HISTORY OF THE DEVELOPMENT OF THE ATOMIC MODEL

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HISTORY OF THE DEVELOPMENT OF THE ATOMIC MODEL

Greeks to Quantum Mechanics

HISTORY OF THE DEVELOPMENT

OF THE ATOMIC MODEL

http://www.teachersdomain.org/asset/phy03_vid_atoms/


The Greeks

History of the Atom

§ Not the history of atom, but

the idea of the atom

§ In 400 B.C the Greeks tried to

understand matter (chemicals)

and broke them down into

earth, wind, fire, and air.


Fire Water Earth Air


Fire Water Earth Air


Greek Model

Democritus

§ Greek philosopher

§ Idea of atomos

ú Atomos = indivisible

ú Atom is derived

To understand the very large,

we must understand the very small.

§ No experiments to support idea

Democritus 400 B.C.

Democrituss model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE


Alchemy

§ After that chemistry was

ruled by alchemy.

§ They believed that they

could take any cheap metals

and turn them into gold.

§ Alchemists were almost like

magicians.

ú elixirs, physical immortality


Alchemy

Alchemical symbols for substances…

.

.

.



.

.


.

.




.

. .



.

.

.

GOLD SILVER COPPER IRON SAND

transmutation: changing one substance into another

Δ

In ordinary chemistry, we cannot transmute elements.


Contributions

of alchemists:

Information about elements

-­‐ the elements mercury, sulfur, and antimony were discovered

-­‐ properties of some elements

Develop lab apparatus / procedures / experimental techniques

-­‐ alchemists learned how to prepare acids.

-­‐ developed several alloys

-­‐ new glassware


Early Ideas on Elements

Robert Boyle stated...

ú A substance was an

element unless it could be

broken down to two or

more simpler substances.

ú Air therefore could not be

an element because it

could be broken down in

to many pure substances.

Robert Boyle


Modern Chemistry

• Beginnings of modern chemistry were seen in the sixteenth

and seventeenth centuries, where great advances were made

in metallurgy, the extraction of metals from ores.

• In the seventeenth century, Boyle described the relationship

between the pressure and volume of air and defined an

element as a substance that cannot be broken down into two

or more simpler substances by chemical means.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.


Modern Chemistry

• During the eighteenth century, Joseph Priestley

discovered oxygen gas and the process of

combustion where carbon-­‐containing materials

burn vigorously in an oxygen atmosphere.

• In the late eighteenth century, Lavoisier discovered

respiration and wrote the first modern chemistry

text. His most important contribution was the

law of conservation of mass, which states

that in any chemical reaction, the mass of

the substances that react equals the mass

of the products that are formed. He is known

as the father of modern chemistry.

Lavoisier

Priestley

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.


• The transformation of a substance

or substances into one or more new

substances is known as a chemical

reaction.

• Law of conservation of mass:

mass is neither created nor

destroyed during ordinary chemical

reactions or physical changes

total mass of the products = total mass of the reactants


LAW OF CONSERVATION OF MASS


• Law of definite proportions: a

chemical compound contains the

same elements in exactly the same

proportions by mass regardless of

the size of the sample or source of

the compound-­‐ Joseph Proust 1799


Law of Definite Proportions con’t

Example:

• all samples of water (H 2 O) contain a ratio

of 8 g oxygen to 1 g hydrogen

• all samples of iron sulfide (FeS) contain a

ratio of 7 g iron to 4 g sulfur

• If 4 g of element A combine with 20 g of

element B, how many grams of element A

would react with 50 g of element B


• Law of multiple proportions: if two

or more different compounds are

composed of the same two elements,

then the ratio of the masses of the

second element combined with a

certain mass of the first element is

always a ratio of small whole

numbers-­‐ John Dalton1803


Law of Multiple Proportions-­‐ con’t

Example:

2 compounds of hydrogen and oxygen:

H 2 O and H 2 O 2

H 2 O 8 g oxygen : 1 g hydrogen

H 2 O 2 16 g oxygen : 1 g hydrogen


DALTON’S ATOMIC THEORY

• All matter is composed of extremely

small particles called atoms.

• Atoms of a given element are identical

in size, mass, and other properties;

atoms of different elements differ

in size, mass, and other properties.

• Atoms cannot be divided, created,

or destroyed


DALTON’S ATOMIC THEORY,

CONTINUED

• Atoms of different elements

combine in simple whole-­‐number

ratios to form chemical

compounds.

• In chemical reactions, atoms are

combined, separated, or

rearranged.


John Dalton(1766-­‐1844)


MODERN ATOMIC THEORY

• Not all aspects of Daltons atomic theory have proven to be correct.

We now know that:

• Atoms are divisible into even smaller particles.

• A given element can have atoms with different

masses.

• Some important concepts remain unchanged.

• All matter is composed of atoms.

• Atoms of any one element differ in properties from atoms

of another element.


1 st Model of the Atom

Billiard Ball Model


Benjamin Franklin

§ American scientist, politician, inventor

§ 1706-­‐1790

§ Flew kite -­‐ 2 kinds of electrical charge

§ Named + positive, -­‐ negative


Michael Faraday

§ English chemist

§ 1791-­‐1867

§ 1839 suggested atom structure related to

electricity


DISCOVERY OF THE ELECTRON

• Experiments in the late

1800s showed that cathode

rays were composed of

negatively charged particles.

J.J.Thomson(1856-­‐1940)

These particles were named

electrons


** William Crookes

(1870s):

Rays causing

shadow were

emi>ed from

cathode.

Maltese cross CRT

radar screen

television

computer

monitor


q The Thomsons (~1900)

J.J. Thomson discovered

that cathode rays are…

…deflected by electric

and magneJc fields

electric field lines

cathode rays

+ + + + + +

J.J. Thomson

Crookes tube

… (–) parJcles

– – – – – –

electrons

phosphorescent

screen


2 nd Model of the Atom

§ Thomson

introduced his

model in 1904

§ a sphere of

positive charge

with e-­‐ embedded

in it called

Plum Pudding

Model


• Robert A. Millikans oil drop

experiment measured the

charge of an electron.


Henri Becquerel

§ French physicist

§ 1852-­‐1908

§ 1896 discovered radioactivity


DISCOVERY OF THE

ATOMIC NUCLEUS

Ernest Rutherford

• More detail of the atoms structure was provided in

1911 by Ernest Rutherford and his associates Hans

Geiger and Ernest Marsden.

• The results of their gold foil experiment led to the

discovery of a very densely packed bundle of matter

with a positive electric charge.

• Rutherford called this positive bundle of matter the

nucleus.


Gold Foil Experiment

gold foil

beam of alpha particles

radioactive

substance

fluorescent screen

circular - ZnS coated


q Ernest Rutherford (1909)

q Gold Leaf Experiment

Beam of α-­‐parJcles (+) directed at

gold leaf surrounded by

phosphorescent (ZnS) screen.

α-­‐source

parJcle

beam

gold

leaf

lead

block

ZnS

screen


History of Atomic Theory

§ Rutherford with his assistant W.H. Geiger


History of Atomic Theory

§ Rutherford theorized that if the

Plum Pudding model was correct.

The number of positive and

negative particles in an atom

would be the same with the

particles evenly spread within the

atom


History of Atomic Theory

QuickTime and a

Graphics decompressor

are needed to see this picture.


History of Atomic Theory

§ the result was completely

unexpected by Rutherford and his

assistant Geiger

§ Rutherford was forced to rethink

the model realizing that the only

explanation was that all the

positive particles must have been

together in a nucleus


3 rd Model of the Atom

The Rutherford Model

-­‐

-­‐

-­‐

-­‐

-­‐

-­‐

-­‐

-­‐


3 rd Model of the Atom

§ Ernest Rutherford

§ Rutherford Model

§ Developed 1908 with the

discovery of the proton

§ Positive nucleus surrounded

by negative electrons


History of Atomic Theory

§ A few years later, experiments

were conducted indicating that

Rutherford's model had some

problems, namely that it did not

explain why the electrons would

not crash into the nucleus when it

inevitably lost energy


Henry Moseley

§ British chemist and physicist

§ 1887-­‐1915

§ Thru table each more positive than last

§ Individual positive charge -­‐ “proton”

§ The number of protons -­‐ atomic number


Niels Bohr

§ Danish Physicist,

Niels Bohr(1913),

solved the problem

by adding to the

Rutherford Model

his idea of relatively

fixed orbits-­‐

Planetary Model


The Bohr Model

§ electrons traveled in orbits around

the nucleus

§ each e-­‐ has a minimum amount of

energy that will keep it in its

lowest orbit and not allow it to

crash into the nucleus(Ground

State)


4 th Model of the Atom

Bohr’s Model (Planetary Model)


4 th Model of the Atom

§ Developed by Niels Bohr

§ Called the Planetary Model or

Bohr Model

§ Introduced in 1913

§ Positive nucleus with electrons

traveling in orbits


James Chadwick

§ British physicist -­‐ 1935 Nobel Physics

§ 1891-­‐1974

§ 1932 discovered existence of “neutrons”

§ uncharged -­‐ slightly heavier than protons

§ found in nucleus


Quantum Mechanics Model


5th Model of the Atom

§ Developed by many scientists (Schroedinger &

Heisenburg)

§ Quantum Mechanics Model

§ Introduced in the 1930s and 1940s

§ Mathematical model – positive nucleus with a

fuzzy cloud of electrons


q Thomsons Daltons q Rutherfords (also Plum Pudding Greek) Model Model












+


+

+ +




+ +


+

N


+

– –

+ + –



+


Bohr Model


Quantum Mechanics Model


To Be Continued…

§

http://www.teachersdomain.org/asset/phy03_vid_quantum/


Review of the Atomic Models

Draw all the Atomic Models

through history and name the

scientist who is responsible for

that model.

http://www.teachersdomain.org/asset/

phy03_vid_uncertainty/

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