1. Discovering the Atom

2. Greek Model Democritus Greek philosopher Idea of ‘democracy’
“To understand the very large,
we must understand the very small.”
Democritus
Greek philosopher
(460 B.C B.C.)
Idea of ‘democracy’
Idea of ‘atomos’
Atomos = ‘indivisible’
‘Atom’ is derived
No experiments to support idea
Democritus’ model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE

3. Dalton’s Atomic Theory 1803
1. All matter is made of tiny indivisible particles called atoms.
2. Atoms of the same element are identical, those of different atoms are different.
3. Atoms of different elements combine in whole number ratios to form compounds
4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

4. Radioactivity
One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie ( ).
She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

5. Thomson
1897 – discovered the electron using a cathode-ray tube

6. Cathode-ray Tube Animation

7. Thomson’s Plum Pudding Model of an Atom

8. Millikan http://www.youtube.com/watch?v=XMfYHag7Liw
Determined the charge of an electron
Oil Drop Experiment

9. Rutherford’s Gold Foil Experiment in 1909

10. Rutherford’s Gold Foil Exp

11. Results of foil experiment if plum pudding model had been correct

12. Actual results of foil experiment
Rutherford’s conclusion: atoms are mostly empty space with a positive nucleus

13. Bohr Model of Atom
Increasing energy
of orbits
n = 3 e-
n = 2
n = 1 e- e-
A photon is emitted
with energy E = hf
Bohr proposed that electrons are in specific circular orbits with specific energy levels around the nucleus

14. Electrons as Waves Louis de Broglie (1924)
Applied wave-particle theory to electrons
electrons exhibit wave properties
Fundamental mode
Second Harmonic or First Overtone
Standing Wave
200
150
100 50
- 50
-100
-150
-200
200
150
100 50
- 50
-100
-150
-200
200
150
100 50
- 50
-100
-150
-200

15. Quantum Mechanics Heisenberg Uncertainty Principle
Impossible to know both the velocity and position of an electron at the same time g
Microscope
Electron

16. Quantum Mechanics Schrödinger Wave Equation (1926)
finite # of solutions  quantized energy levels
defines probability of finding an electron
Courtesy Christy Johannesson

17. Quantum Mechanics Orbital (“electron cloud”)
Region in space where there is 90% probability of finding an electron
90% probability of
finding the electron
Orbital
Electron Probability vs. Distance 40 30
Electron Probability (%) 20 10 50
100
150
200
250
Distance from the Nucleus (pm)
Courtesy Christy Johannesson

18. Quantum Mechanical Model
Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

19. Confirmed existence of neutron
Chadwick
Confirmed existence of neutron
3min

20. Modern View The atom is mostly empty space Two regions Nucleus
protons and neutrons
Electron cloud
region where you might find an electron

21. Models of the Atom - Greek model (400 B.C.) Thomson’s plum-pudding
"In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg + - e +
Greek model
(400 B.C.)
Dalton’s model
(1803)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

22. Models of the Atom - Dalton’s model (1803) Greek model (400 B.C.)+ + -
Dalton’s model
(1803)
Greek model
(400 B.C.)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1897 J.J. Thomson, a British
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
1911 New Zealander
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1926 Erwin Schrodinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
1924 Frenchman Louis
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
1932 James
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

23. + - e + A
Dalton’s model B C D E