of the atom was that it could explainall three of these laws and much more. Dalton’s theory was that the smallest piece ofmatter was an atom that was indivisible, and that an atom was different from one ele-ment to another. All atoms of a particular element were thought to be exactly the same.Dalton’s model of the atom was that of a featureless sphere — by analogy, a billiard ball(Figure 1). Dalton’s atomic theory lasted for about a century, although it came underincreasing criticism during the latter part of the 1800s.

The Thomson Atomic ModelThe experimental studies of Svante Arrhenius and Michael Faraday with electricity andchemical solutions and of William Crookes with electricity and vacuum tubes suggestedthat electric charges were components of matter. J. J. Thomson’s quantitative experi-ments with cathode rays resulted in the discovery of the electron, whose charge was latermeasured by Robert Millikan. The Thomson model of the atom (1897) was a hypoth-esis that the atom was composed of electrons (negative particles) embedded in a posi-tively charged sphere (Figure 2(a)). Thomson’s research group at Cambridge Universityin England used mathematics to predict the uniform three-dimensional distribution of

Figure 1In Dalton’s atomic model, an atom isa solid sphere, similar to a billiardball. This simple model is still usedtoday to represent the arrangementof atoms in molecules.

Creating the Dalton Atomic Theory(1805)SUMMARY

Table 1

Key experimental work Theoretical explanation Atomic theory

Law of definite composition: Each atom has a particular Matter is composed ofelements combine in a combining capacity. indestructible, indivisiblecharacteristic mass ratio atoms, which are identical

Law of multiple proportions: Some atoms have more for one element, but

there may be more than than one combining different from other

one mass ratio capacity. elements.

Law of conservation of Atoms are neither createdmass: total mass remains nor destroyed constant

in a chemical reaction.

William Crookes (1832–1919)William Crookes was the eldest ofsixteen children and inherited hisfather’s fortune, made in real estate.This enabled him to lead a leisurelylife, and also to conduct scientificresearch in many areas of chemistryand physics. Crookes is best knownfor his cathode ray tube, which wasmade possible by his improvementsto the vacuum pump and Volta’sinvention of the electric cell. Hisvacuum techniques later made massproduction of the light bulb practical.

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Atomic Theories 163NEL

the electrons throughout the atom. The Thomson model of the atom is often commu-nicated by using the analogy of a raisin bun, with the raisins depicting the electrons andthe bun being the positive material of his atom (Figure 2(b)).

Section 3.1

The Rutherford Atomic TheoryOne of Thomson’s students, Ernest Rutherford (Figure 3), eventually showed that someparts of the Thomson atomic theory were not correct. Rutherford developed an expertisewith nuclear radiation during the nine years he spent at McGill University in Montreal.He worked with and classified nuclear radiation as alpha (�), beta (�), and gamma (�)— helium nuclei, electrons, and high-energy electromagnetic radiation from the nucleus,respectively. Working with his team of graduate students he devised an experiment to testthe Thomson model of the atom. They used radium as a source of alpha radiation,which was directed at a thin film of gold. The prediction, based on the Thomson model,was that the alpha particles should be deflected little, if at all. When some of the alphaparticles were deflected at large angles and even backwards from the foil, the prediction

The Nature of Cathode Rays(p. 209)The discovery of cathode rays led toa revision of the Dalton atomicmodel. What are their properties?

INVESTIGATION 3.1.1

Figure 2(a) In Thomson’s atomic model, the

atom is a positive sphere withembedded electrons.

(b) This model can be compared toa raisin bun, in which theraisins represent the negativeelectrons and the bun repre-sents the region of positivecharge.

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(a) (b)

Creating the Thomson Atomic Theory(1897)SUMMARY

Table 2

Key experimental work Theoretical explanation Atomic theory

Arrhenius: the electrical Atoms may gain or lose Matter is composed ofnature of chemical electrons to form ions in atoms that contain solutions solution. electrons (negative

Faraday: quantitative Particular atoms and ions particles) embedded in a

work with electricity and gain or lose a specific positive material. The kind

solutions number of electrons. of element is characterized

Crookes: qualitative Electricity is composed by the number of electrons

studies of cathode rays of negatively charged in the atom.

particles.

Thomson: quantitative Electrons are astudies of cathode rays component of all matter.

Millikan: charged oil Electrons have a specific drop experiment fixed electric charge.

Figure 3Rutherford’s work with radioactivematerials at McGill helped preparehim for his challenge to Thomson’satomic theory.

Rutherford Quotes• “You know it is about as incred-

ible as if you fired a 350-mmshell at a piece of tissue paperand it came back and hit you.”

• “Now I know what the atomlooks like.” 1911

• The electrons occupy most of thespace in the atom, “like a fewflies in a cathedral.”

• The notion that nuclear energycould be controlled is “moon-shine.” 1933

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164 Chapter 3 NEL

was shown to be false, and the Thomson model judged unacceptable (Figure 4).Rutherford’s nuclear model of the atom was then created to explain the evidence gath-ered in this scattering experiment. Rutherford’s analysis showed that all of the positivecharge in the atom had to be in a very small volume compared to the size of the atom.Only then could he explain the results of the experiment (Figure 5). He also had tohypothesize the existence of a nuclear (attractive) force, to explain how so much posi-tive charge could occupy such a small volume. The nuclear force of attraction had to bemuch stronger than the electrostatic force repelling the positive charges in the nucleus.Even though these theoretical ideas seemed far-fetched, they explained the experimentalevidence. Rutherford’s explanation of the evidence gradually gained widespread accept-ance in the scientific community.

Figure 4Rutherford’s experimental observa-tions were dramatically differentfrom what he had expected basedon the Thomson model.

alpha particles

metal foil

Prediction

alpha particles

metal foil

Evidence

Figure 5To explain his results, Rutherford sug-gested that an atom consisted mostlyof empy space, explaining why mostof the alpha particles passed nearlystraight through the gold foil.

atom

nucleus

Rutherford’s Gold FoilExperiment (p. 210)Rutherford’s famous experimentinvolved shooting “atomic bullets” atan extremely thin sheet of gold. Youcan simulate his experiment.

ACTIVITY 3.1.1

Protons, Isotopes, and NeutronsThe Thomson model of the atom (1897) included electrons as particles, but did notdescribe the positive charge as particles; recall the raisins (electrons) in a bun (positivecharge) analogy. The Rutherford model of the atom (1911) included electrons orbiting apositively charged nucleus. There may have been a hypothesis about the nucleus beingcomposed of positively charged particles, but it was not until 1914 that evidence was gath-ered to support such a hypothesis. Rutherford, Thomson, and associates studied positiverays in a cathode ray tube and found that the smallest positive charge possible was fromionized hydrogen gas. Rutherford reasoned that this was the fundamental particle of pos-itive charge and he named it the proton, meaning first. (Again Rutherford showed hisgenius by being able to direct the empirical work and then interpret the evidence theo-retically.) By bending the hydrogen-gas positive rays in a magnetic field they were able todetermine the charge and mass of the hypothetical proton. The proton was shown to havea charge equal to but opposite to that of the electron and a mass 1836 times that of anelectron. All of this work was done in gas discharge tubes that evolved into the version ofthe mass spectrometer (Figure 6) developed by Francis Aston during the period 1919–1925.

Evidence from radioactivity and mass spectrometer investigations falsified Dalton’stheory that all atoms of a particular element were identical. The evidence indicated that

Creating the Rutherford Atomic Theory(1911)SUMMARY

Table 3

Key experimental work Theoretical explanation Atomic theory

Rutherford: A few positive The positive charge in the An atom is composed of a alpha particles are atom must be concentrated very tiny nucleus, which deflected at large angles in a very small volume contains positive charges when fired at a gold foil. of the atom. and most of the mass of

Most materials are very A very strong nuclear the atom. Very small

stable and do not fly force holds the positive negative electrons occupy

apart (break down). charges within the nucleus. most of the volume of the

Rutherford: Most alpha Most of the atom is atom.

particles pass straight empty space.through gold foil.

proton ( 10p or p+) a positively

charged subatomic particle found inthe nucleus of atoms

Atomic Theories 165NEL

there were, for example, atoms of sodium with different masses. These atoms of dif-ferent mass were named isotopes, although their existence could not yet be explained.

Later, James Chadwick, working with Rutherford, was bombarding elements withalpha particles to calculate the masses of nuclei. When the masses of the nuclei werecompared to the sum of the masses of the protons for the elements, they did not agree.An initial hypothesis was that about half of the mass of the nucleus was made up ofproton–electron (neutral) pairs. However, in 1932 Chadwick completed some carefulexperimental work involving radiation effects caused by alpha particle bombardment.He reasoned that the only logical and consistent theory that could explain these resultsinvolved the existence of a neutral particle in the nucleus. According to Chadwick, thenucleus would contain positively charged protons and neutral particles, called neutrons.The different radioactive and mass properties of isotopes could now be explained bythe different nuclear stability and different masses of the atom caused by different num-bers of neutrons in the nuclei of atoms of a particular element.

Section 3.1

magnet

slitgas discharge tube

sample inlet

detector

slit

beam of positive ions

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magnet

Figure 6A mass spectrometer is used todetermine the masses of ionizedparticles by measuring the deflectionof these particles as they passthrough the field of a strong magnet.

isotope ( ZA X) a variety of atoms of

an element; atoms of this varietyhave the same number of protons asall atoms of the element, but a dif-ferent number of neutrons

neutron ( 10n or n) a neutral

(uncharged) subatomic particlepresent in the nucleus of atoms

• An atom is made up of an equal number of negatively charged electrons and postivelycharged protons.

• Most of the mass of the atom and all of its positive charge is contained in a tiny core regioncalled the nucleus.

• The nucleus contains protons and neutrons that have approximately the same mass.

• The number of protons is called the atomic number (Z).

• The total number of protons and neutrons is called the mass number (A).

Rutherford ModelSUMMARY

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Section 3.1 QuestionsMaking Connections

9. State some recent examples of stories in the news mediathat mention or refer to atoms.

10. Describe some contributions Canadian scientists and/orscientists working in Canadian laboratories made to theadvancement of knowledge about the nature of matter.

Extension

11. Rutherford’s idea that atoms are mostly empty space isretained in all subsequent atomic theories. How can solidsthen be “solid”? In other words, how can your chair supportyou? Why doesn’t your pencil go right through the atomsthat make up your desk?

12. When you look around you, the matter you observe can besaid to be made from electrons, protons, and neutrons.Modern scientific theories tell us something a little differentabout the composition of matter. For example, today pro-tons are not considered to be fundamental particles; i.e.,they are now believed to be composed of still smaller parti-cles. According to current nuclear theory, what is the com-position of a proton? Which Canadian scientist received ashare of the Nobel Prize for his empirical work in verifyingthis hypothesis of sub-subatomic particles?

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Creating the Concepts of Protons,Isotopes, and NeutronsSUMMARY

Table 4

Key experimental work Theoretical explanation Atomic theory

Rutherford (1914): The The smallest particle of Atoms are composed oflowest charge on an positive charge is the protons, neutrons, and ionized gas particle is proton. electrons. Atoms of the from the hydrogen ion same element have the

Soddy (1913): Radioactive Isotopes of an element same number of protons

decay suggests different have a fixed number of and electrons, but may

atoms of the same protons but varying have a varying number of

element stability and mass. neutrons (isotopes of the

Aston (1919): Mass The nucleus contains element).

spectrometer work neutral particles called indicates different neutrons.masses for some atoms of the same element

Radiation is produced by bombarding elements with alpha particles.

Understanding Concepts

1. Summarize, using labelled diagrams, the evolution ofatomic theory from the Dalton to the Rutherford model.

2. Present the experimental evidence that led to theRutherford model.

3. How did Rutherford infer that the nucleus was(a) very small (compared to the size of the atom)?(b) positively charged?

4. (a) State the experimental evidence that was used in thediscovery of the proton.

(b) Write a description of a proton.

5. (a) State the experimental evidence that was used in thediscovery of the neutron.

(b) Describe the nature of the neutron.

Applying Inquiry Skills

6. What is meant by a “black box” and why is this an appro-priate analogy for the study of atomic structure?

7. Theories are often created by scientists to explain scientificlaws and experimental results. To some people it seemsstrange to say that theories come after laws. Compare thescientific and common uses of the term “theory.”

8. What is the ultimate authority in scientific work (what kindof knowledge is most trusted)?