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Glencoe: Chapter 4
The Structure of the Atom
Section One: Early Ideas about
Matter
 Atomists and Democritus : 400 B.C. From
Thrace in Greece.
 Atoms- “Uncut-Table”
Indivisible parts which cannot be broken down
further.
 Aristotle proposed that matter was
continuous and was not made up of smaller
particles.
 Fall of the Roman Empire; Dark Ages
Medieval Period or the Middle Ages (4001400).
 Accepted until the 17th century (1600’s).
Why?
Renaissance
 Then came Isaac Newton and Robert Boyle.
 Robert Boyle : proved gas pressure and
volume were related mathematically.
John Dalton
• logical hypothesis in 1800’s.
Antoine Lavoisier
French Chemist that discovered elemental
oxygen
Conservation of mass
In ordinary chemical reactions, matter can be
changed in many ways but it cannot be
created or destroyed.
Law of definite proportions
• Joseph Proust
specific substances always contain elements
in the same ratio by mass.
Law of Multiple Proportions
Based on atomic theory but no experiment
evidence at the time
• The ratio of the masses of one element that
combine with a constant mass of another
element can be expressed in small whole
numbers.
Dalton’s Hypothesis
What did Lavoisier and Proust finding explain?
Basis for atomic theory.
• All matter is composed of extreme small particles
called atoms, which cannot be subdivided, created, or
destroyed.
• Atoms of a given element are identical in their
physical and chemical properties.
• Atoms of different elements differ in their physical
and chemical properties.
• Atoms of different elements combine in simple
whole-number ratios to form compounds.
• In chemical reactions, atoms are combined,
separated, or rearranged but never created,
destroyed, or changed.
Glencoe Chemistry: Chapter 4 Section Two
William Crooke (1890’s)
Used a cathode ray tube. An apparatus
that helped discover the electron.
• Electrode :
metal piece in a cathode ray tube
• Anode : positive electrode
• Cathode : Negative electrode
Cathode rays
begin at the cathode and traveled
toward the anode.
J.J. Thomson (English) :
• Researched with cathode rays and was
credited with the discovery that
cathode rays were actually electrons.
• He was able to determine the ratio of an
e- charge to its mass.
Robert Millikan :
• Used an oil drop experiment to obtain
the first accurate measurements of an echarge.
• Standard unit of negative charge (-1)
symbol e-
J. J. Thomson :
• Found rays traveling in opposite direction of
cathode rays. He showed these particles
possessed a positive charge (+ charge).
• Credited with discovery of proton (+ charge).
It was a mass of 1836 times the mass of an
e-.
• 1920 ; Lord Rutherford (English) predicted a
3rd particle.
• 1930 ; Walter Bothe - 1st evidence of this
3rd particle.
• James Chadwick in 1932 found a high
energy particle with no charge and the
same mass of a proton; neutron.
• The discovery of the neutron changed
John Dalton’s atomic Theory.
• The Gold Foil Experiment
Page 112 Figure 4.12 & 4.13
• Subjecting a very thin sheet of Gold Foil to
a stream of positively charged subatomic
particles.
• Found out that most of the particles
passed through the sheet concluded that
the atom was mostly empty space.
 Also found that a few particles
were deflected. Rutherford
explained the atom was
composed of a small “core”
containing all the positive
charged and almost all of the
mass, called the nucleus.
 The nucleus occupies about a
trillionth of the volume of an atom
(10-12).
Hydrogen nucleus is the size of a
ping-pong ball. The electron is
roughly the size of a tennis ball,
about 1.35 km away.
• Atoms of the same element that differ
in mass are called Isotopes.
# Protons
# Neutrons
Neon-20
10
10
Neon-21
10
11
Neon-22
10
12
Henry Moseley (1913)
Studied x-rays and found that the
wavelength of the x-rays depend on
the number of protons in the nucleus
and always the same for a given
element.
 Atomic number (Z) = number of
protons.
Number of protons determines the
identity of the element.
 Nuclide
a particular kind of atom containing a
definite number of protons and
neutrons.
 Mass number - Atomic number (Z)
= number of Neutrons
Isotopes of Hydrogen
#Protons #Neutrons Mass#
Protium
Deuterium
Tritium
The Nuclear Atom
Under Rutherford’s direction of Bohr,
Geiger, and Marsden ; these scientists
conducted experiments that are the basis
for modern concept of atomic structure.
Radioactivity
Becquerel (1896) – Found that matter
containing uranium exposes photographic film.
Marie Curie and Pierre Curie found that rays
were given off by the elements U and Ra
Radioactivity is the phenomenon of “rays”
beings produced spontaneously by unstable
atomic nuclei.
1) Alpha particle: Helium nucleus consisted of
2p + 2n.
2) Beta particles – high speed electron released
from a radioactive nuclei. β
3) Gamma Rays - high energy x-ray. γ
Unstable nuclei that emit rays or particles to
become stable are said to decay.
Page 648 and Page 651.