Scientist that changed our view of the atom

Evolution of Atomic Theory

1. Evolution of Atomic
Theory
Scientists that changed our
view of the atom
Mrs. Baldessari
Chemistry
?Greatest Chemistry Discoveries?? –YouTube

2. What: All matter is a combo of fire, air, earth or water
When: 350 B.C.
Where: Greece
Aristotle

3. 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

4. Democritus
•Did not conduct any
experiments
•All matter is made of
(atomos = indivisible)
atoms
•Atoms are indivisible
and indestructible
•ATOMIC STRUCTURE

5. 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.

6. John Dalton
John Daltonwas born on
September 6, 1766 in Cumbria,
England.
He died on July 27, 1844
Daltonwas a chemist,
meterologist, and physicist.
Attended college in Manchester.
He provided us with the first
concrete ideas about the atom.

7. John Dalton –solid sphere model
Four points to his theoryin 1805
1.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

8. Joseph John Thomsonwas 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

9. JJ Thomson
•Conducted experiment with a
Cathode Ray Tube
•Model = plum pudding
•When: 1897
•Where: England

10. Cathode-ray Tubes

11. 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

12. JJ Thomson –His Experiment
Cathode Ray Tube

13. 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

14. 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”

15. Goldstein
Eugene Goldsteinwas 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.

16. 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.

17. 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

18. 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.

19. Watch Millikan's oil drop
experiment
Millikan’s 1910 Oil Drop
Experiment

20. 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.

21. 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

22. Rutherford
Ernest Rutherfordwas 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

23. Rutherford’s 1910 Gold Foil
Experiment
watch Rutherfords gold foil experiment

24. Results of foil
experiment if
Plum Pudding
model had been
correct

25. What Actually Happened

26. 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)

27. 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

28. 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

29. 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.

30. 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

31. 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

32. James Chadwick
Chadwickwas 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

33. 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.

34. 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

35. 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