Transcript History of the Atomic Model

3.1 Early Atomic Theory
Early Greek Theories
Aristotle (350 B.C.)
• 4 Elements
Democritus (400 B.C)
• Atoms and a void (empty space)
• Atoms are indivisible
Dalton’s Billiard Ball Model (1805)
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All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in
constant ratios to form compounds.
• Atoms are rearranged in reactions, but are
not created nor destroyed.
John Dalton (1766-1844)
English chemist, physicist
• Matter is composed of
indestructible, indivisible
atoms
Thomson’s Blueberry Muffin Model
(1897)
• Materials, when rubbed, can develop a charge
difference.
• This electricity was called “cathode rays”
• These rays have a small mass and are
negatively charged.
• Thomson noted that these negative subatomic
particles (electrons) were a fundamental part of
all atoms.
JJ Thomson
(1856-1940)
English
physicist
Rutherford’s Nuclear Model
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through. So,
atoms are mostly empty.
Some positive -particles deflected
or bounced back!
Thus, a “nucleus” is positive
(protons) & holds most of an atom’s
mass.
Rutherford Experiment Animation
Ernest Rutherford
(1871-1937)
British physicist
Summary of Rutherford’s Nuclear Model
1. The atom has a tiny, dense, positively
charged centre called the nucleus. The
protons are found in the nucleus.
2. Electrons move around the nucleus similar
to planets orbiting around the Sun (also
called “Planetary Model of the Atom”)
Limitations to Rutherford’s Model
• According to electromagnetic theory, orbiting
electrons (accelerating charged particles)
should emit radiation, lose energy and therefore
slow down
• This slowing down in a spiral path should cause
the electrons to collapse into the nucleus
• However most atoms are very stable and will
not self-collapse
Exploring Radioactivity
Radioactivity: the spontaneous decay of the
nucleus of an atom
Rutherford’s Contribution:
- First proposed that radioactivity results
from disintegration of atoms
- Discovered the alpha particle and named
the beta particle and gamma ray
3 Types of Radioactive Emissions
Isotopes
Isotopes: atoms with the same number of protons but
different number of neutrons
Ex. Carbon-12 vs Carbon-14
• C-12 is stable and C-14 is unstable (too many neutrons
in nucleus)
Radioisotope: isotope that emits radioactive gamma rays
or subatomic particles (Ex. C-14)
• Scientists use radioisotopes in many applications like
carbon dating, nuclear energy and medicine
The Nature of Matter and Energy
Before 1900:
Classical theory of physics (Newtonian physics)
• Describes behaviour of physical objects (bigger
than atomic level)
After 1900:
Quantum Theory
• Born out of the inability of classical theory to
explain how all matter behaves
• Takes into account atomic particles, wave theory,
quanta and radiation
.
Key Breakthroughs in Quantum Theory
Photoelectric Effect: when light shines upon a metal, it will
emit electrons (called photoelectrons)
Max Planck (1858-1947, German physicist)
• Initial founder of Quantum theory
• Postulated that light is emitted in discrete packets of energy
called quanta
Albert Einstein (1879-1955, German physicist)
• Photon: unit of light energy
• Proposed that during the photoelectric effect, an electron
was emitted from the metal because a photon collided with
the electron (thus transferring its kinetic energy)
• To free an electron from an atom requires at least 1 photon
Homework
Page 142 #2, 6, 8