Our understanding of atomic structure has changed significantly over time. Originally, Dalton proposed that atoms were indivisible spheres (1808). Later, experiments revealed atoms have internal structure including a small, dense nucleus and subatomic particles like electrons, protons, and neutrons. The modern atomic model depicts atoms as a small, positively charged nucleus surrounded by negatively charged electrons.

1. Honors Nuclear Radiation
 In your notebook, write a paragraph
describing how our understanding of the
atom has changed over time.

2. Getting Inside the Atom
Atomic Structure

3. Dalton’s Atomic Theory
in 1808
 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 subdivided, created, or destroyed.

4. 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.

6. Modern Atomic Theory
 Not all aspects of Dalton’s 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.
• Atoms of any one element differ in properties from
atoms of another element.
• All matter is composed of atoms.

7. Discovery of the Subatomic
Particles
 For the better part of 70 years Dalton’s ideas remained unchanged.
 In 1897, using Cathode ray tubes, J.J. Thomson reported that
cathode rays were made up of negatively charged particles in
motion.

8. Figure 3.7: Schematic of a cathode ray tube.

10. Plum Pudding model of an atom.

11. Discovery of the Atomic
Nucleus
 More detail of the atom’s 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.

12. Figure 3.5: Rutherford’s
experiment.

14. Figure 3.6: Results of foil experiment if Plum
Pudding model had been correct.

15. Figure 3.6: Actual Results.

16. Discovery of the Proton
 1919
 Ernest Rutherford

18. Discovery of the neutron
 1932
 James Chadwick

20. The Structure of the Atom
An atom is the smallest particle of an element
that retains the chemical properties of that
element.
The nucleus is a very small region located at the
center of an atom.
The nucleus is made up of at least one positively
charged particle called a proton and usually one
or more neutral particles called neutrons.

21. Fundamental Particles
(The Standard Model)

23. Part I
Fundamental Particle – The Meaning

25. It is understood that the universe is
composed of particles.

26. Currently, the leptons and quarks appear
to qualify as the true
fundamental/elementary particles;
meaning without structure.

27. The organization of known fundamental
particles – The Standard Model

29. “The standard model of particle physics
is a theory which [currently] describes
three of four known fundamental
interactions between the elementary
particles that make up all matter…

30. It is a quantum field theory developed
between 1970 and 1973 which is
consistent with both quantum mechanics
and special relativity.”

31. “…the standard model falls short of
being a complete theory of fundamental
interaction, primarily because of it’s lack
of inclusion of gravity, the fourth known
fundamental interaction.”

32. The particle nature of the standard
model consists of two groups:
- Matter Particles
- Force-Mediating Particles

33. Matter Particles

36. The matter component of the Standard
Model is comprised of twelve particles.

38. These particles all have an intrinsic spin
value of ½, making them conform to the
Pauli Exclusion Principle.

40. All matter particles of the Standard Model also
have corresponding antimatter particles. These
particles breakdown into groups of quarks (up,
down, strange, charm, top, and bottom) and
leptons (electron, muon, tau, and
corresponding neutrinos).
Quarks and leptons are further grouped into
sets known as generations.

41. Quarks carry color charges (red, blue, or
green) so they participate in strong
interactions.

42. The up, charm, and top quarks carry the
electric charge (+2/3).
The down, strange, and bottom quarks
carry the electric charge (-1/3).
This allows the quarks to participate in
electromagnetic interaction.

43. Leptons are color neutral and do not
participate in strong interaction.

44. The electron, muon, and tau particles
carry the electric charge (-1) and
participate electromagnetic interaction.

45. Neutrinos have no electric charge and do
not participate in electromagnetic
interactions.

46. Quarks and leptons carry flavor charges
and participate in weak nuclear
interactions.

48. Force-Mediating Particles

51. Force-mediating particles of the
Standard Model group into three
categories that correspond with three of
the four fundamental interactions.
All three mediating particles are bosons
and have intrinsic spins of (1).

53. This allows these particles to not conform
to the Pauli Exclusion Principle.

54. Photons are electromagnetic force
mediators involving charge particles.
Photons are considered massless
particles.

55. W+, W-, and Zo gauge bosons are weak
nuclear mediators involving particles of
different flavors (quarks and leptons).
W+, W-, and Z0 bosons are massive
particles.

56. Gluons are strong nuclear force-
mediators involving color charged
particles (quarks).
Gluons are considered massless particles.

57. Part III
The Higgs particle – The Challenge

59. The Standard Model predicts the
existence of one more particle known as
the “Higgs boson.”
The Large Hadron Collider (LHC) at
CERN is hoped to confirm the existence
of the Higgs boson in the near future.

60. Quarks
• Protons and neutrons are composed of quarks.
• There are six different types of quarks.
u up (+2/3)
d down (-1/3)
t top (truth)
b bottom (beauty)
c charm
s strange

61. Atomic Structure
u
d u
u
d d