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Phy 102: Fundamentals of
Physics II
Chapter 11: The Atomic Nature of Matter
The Atomic Hypothesis
• All matter is made up of small invisible particles
called atoms
• Atoms can combine to form larger blocks of
matter called molecules
• Atoms have 2 primary regions:
– The nucleus
– The electron cloud
• The atomic nucleus contains protons & neutrons
• The electrons are held to nucleus by the electric
force (attractive) between the protons & the
electrons
History of the Atomic Hypothesis
• Democritus proposes the concept of “atoms”
• Dalton proposes his “atomic theory of matter” &
re-introduces the concept of the atom
• Rutherford introduces the nuclear atom hypothesis
• Bohr proposes the “planetary” model of the atom
• De Broglie suggests that electrons can act like
waves
– leads to the wave theory of the electron cloud
• Schrodinger & Heisenberg simultaneously work
out the foundations for the quantum theory of the
atom (referred to as quantum mechanics)
• Gell-Mann proposes the quark hypothesis
John Dalton (1776-1844)
• English Chemist
• Considered by many to be the
“Father of Modern Chemistry”
• Re-introduced the atomic hypothesis of matter
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Matter is never created nor destroyed
The basic units of matter are called atoms
Atoms cannot be broken down into smaller units
Substances composed of only one type of atom are called
elements
– Atoms of a given element are identical
– Atoms of different elements have different masses
– Atoms may combine in defined whole-number ratios to form
molecules
Ernest Rutherford (1871-1937)
• Celebrated experimental physicist
• Considered the “Father of Nuclear
Physics”
• Nobel Prize in Chemistry (1908)
• Guided many of his students to win
Nobel Prizes
• Contributions included:
– Proposed the model of the nuclear atom
– Explained radioactivity as the disintegration
of atoms
– The first to split the atom (O to N)
– Invented the 1st method to measure
radioactive particles by electrical means
– Helped invent the modern smoke detector
Neils Bohr (1885-1962)
• Danish physicist
• Won the Nobel Prize (1922) for his work
on the structure of atoms
• Proposed a “planetary” model of the atom,
referred to as the Bohr Model
• Worked on the Manhattan Project at Los
Alamos
• Later in life promoted the peaceful use of
nuclear energy
Murray Gell-Mann (1929- )
• Won the Nobel Prize (1969) for his
contributions to elementary particle
physics (the quark theory)
• Numerous contributions to the field of
elementary particle physics including:
– Classification of elementary particles & their
interaction
– Proposed the existence of “quarks”
– Developed the following theories:
• Strangeness Theory”
• Eightfold Way Theory
• Quantum Chromodynamics
• Considered by many to be “the greatest
living theoretical physicist”
Elements
• Substances made of only one type of atom
• Matter that cannot be broken down into new
substances
• Atoms of a particular element have the same # of
protons
Atomic Number = # of Protons
• Atoms of a particular element can have different #
of neutrons
Atomic Mass = (# of Protons) + (# of Neutrons)
The Electron
• First discovered by J.J. Thomson (1897)
• Small negatively charged “particle”
– Mass of electron = 9.11x10-31 kg
– Charge of electron = -1.60x10-19 C
• Determine for the chemical properties of a
substance
• Form a “cloud” around the nucleus of the atom
• Establish the volume of an atom
• Flow of electrons in conducting material is
referred to as “electrical current” (or electricity)
The Atomic Nucleus
• The “nuclear atom” hypothesis proposed by
Earnest Rutherford (1909)
• Dense, positively charged “core” of the
atom
• Contains protons & neutrons
• Determine the mass of the atom
The Proton
• First observed by Eugene Goldstein (1869)
– He referred to them in his experiments as “canal rays”
• “heavy” positive charged particle
– About 2000 times as massive as an electron
• Mass of Proton = 1.67x10-27 kg
– Same magnitude of charge as electron (opposite sign)
• Charge of Proton = +1.60x10-19 C
• Number of protons in the nucleus is unique for
each element
• # of protons is the same as the # of electrons (except
for ions)
The Neutron
• Discovered by James Chadwick (1932)
• “heavy” neutral charge particle
– Mass slightly larger than proton
– Mass of Neutron = 1.67x10-27 kg
• Provide the “glue” that holds the protons
together in the nucleus
• Atoms of the same element with different
amounts of neutrons are called isotopes
The Periodic Table
• The master “menu” of matter
• First proposed by Dmitri Mendeleev
• All known elements are organized by
chemical properties
• There are currently ~120 elements have
been identified
• Columns are called Groups
• Rows are called Periods
Quarks
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First proposed by Murray Gell-Mann (1963)
Along with Leptons (electrons, neutrinos,…) form the
“fundamental particles”
Combine in combinations to form protons &
neutrons
Two Classes of quarks
Charge = -1/3
• Down
• Strange
• Bottom (Beauty)
Charge = +2/3
• Up
• Charmed
• Top (Truth)
Elements, Compounds &
Mixtures
• Matter can be classified as
– Pure substances (elements & compounds)
– Mixtures (homogeneous & heterogeneous)
• Atoms of elements combine to form
compounds
• Molecules are the basic units of compounds