This document summarizes the history of the atomic model from ancient Greek philosophers to modern particle physics. It describes how Democritus first proposed the idea of indivisible atoms in 460 BC. In the early 1900s, J.J. Thomson's plum pudding model depicted atoms as electrons scattered within a uniform positive charge. Rutherford's gold foil experiment in 1910 showed that the positive charge and most of the mass are concentrated in a tiny nucleus. Neutrons were discovered by Chadwick in 1932. Modern atomic structure consists of a small, dense nucleus of protons and neutrons surrounded by electrons.

1. Atomic Structure
Derek Murphy

2. HISTORY OF THE ATOM
460 BC Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMA
(greek for indivisible)

3. HISTORY OF THE ATOM
1808 John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS

4. Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small
particles called atoms. All atoms of a given
element are identical. The atoms of one element
are different from the atoms of all other elements.
2. Compounds are composed of atoms of more
than one element. The relative number of atoms
of each element in a given compound is always
the same. Law of Multiple Proportions
3. Chemical reactions only involve the rearrangement
of atoms. Atoms are not created or destroyed in
chemical reactions. Law of Conservation of Mass
2.1

5. 2
2.1

6. 8 X2Y16 X 8 Y+
2.1

7. History of the Atom
William Crookes
(1832-1919)
1875: Identified a
negatively charged
radiation emmiting
from the cathode:
cathode rays.

8. HISTORY OF THE ATOM
1898 Joseph John Thompson
Demonstrated that cathode rays are
negatively charged particles
ELECTRON

9. Cathode Rays are streams of negatively charged particles: electrons
Cathode ray particles are attracted toward positive plate.

10. Ratio of Charge to Mass
Next, Thompson measured how much electrons are deflected by a magnetic
field and compared this with the electric deflection.
He found that the mass to charge ratio was over a thousand times lower than
that of a hydrogen ion (H+), suggesting either that the particles were very light
and/or very highly charged.
charge of electron
mass of electron
= 1.76 x 1011 coulombs per kg
Units for quantity of charge

12. Robert Millikan’s Oil Drop Experiment
Determined charge on the electron: 1.6 x 10-19 coulomb
charge of electron
mass of electron
= 1.76 x 1011 coulombs per kg
Mass of electron = 9.1 x 10-31 kgCalculate Mass of electron

13. http://www.youtube.com/watch?v=XMfYHag7Liw

14. HISTORY OF THE ATOM
Thompson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere surrounded
by a soup of positive charge to balance the electron's charge
1904
like plums surrounded by pudding.
PLUM PUDDING
MODEL

15. HISTORY OF THE ATOM
1910 Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them
passed through. About 1 in 10,000 hit
something

16. Atoms’ positive charge is concentrated in a nucleus 2.2
The Rutherford experiment
“It was about as credible as if you had fired a 15-inch [artillery] shell
at a piece of paper and it came back and hit you.” -- Rutherford

17. Expected Result based on
Thompson Model of Atom.
Model to explain observed
results.
Nucleus of Atom containing positively charged proton(s)
The only way to account for the observations was to conclude that all of the
positive charge and most of the mass of the atom are concentrated in a very small
region. Rutherford called this tiny atomic core the nucleus.
Positive charge, mass = 1.673 x 10-24 g (1800 x electron)

18. nuclear radius of gold atom = 1 x 10-13 cm
Rutherford’s Model of
the Atom
This makes the nucleus about 10,000 times smaller than the atom.

19. Chadwick’s Experiment (1932)
Detected non-charged particle with enough
mass to displace proton --- Neutron!
+ive
-ive
No charge, mass = 1.679 x 10-24 g

20. Subatomic Particles
• Protons and neutrons make up the nucleus, providing
most of the atom’s mass; the protons provide all of its
positive charge.
• The nuclear radius is approximately 10,000 times
smaller than the radius of the entire atom.
• Negatively charged electrons outside the nucleus
occupy most of the volume of the atom, but contribute
very little mass.
• A neutral atom has no net electrical charge because
the number of electrons outside the nucleus equals the
number of protons inside the nucleus.

21. Properties of Subatomic Particles
Relative Charge Relative Mass Location
Proton +1 1 Nucleus
Neutron 0 1 Nucleus
Electron -1 1/1838 Outside Nucleus