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


§ 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


QuickTime and a 

Graphics decompressor 

are needed to see this picture.


§ 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


§ 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/

HISTORY OF THE DEVELOPMENT OF THE ATOMIC MODEL

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