Chapter 2.1-2.2 The Discovery of the Atomic Structure

The Discovery of the Atomic Structure

Advanced Chemistry

Early Models of the Atom

• An atom is the smallest particle of an element that retains its identity in a chemical reaction.

• Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.

Democritus (460-370 BC)

•Described the material world as made up of tiny indivisible particles they called atomos.

• Indivisible or uncuttable

•Plato & Aristotle believed there can be no ultimately indivisible particles.

John Dalton

• Idea of atoms reemerged in Europe during 17th century.

• Chemists could measure the amount of elements that reacted with one another to form new substances.

• John Dalton developed the atomic theory that was based on four postulates

Dalton's Postulates

Each element is composed of extremely small particles called atoms.

Dalton's Postulates

All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

Dalton's Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

Dalton's Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Laws of Chemical Combination

• Dalton explains several laws of chemical combination in his atomic theory.

1. Law of constant composition

2. Law of conservation of mass

3. Law of multiple proportions

Law of Constant Composition

In a given compound, the relative numbers and kinds of atoms are

constant.

Law of Conservation of Mass

The total mass of materials present after a chemical reaction is the same as the total mass

present before the reaction.

Law of Multiple Proportions

If two elements A and B combine to form more than one compound, the masses of B that can combine

with a given mass of A are in the ratio of small whole numbers.

Much of Dalton’s atomic theory is accepted today.

• One important change, however, is

that atoms are now known to be

divisible.

• They can be broken down into even

smaller, more fundamental particles,

called subatomic particles.

Three kinds of subatomic particles are electrons, protons, and neutrons.

In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron.

• Electrons are negatively charged

subatomic particles.

Electrons

ElectronsThomson performed experiments that involved passing electric current through gases at low pressure.

• He sealed the gases in glass tubes fitted at

both ends with metal disks called

electrodes.

• The electrodes were connected to a source

of electricity.

Electrons

• One

electrode,

the anode

became

positively

charged.

• The other electrode, the cathode, became

negatively charged.

Electrons

The result was

a glowing

beam, or

cathode ray,

that traveled

from the

cathode to the

anode.

Electrons

Thomson found

that a cathode

ray is deflected

by electrically

charged metal

plates.

• A positively charged plate attracts the cathode

ray, while a negatively charged plate repels it.

Thompson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed.

• Thompson called these particles

corpuscles.

• Later they were named electrons.

Electrons

The U.S. physicist Robert A. Millikan (1868–1953) carried out experiments to find the quantity of an electron’s charge.

• In his oil-drop experiment, Millikan

suspended negatively charged oil droplets

between two charged plates.

• He then changed the voltage on the plates

to see how this affected the droplets’ rate of

fall.

Electrons

The U.S. physicist Robert A. Millikan (1868–1953) carried out experiments to find the quantity of an electron’s charge.

• From his data, he found that the

charge on each oil droplet was a

multiple of 1.60 10–19 coulomb,

meaning this must be the charge of

an electron.

Electrons

Radioactivity

•Radioactivity is the spontaneous emission of radiation by an atom.

• It was first observed by Henri Becquerel.

•Marie and Pierre Curie also studied it.

Radioactivity• Three types of radiation were discovered by

Ernest Rutherford:

• particles – attracted to negatively charged plate

• particles – attracted to positive charged plate

• rays – carries no charge

When subatomic particles were discovered, scientists wondered how the particles were put together in an atom.

• Most scientists—including J. J. Thompson—

thought it likely that the electrons were evenly

distributed throughout an atom filled uniformly

with positively charged material.

– In Thomson’s atomic model, known as the “plum-

pudding model,” electrons were stuck into a lump

of positive charge, similar to raisins stuck in

dough.

Plum Pudding Model

This model of the atom turned out to be short-lived, however, due to the work of a former student of Thomson, Ernest Rutherford (1871–1937).

Rutherford’s Gold-Foil ExperimentIn 1911, Rutherford and his co-workers wanted to test the existing plum-pudding model of atomic structure.

• They devised the gold-foil experiment.

• Their test used alpha particles, which are helium

atoms that have lost their two electrons and

have a double positive charge because of the

two remaining protons.

Rutherford’s Gold-Foil Experiment

In the experiment, a narrow beam of alpha particles was directed at a very thin sheet of gold.

Rutherford’s Gold-Foil Experiment

In the experiment, a narrow beam of alpha particles was directed at a very thin sheet of gold.

• According to the prevailing

theory, the alpha particles

should have passed easily

through the gold, with only

a slight deflection due to

the positive charge thought

to be spread out in the gold

atoms.

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Rutherford’s Gold-Foil Experiment

Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.

Copyright © Pearson

Education, Inc., or its affiliates. All

Rights Reserved.

Rutherford’s Gold-Foil Experiment

Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.

• What was surprising

is that a small

fraction of the alpha

particles bounced off

the gold foil at very

large angles.

• Some even bounced

straight back toward

the source.

Based on his experimental results, Rutherford suggested a new theory of the atom.

• He proposed that the atom is mostly empty

space.

– Thus explaining the lack of deflection of most

of the alpha particles.

Based on his experimental results, Rutherford suggested a new theory of the atom.

• He concluded that all the positive charge and

almost all of the mass are concentrated in a

small region that has enough positive charge to

account for the great deflection of some of the

alpha particles.

Neutrons

In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.

• Neutrons are subatomic particles with no

charge but with a mass nearly equal to that

of a proton.