Atomic History

, who was from was the first person to use

the term atom during B.C. Democritus defined the atom as being .

opposed Democritus because believed that atoms and that matter was .

Ancient HistoryDemocritus Greece

400

indivisible

Aristotle Aristotle

did not exist continuous

Chemists accepted the modern definition of an element during the late .

An is a substance that cannot be broken down by ordinary chemical means.

Chemists could not agree whether elements always combine in the

when forming a compound.

Foundations of Atomic Theory

1700s

element

same ratio

Chemists studied during the 1790s in order to find out whether elements always combine in the same ratio.

They discovered the law of , which states that

Foundations of Atomic Theory

chemical reactions

conservation of m

ass

Chemists studied during the 1790s in order to find out whether elements always combine in the same ratio.

They discovered the law of , which states that

cannot be or during chemical and

physical changes.

Foundations of Atomic Theory

chemical reactions

conservation of m

ass

masscreated destroyed

45 g H2O

Conservation of Mass

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

Highvoltage

Before reaction

electrodes

glasschamber

5.0 g H280 g O2+85 g total

H2O2

Highvoltage

After reaction

0 g H2

40 g O2+85 g total

O2

H2O

The law of definite proportions deals with the compound.

The fact that a compound consists of the percentages by regardless of

of each element you start with, the compound is formed, or the compound is formed.

Law of Definite Proportionssame

alwayssame

mass how much

wherehow

In the laboratory…

In nature… (copper ore in Nevada)

Example of the law of definite proportions

+ +

51.44%copper

38.85% oxygen

copper carbonate

9.72% carbon

51.44% Cu38.85% O9.72% C

The law of multiple proportions states: when two of the elements combine to form

compounds and the first element has a mass, the

of masses between the element will be a

number.

Law of Multiple Proportions

samedifferent

fixed ratiosecond element

whole

Two compounds containing carbon and oxygen

Example of the law of multiple proportions

Carbon monoxide (CO)

Carbon dioxide (CO2)

1.0g C1.33g O

1.0g C2.66g O

1.33𝑔𝑂𝑖𝑛𝐶𝑂2.66𝑔𝑂 𝑖𝑛𝐶𝑂2

=2

1. All matter is made up of .

2. Atoms of the element are and atoms of elements

are .

3. Atoms cannot be , , or .

4. Atoms of different elements combine in to form compounds.

5. In chemical reactions, atoms are , or .

Dalton’s Atomic Theoryatoms

sameidentical different

subdivided

different

created destroyed

whole-number ratios

combined separated rearranged

1. All matter is made up of .

2. Atoms of the element are and atoms of elements

are .

3. Atoms cannot be , , or .

4. Atoms of different elements combine in to form compounds.

5. In chemical reactions, atoms are , or .

Modern Atomic Theoryatoms

sameidentical different

subdivided

different

created destroyed

whole-number ratios

combined separated rearranged

2. Atoms of different elements are different

conducted the cathode ray experiment in .

Cathode Ray Experiment

Joseph John (J.J.) Thomson1897

Cathode Ray Experiment

+-

Cathode ray tube

metal disks

• are tubes that are filled with low pressure gas and contain two metal disks.

Cathode ray tubes

Cathode Ray Experiment

+-

vacuum tube

metal disks

voltage source

Cathode Ray Experiment

+-voltage sourceOFF

ON

Passing an electric current through a cathode ray tube causes the tube to

.glow

• The glow was caused by a of called a

cathode ray. • A cathode ray is by a magnetic field.

Modern Atomic Theorystream

particles

deflected

Thomson measured the ratio of the cathode ray particles and found that this ratio was ALWAYS the for every element.

Thomson concluded that cathode rays are composed of identical . charged .

Findings from the Cathode Ray Experiment

charge:mass

same

negativelyparticles

studied electrons in .

Millikan found that the of an electron is of the mass of a atom.

He confirmed that an electron carries a charge.

Millikan concluded that are present in atoms of elements.

Charge and Mass of the Electron

Robert A. Millikan1909

mass1/2000

hydrogen

negativeelectrons

all

The discovery of the electron led chemists to infer the following during the early 1900s:◦Atoms must contain a charge in order to the

because atoms are electrically .

◦Atoms must contain other that account for most of their because of the tiny mass of the electron

Charge and Mass of the Electron

positivebalance

electronsneutral

particlesmass

conducted the gold foil experiment in

. are positively charged particles.

Rutherford and associates shot a thin piece of with fast-moving

. They assumed that and

were distributed throughout the gold foil.

The Gold Foil Experiment Rutherford and associates

1911Alpha particles

gold foilalpha particles

masscharge uniformly

What he expected…

He expected that the alpha particles would the gold foil pass through

What he got…

He found that the of the alpha particles passed through while other alpha particles were

majority

redirected

Rutherford explained that the alpha particles were redirected because they experienced a , which occupies a very

of space. He concluded that the force was due to a bundle of matter with a

charge, which he called the .

Findings from the Gold Foil Experiment

powerful forcesmall amount

densepositive

nucleus

observed cathode ray tubes in and found rays that traveled in the

direction of cathode rays

He concluded that these oppositely traveling rays contained charged particles.

discovered the existence of the in .

Other discoveries Goldstein

1886

opposite

positively

Chadwickneutron

1932

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton    Neutron    Electron    

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  Neutron    Electron    

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  Neutron  0  Electron    

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  Neutron  0  Electron  -1  

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  1Neutron  0Electron  -1

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  1Neutron  0 1

Electron  -1

Composition of the Atomic Nucleus

Subatomic Particle

Charge Mass number

Proton  +1  1Neutron  0 1

Electron  -1 0

Atoms are electrically neutral because they contain the number of and

. The nuclei of atoms of different elements differ in the

.

Composition of the Atomic Nucleus

sameprotons electrons

number of protons

Atoms with two or more protons do not fall apart because there is a strong between them.

The are the short-range forces between protons and protons, neutrons and neutrons, and protons and neutrons that hold the nucleus together.

Forces in the Nucleus

attractionnuclear forces

The is a region occupied the electrons.

Atomic radii are expressed in and they range from to in length.

Forces in the Nucleus

electron cloud

picometers (pm)40pm 270pm

What are the two most important ideas of Dalton’s atomic theory?

Which of the following did Thomson NOT discover about the cathode ray? a) The charge to mass ratio of a cathode ray is

largeb) The mass of a cathode ray is 1/2000 the mass of

a hydrogen atomc) Cathode rays are made up of negatively charged

particles What did Rutherford NOT discover about an

atom? a) The nucleus contains neutronsb) The nucleus contains positively charged particlesc) The volume of a nucleus is very tiny

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