Evolution of Atomic

Theory

Scientists that changed our

view of the atom

Mrs. Baldessari

Chemistry

?Greatest Chemistry Discoveries?? – YouTube

What: All matter is a combo of fire, air, earth or water

When: 350 B.C.

Where: Greece

Aristotle

Democritus

• 460BC-370BC

•Born in Abdera, Thrace.

• Greek philosopher who

developed the atomic theory

of the universe.

•There was no Atomic Theory

before Democritus - he

developed the first theory

Democritus

• Did not conduct any

experiments

• All matter is made of

(atomos = indivisible)

atoms

• Atoms are indivisible

and indestructible

• ATOMIC STRUCTURE

Democritus’ Atomic

Theory

Atoms differ in shape, size, weight, sequence, and

position.

Atoms spin and collide, creating larger aggregations

of matter.

Democritus performed no experiments and had

almost no evidence for his theory of the atomic

model.

John Dalton

John Dalton was born on

September 6, 1766 in Cumbria,

England.

He died on July 27, 1844

Dalton was a chemist,

meterologist, and physicist.

Attended college in Manchester.

He provided us with the first

concrete ideas about the atom.

John Dalton – solid sphere model

Four points to his theory in 18051. All matter is composed of atoms that are

indestructible and indivisible

2. Atoms of one element are all the same; atoms of a different element are different (Au atoms are different from Ag atoms)

3. Atoms cannot be changed into different atoms by chemical of physical changes (Pb atoms cannot be turned into Au atoms)

4. All atoms are like Billiard Balls – a solid, sphere

5. Atoms react in whole numbers

Joseph John Thomson was born in

Cheetham Hill, a suburb of Manchester

on December 18, 1856.

He is given credit for the discovery of the

electron (particles with a negative

charge).

Guess What?

Atoms are not the smallest particles

JJ Thomson

• Conducted experiment with a

Cathode Ray Tube

• Model = plum pudding

• When: 1897

• Where: England

Cathode-ray Tubes

JJ Thomson – His Experiment 1897

watch JJ Thomson’s cathode ray tube

experiment

Quick Facts:

Born: December 18,1856 Cheetham Hill Manchester, UK

Died: August 30, 1940 Cambridge UK

Nationality: British

Fields: Physics

Institutions: Cambridge University

Known for: Plum pudding model, discovery of electron,

discovery of isotopes

Awards: Nobel Prize for Physics 1906

JJ Thomson – His Experiment

Cathode Ray Tube

Thomson’s Plum

pudding model

The atomic model before his work was thought to of had resembled a

billiard ball a small, solid sphere. Thomson’s work suggested that it

wasn’t one particle but a “jigsaw puzzle” of smaller pieces.

Thomson discovered the electron by using a cathode-ray tube, He

interpreted that the deflection of the rays by electrically charged plates

and magnets as evidence of "bodies much smaller than atoms“. All

gases contained these particles and the charge: mass ratio was the

same for all gases and metals used.

In 1904 he suggested a model (By which some called the “Plum

pudding model”) of the atom as a sphere of positive matter in which

electrons are positioned by electrostatic forces. They called it the Plum

pudding model because the pudding was the atom and the bits of

plum were the electrons positioned there.

Plum Pudding Model

JJ Thomson

3 Points to his theory

1. * All gases had the

same mass: charge

ratio

2. * All atoms contain

small, charged

particles called

electrons

3. * The atom now looked

like “Plum Pudding”

Goldstein

Eugene Goldstein was born in 1850 in

Poland

Died December 25, 1930 in Berlin

Germany

Goldstein's with cathode-ray and

worked with anode rays. He observed

particles moving toward the cathode

(negative terminal) to suggested the

presence of the proton -- a positively

charged particle.

Goldstein – His Experiment 1886-

1887

Cathode Ray Tube

Eugene Goldstein discovered positive particles by using a tube filled with

hydrogen gas (this tube was similar to Thomson's tube). This resulted in The

positive particle had a charge equal and opposite to the electron. The positive

particle was named the proton.

Goldstein

Major Point to his theory

He is given credit for the discovery of a positively

charged particle = Proton

The atom was still thought to resemble

only

Only now there are also positive charges

in the Plum Pudding

Robert A. Millikan

Born March 22, 1868 Morrison

Illinois, USA

Died December 19, 1953 (Age 85)

Attended Oberlin College 1891 and

Columbia University in 1895

He discovered from his

measurements and Thomson’s

charge: mass ratio the charge of an

electron and its mass.

Millikan received the 1932 Nobel

Prize in Physics for this discovery.

Watch Millikan's oil drop

experiment

Millikan’s 1910 Oil Drop

Experiment

Millikan’s Experiment

Millikan discovered the mass of an electron by introducing charged oil

droplets into an electrically charged field. The charge of the electron

was found to be 1.602E-19 coulombs. Using Thomson's mass ration,

Millikan found the mass of one electron to be 9.11E-28 grams.

In his Oil Drop experiment he took a atomizer from a perfume bottle

and sprayed drops of oil or water into the sample chamber, Then

some droplets fell threw a pinhole and ended up between two plates.

One of the plates were positive and the other was negative. The

middle chamber was ionized by x-rays. Particles that did not capture

any electrons fell to the bottom plate due to gravity. The others that

picked up an electron would stick to the upper plate or fall noticeably

slower.

Millikan

2 Points to his theory

1. All electrons have the same mass

About 1/2000 atomic mass unit

2. All electrons have the same charge

-1.6 x 10 -19 Coulomb

No new model - The atom was still thought to resemble

Plum Pudding

Rutherford

Ernest Rutherford was born

August 30, 1871 in Nelson,

New Zealand

Died: October 19, 1937 in

Cambridge, England

Proved nucleus is dense,

positively charged core of

atom, in 1911

Won Nobel Prize in

Chemistry, 1908

Rutherford’s 1910 Gold Foil

Experiment

watch Rutherfords gold foil experiment

Results of foil

experiment if

Plum Pudding

model had been

correct

What Actually Happened

Rutherford’s observations and

conclusions

3 Points to his theory

1. * Most alpha particle went through = Atom is mostly empty space (area of electrons)

2. * Some alpha particles bounced straight back = atom has a solid mass (nucleus)

3. * Some alpha particles reflected= nucleus is positively charged

JJ Thomson’s Plum pudding model thrown out

New Model = Nuclear Model (stationary planetary model)

Gold Foil Experiment

Observations and Conclusions:

Most of the alpha particle went straight through with no deflection – the atom is mostly empty space

A few alpha particles bounced straight back – The atom contains a small concentrated area that contains most of the mass (nucleus)

Some alpha particles were deflected at slight angles – the nucleus contained a positive charge which repelled the alpha particle as it came close

New model: Nuclear Model

Schrödinger

Erwin Schrödinger

born August 12 in

Vienna, Austria in

1887.

Died: January 4,

1961 in Vienna

Fields: Physics

Won Nobel Prize in

Physics in 1933

video of Schrodinger's cat

Schrödinger

Schrödinger hypothesized that the cat can exist as both alive and dead until it is

observed. Once it is observed, the characteristics of the cat have changed.

He also believed this to be true of the motion of the electrons.

Schrödinger

Schrödinger used wave mechanics to define the probability regions (Orbitals) of finding an electron of a given energy.

Once we measure the electron, some of its characteristics must be changed.

New Model: Quantum Mechanical Model

Schrödinger

Major points

1. Electrons do not follow fixed paths

2. They move randomly in areas of probability (orbitals)

3. There are specific energies associated with each orbital

New Model =

Quantum Mechanical Model

James Chadwick

Chadwick was born in Cheshire,

England, on 20th October, 1891.

Died: July 24, 1974 in

Cambridge, England

Discovered the neutron

Chadwick’s discovery allowed

the creation of elements heavier

than uranium in the laboratory.

Won Nobel Prize in Physics in

1935

Chadwick – His 1932 Experiment

Experiment:

1. Bombarded a Be atom with large alpha particles

2. This caused an unknown “radiation” to be emitted. (many

thought it was a gamma ray – which has no charge and

no mass)

3. The unknown radiation was allow to bombard paraffin

wax.

4. This dislodged protons into a detector.

Chadwick

2 Points to his theory

1. Discovered the Neutron – Same mass

as a proton, but without a charge

2. Mass could be converted into energy

Model was still thought to resemble the Quantum

Mechanical Model

Importance of Chadwick’s

1932 Discovery

Conclusions:

1. The unknown “radiation” did not carry a charge

and was not deflected by magnetic fields

2. It could not be gamma rays because the particle

contained enough mass to dislodge a proton.

3. New particle – Neutron – no charge; mass ~ the

same as a Proton.

No new model: Quantum Mechanical Model