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1.5: The History of
Atomic Structure
GPS Standards:
SC3. Students will use the modern atomic theory to explain the characteristics of atoms.
a. Discriminate between the size, charge, and position of protons, neutrons, and electrons in the atom.
f. Relate light emission and the movement of electrons to element identification
9/19/13
Part I: The Road to Modern Atomic Theory
• Early Theories - 400 B.C.
• common Greek theory was that all matter
consisted of four "elements"—earth, air,
fire, and water.
• Democritus (460-360 BC) = a philosopher
who theorized that all matter was made
of indivisible particles (atoms).
Democritus
• his theory was based on logical reasoning, not data.
• philosophers/scientists of this time period did not think
doing experiments was necessary—you could reach the
truth by pure logical reasoning.
• alchemy (the process of changing base metals to gold) was
the main form of experimentation from this time period until the
late 1600's.
• alchemy (the process of changing base metals to gold) was
the main form of experimentation from this time period until the
late 1600's.
• Modern Era Theories/Laws—1600’s – early 1800’s
• Robert Boyle, in the mid-1600’s, defined
“element” as a substance that cannot be broken
down into simpler substances.
• Antoine Lavoisier – mid 1700’s – considered
to be the pioneer of modern chemistry.
• used the scientific method and
Boyle
did careful, controlled experiments
involving the combustion of metals.
• law of conservation of mass: mass is neither
created nor destroyed in an average chem. rxtn.
(this means burning, rusting, etc.  destroying
matter)
Lavoisier
• law of conservation of mass: mass is neither created nor
destroyed in an average chem. rxtn. (this means burning,
rusting, etc.  destroying matter)
• Joseph Proust—published Law of Definite
Proportions in 1794:
• a chemical compound contains the same
elements in exactly the same proportions
(by mass) regardless of the size or source
of the sample. (this means water = H2O
Proust
wherever you get it, however much you have)
• law of multiple proportions: picture two different compounds
with the same two elements in them—i.e. carbon monoxide,
carbon dioxide—these compounds will have the same small,
whole number ratio in their required masses as they do in
their chemical formulas.
• law of multiple proportions: picture two different compounds
with the same two elements in them—i.e. carbon monoxide,
carbon dioxide—these compounds will have the same small,
whole number ratio in their required masses as they do in
their chemical formulas.
CO = 1 oxygen,
CO2 = 2 oxygens,
1:2 ratio in oxygen atoms
CO = 16g of oxygen,
CO2 = 32 g of oxygen,
1:2 ratio in oxygen grams
Part II: Early Atomic Models
• Dalton, John—“Solid marble” model
• English schoolteacher (1808), proposed
explanation for the 3 laws above
• thought elements were composed of
atoms, which were indivisible
Dalton
Part II: Early Atomic Models
• Dalton, John—“Solid marble” model
• English schoolteacher (1808), proposed
explanation for the 3 laws above
• thought elements were composed of
atoms, which were indivisible
Dalton
• Dalton’s Atomic Theory: (five parts)
1. all matter is composed of very tiny particles called atoms
2. atoms of a given element are identical in size/mass/other
properties; other elements’ atoms are different
3. atoms cannot be subdivided, created, or destroyed
4. in chem rxtn, atoms are combined/separated/rearranged
5. atoms of diff. elements combine in small, whole-# ratios
to form compounds
• “black box” model of the atom. Even after 2000 years of the
atomic concept, the structure of the atom was still unknown.
• “black box” model of the atom. Even after 2000 years of the
atomic concept, the structure of the atom was still unknown.
• at this point, the atom was still thought to be the smallest
unit of matter.
• Thomson, Joseph John—“Plum Pudding”
model (or “cookie dough”)
• English physicist (1897), performed
cathode-ray tube experiments
• discovered the electron and its charge
Thomson
• cathode-ray tube: glass vacuum tube (attached to a voltage
source) through which electricity flows, producing a glow.
• electrons travel towards anode (opposite charges attract)
• paddle wheel placement shows that particles were
moving, had mass, and had a negative charge
• cathode-ray tube: glass vacuum tube (attached to a voltage
source) through which electricity flows, producing a glow.
• electrons travel towards anode (opposite charges attract)
• paddle wheel placement shows that particles were
moving, had mass, and had a negative charge
• this model shows the electrons on the outside of the
positively-charged atom (if there were negative parts to
atoms, then there must be positive parts as well to balance
it, because most matter is neutral).
• this model shows the electrons on the outside of the
positively-charged atom (if there were negative parts to
atoms, then there must be positive parts as well to balance
it, because most matter is neutral).
• cathode rays were not like ordinary light—they could be
deflected by electric or magnetic fields, and they could
pass through thin metal foils, since they were so small.
• cathode rays were not like ordinary light—they could be
deflected by electric or magnetic fields, and they could
pass through thin metal foils, since they were so small.
• Millikan, Robert A.—see Thomson model
• American physicist (1909), performed
experiments that determined the size of
electron to be about 1/1837 the mass of
the hydrogen atom, or about 9.109 x 10-31 kg.
Millikan
• because atoms are electrically neutral, they must contain
some + charge to offset all the - charges from the e-.
• because e- have so much less mass than the total atom
itself, there must be other, much heavier particles make
up most of the mass.
• Rutherford, Ernest—“Electron cloud, positive
nucleus” model
• New Zealander, along with Hans Geiger
and Earnest Marsden (1911) performed the
gold-foil experiment, leading to the discovery
of the positively-charged nucleus.
• experiment = bombard a thin gold foil with
Rutherford
alpha-particles (He+).
• they did this assuming that the
charge and mass within each
atom was uniformly distributed
throughout the atom, and
expected the alpha particles to go
right through.
• most did, but some (1/8000)
actually bounced back toward
the particle source!
• they did this assuming that the charge and mass within each
atom was uniformly distributed throughout the atom, and
expected the alpha particles to go right through.
• most did, but some (1/8000) actually bounced back toward the
particle source!
• Rutherford concluded that the reason some
particles bounced back was that there was
a very dense positively-charged area that
occupied only a very small amount of space
within the core of the atom (nucleus).
• to explain this, Rutherford proposed a new
model for the atom. He imagined the atom
as a miniature solar system with a nucleus as the sun and
electrons orbiting like planets.
• to explain this, Rutherford proposed a new model for the atom.
He imagined the atom as a miniature solar system with a
nucleus as the sun and electrons orbiting like planets.
• although the nucleus was only one million millionth the
volume of the atom, it had over 99.9% of the mass.
• Rutherford: "It was as though you had fired a fifteen-inch shell at
a piece of tissue paper and it had bounced back and hit you."
• Bohr, Neils—“Quantized energy level” model
• Danish physicist (1913), new model that
explained why negative electrons do not
fall in towards the positive nucleus
• Bohr used Rutherford's ideas to explain the
behavior of the simplest atom with one
electron—hydrogen.
• he suggested that an electron was restricted to
Bohr
certain allowed orbits round the nucleus.
• if it jumped from one orbit to another, the atom emitted or
absorbed light.
• thus, electrons only exist in specific energy levels (orbits
around the nucleus) similar to the rungs of a ladder.
• these levels reflect quantum amounts of energy that are
required to move electrons from one level to another.
• these levels reflect quantum amounts of energy that are
required to move electrons from one level to another.
• unfortunately, this model only works for 1 atom—hydrogen.
• Chadwick, James—“2 subatomic nucleus particles” model.
• discovered the neutron in the nucleus
• he used work previously done by Rutherford to establish the existence of the proton:
• using α particles to bombard N, he found
nuclei of H which had been chipped off.
• in 1920 he speculated that hydrogen nuclei
were a building block of all nuclei. He
Chadwick
suggested the name proton for this unit.
• other evidence for the proton came from the electric
charges and masses of nuclei which change by whole
units from element to element.
• other evidence for the proton came from the electric
charges and masses of nuclei which change by whole
units from element to element.
• these new positive particles in the nucleus were found to be
quite heavy (about 2000 times the mass of the e-).
• However, the total mass of most atoms did not add up to
match the weight of just the protons and electrons together.
Therefore, there must be another, non-charged particle in the
atom that adds the additional weight.
• around 1930, Irene Joliot-Curie (daughter of
Pierre and Marie Curie) performed an
experiment in which beryllium was bombarded
with alpha-particles, and a beam of particles
with high penetrating power emerged.
Joliot-Curie
• around 1930, Irene Joliot-Curie (daughter of
Pierre and Marie Curie) performed an experiment
in which beryllium was bombarded with alphaparticles, and a beam of particles with high
penetrating power emerged.
• she was conducting many radiation experiments
during this time. However, she was not
Joliot-Curie
searching for neutrons.
• Chadwick realized that some of her experiments might be
used to find the elusive neutron, so he repeated her
experiments with this goal in mind.
• he was able to determine that these particles had approximately
the same mass as a proton. When they were exposed to
electric and magnetic fields, and they were not deflected, he
determined that they had no charge, either.
• he was able to determine that these
particles had approximately the same
mass as a proton. When they were
exposed to electric and magnetic fields,
and they were not deflected, he
determined that they had no charge, either.
• he called these high-mass, uncharged
particles “neutrons.”
• Modern Atomic Model
• e- in the modern model are arranged in “orbitals,” not orbits
(Bohr model).
• Orbitals are cloud-like regions around the nucleus of the
atom in which 1 or 2 electrons are most likely to be found.
They are arranged about the nucleus on the x, y, and z axis,
which we will discuss in further detail tomorrow.
The models of the atom over the years:
The
Modern
Model
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