Download History of the Atom

Atomic Theory:
History of the Atom
Who are these men?
In this lesson, we’ll learn
about the men whose quests
for knowledge about the
fundamental nature of the
universe helped define our
views.
Democritus
●
This is the Greek
philosopher Democritus
who began the search for
a description of matter
more than 2400 years ago.
● He asked: Could matter
be divided into smaller
and smaller pieces
forever, or was there a
limit to the number of
times a piece of matter
could be divided?
400 BC
Atomos
●
●
●
His theory: Matter could
not be divided into
smaller and smaller
pieces forever, eventually
the smallest possible
piece would be obtained.
This piece would be
indivisible.
He named the smallest
piece of matter “atomos,”
meaning “not to be cut.”
Atomos
▪
▪
To Democritus, atoms
were small, hard
particles that were all
made of the same
material but were
different shapes and
sizes.
Atoms were infinite in
number, always
moving and capable
of joining together.
Carbon Atoms
Silicon Atoms
Nickel Atoms
Earliest Model of the Atom
●
The atomic
model has
changed
throughout the
centuries,
starting in 400
BC, when it
looked like a
billiard ball →
Democritus Theory
This theory was ignored and
forgotten for more than 2000
years!
Why?
●
The eminent
philosophers of
the time, Aristotle
and Plato, had a
more respected,
(and ultimately
wrong) theory.
Aristotle and Plato favored the earth,
fire, air and water approach to the
nature of matter. Their ideas held sway
because of their eminence as
philosophers. The atomos idea was
buried for approximately 2000 years.
Dalton’s Model
●
In the early 1800s,
the English
Chemist John
Dalton performed a
number of
experiments that
eventually led to
the acceptance of
the idea of atoms.
Dalton’s Theory
●
●
●
●
He deduced that all
elements are composed of
atoms. Atoms are
indivisible and
indestructible particles.
Atoms of the same element
are exactly alike.
Atoms of different elements
are different.
Compounds are formed by
the joining of atoms of two
or more elements.
.
This theory became
one of the
foundations of modern
chemistry.
What were the HOT science
topics of this era?
Radiation
and
Electricity
Benjamin Franklin explored the
phenomenon of electricity in 1746.
He is credited with the concepts of
positive and negative charges.
Another scientist, Michael Faraday,
proposed a connection between
atoms and electricity.
JJ Thomson will use the ideas of
Faraday and Franklin in his
proposal of negative particles –
electrons.
Thomson’s Plum Pudding
Model
●
In 1897, the
English scientist
J.J. Thomson
provided the first
hint that an atom
is made of even
smaller particles.
Subatomic Particles and Atomic Structure
In the late 1800’s, many scientists were doing research involving
radiation, the emission and transmission of energy in the form of
waves.
They commonly used a cathode ray tube, which consists of two
metal plates sealed inside a glass tube from which most of the air
has been evacuated.
Subatomic Particles and Atomic Structure
When metal plates are connected to a high-voltage source, the
negatively charged plate, or cathode, emits an invisible ray.
The cathode ray is drawn to the anode where it passes through a
small hole.
Although invisible, the path is revealed when the ray strikes a
phosphor-coated surface producing a bright light.
Subatomic Particles and Atomic Structure
This prompted him to propose the rays were actually
a stream of negatively charged particles.
These negatively charged particles are called
electrons.
Thomson Model
●
This surprised
Thomson,
because the
atoms of the gas
were uncharged.
Where had the
negative charges
come from?
Where did
they come
from?
Thomson concluded
that the negative
charges came from
within the atom.
A particle smaller
than an atom had to
exist.
The atom was
divisible!
Thomson called the
negatively charged
“corpuscles,” today
known as electrons.
Since the gas was known
to be neutral, having no
charge, he reasoned that
there must be positively
charged particles in the
atom.
But he could never find
them.
Thomson Model
●
●
He proposed a
model of the atom
that is sometimes
called the “Plum
Pudding” model.
Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about,
like raisins in a
pudding.
Thomson -Millikan
●
●
JJ Thomson is credited with the
discovery of the electron. He concluded
that the electron conducts electricity in
the atom.
Robert Millikan is credited with
determining the magnitude of the
electron’s charge (mathematical value)
from his Oil Drop Experiment.
Subatomic Particles and Atomic Structure
R. A. Millikan (1868-1953) determined the charge on an electron
by examining the motion of tiny oil drops.
The charge was determined to be -1.6022 x 10-19 C. Later, we will
learn the charge on a proton is 1.6 x 10-19 C.
The Oil Drop Experiment
What Millikan did was to put a charge on a tiny drop of oil, and
measure how strong an applied electric field had to be in order to
stop the oil drop from falling. Since he was able to work out the
mass of the oil drop, and he could calculate the force of gravity on
one drop, he could then determine the electric charge that the drop
must have.
Subatomic Particles and Atomic Structure
Wilhelm Rontgen (1845-1923) discovered Xrays. They were not deflected by magnetic or
electric fields, so they could not consist of
charged particles.
Antoine Becquerel (1852-1908) discovered
radioactivity, the spontaneous emission of
radiation.
1896 Marie Curie
●
In 1896, French chemist Antoine Henri
Becquerel made an accidental discovery when
he was studying uranium salts exposed to
sunlight and their ability to fog a photographic
film plate. He left one sample that was not
being exposed to the sun on the plate and saw
that it had fogged. His associates Marie Curie
and her husband Pierre Curie showed that the
uranium atoms emitted rays that fogged the
plates.
●
●
●
Marie Curie, a Polish scientist, named the
process that the uranium atoms gave off rays,
radioactivity. The particles and rays given off by
a radioactive substance was called radiation.
This discovery proved that Dalton’s theory that
atoms are indivisible was incorrect.
Radioisotopes, or radioactive atoms, have
unstable nuclei and emit radiation as they
undergo changes.
Curie discovered that the radioactivity of a
substance was in proportion to the amount of
the radioactive material present.
The discovery of radiation allowed Becquerel
and the Curies to be awarded the Nobel prize in
1903.
Subatomic Particles and Atomic Structure
Alpha (α) rays consist of positively charged particles, called α
particles.
Beta (β) rays, or β particles, are electrons so they are deflected away
from the negatively charged plate.
Gamma (γ) rays, like X-rays, have no charge and are unaffected by
external electric or magnetic fields.
The Discovery of the Proton
● Discovered
by Eugen
Goldstein (German) in 1886.
● He
observed “Canal rays”
and found that they are
composed of positive
particles – protons.
Canal Rays
●
Canal Rays passed through holes, or
channels, in the reverse direction as the
cathode ray. This was basically the
counterpart to JJ Thomson’s findings of
electrons.
Canal Rays
Rutherford’s Gold Foil
Experiment
●
In 1908, the
English physicist
Ernest Rutherford
was hard at work
on an experiment
that seemed to
have little to do
with unraveling the
mysteries of the
atomic structure.
●
Rutherford’s experiment Involved
firing a stream of tiny positively
charged particles at a thin sheet of
gold foil (2000 atoms thick)
Subatomic Particles and Atomic Structure
Ernest Rutherford used α
particles to prove the
structure of atoms.
The majority of particles
penetrated the gold foil
undeflected.
Sometimes, α particles were
deflected at a large angle.
Sometimes, α particles
bounced back in the direction
from which they had come.
Gold Foil Experiment
●
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Most of the positively
charged “bullets” passed
right through the gold
atoms in the sheet of gold
foil without changing
course at all.
Some of the positively
charged “bullets,”
however, did bounce away
from the gold sheet as if
they had hit something
solid. He knew that
positive charges repel
positive charges.
Gold Foil Experiment
●
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●
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This could only mean that the gold atoms in the
sheet were mostly open space. Atoms were not
a pudding filled with a positively charged
material.
Rutherford concluded that an atom had a small,
dense, positively charged center that repelled
his positively charged “bullets.”
He called the center of the atom the “nucleus”
The nucleus is tiny compared to the atom as a
whole.
Subatomic Particles and Atomic Structure
Rutherford proposed a new model for the atom: Nuclear Model.
Positive charge is concentrated in the nucleus with the electrons
orbiting around the outside.
The nucleus accounts for most of an atom’s mass and is an
extremely dense central core within the atom.
But wait – there’s more!
James Chadwick
(1932)
Discovered a neutral
(uncharged) particle in the
nucleus. Called it the
“neutron”.
Atom “split” later that year
Atom “split” by
John Cockcroft
and Ernest
Walton, using a
particle
accelerator, in late
1932.
Splitting the atom led to some
very practical consequences
Bohr Model
In 1913, the Danish
scientist Niels Bohr
(Rutherford’s
graduate student)
proposed an
improvement on
Rutherford’s model.
In his model, he
placed each electron
in a specific energy
level.
Bohr Model
●
According to Bohr’s
atomic model, electrons
move in definite orbits
around the nucleus,
much like planets circle
the sun. These orbits,
or energy levels, are
located at certain
distances from the
nucleus. This is called
the Planetary Model.
The position of electrons can’t
be predicted precisely
Werner Heisenberg (1927)
The “Uncertainty Principle”
●
There’s an upper limit to how precisely an
electron’s position and momentum can be known
●
The more precisely one is known, the less
precisely the other can be known
In
Electron Cloud:
●
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An electron cloud is a
space in which electrons
are likely to be found.
Electrons whirl about the
nucleus billions of times in
one second
They are not moving
around in random
patterns.
Location of electrons
depends upon how much
energy the electron has.
Electron Cloud:
●
Depending on the electron’s energy, they are
locked into a certain area in the cloud.
●
Electrons with the lowest energy are found in the
energy level closest to the nucleus.
●
Electrons with the highest energy are found in
the outermost energy levels, farther from the
nucleus.
More Subatomic Particles??
Murray Gell-Mann and George Zweig (1964) - proposed protons and
neutrons are made of smaller particles they named quarks (aces)
●
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Can never be isolated
There are up quarks and down quarks
Protons & neutrons are not
fundamental
●
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Gell-Mann & Zweig hypothesized 6 different
quarks with fractional charge (UP quark has
+2/3 charge, DOWN quark has –1/3)
Protons and neutrons are composed of UP
and DOWN quarks, held together by gluon
particles
What are atoms made of?
Protons: positively charged particles that are housed in the nucleus
of an atom and have significant mass
Neutrons: neutral particles that are housed in the nucleus. They act to
hold the protons in place since like charges repel each
other. Neutrons have significant mass
Electrons have negligible mass, have a negative charge and are allowed
to roam freely in the electron cloud so they take up significant
volume in the atom
Taylor
Atomic Number, Mass Number, and Isotopes
All atoms can be identified by the number of protons and neutrons
they contain.
The atomic number (Z) is the number of protons in the nucleus.
⮚Atoms are neutral, so it’s also the number of electrons.
⮚Protons determine the identity of an element. For example,
nitrogen’s atomic number is 7, so every nitrogen has 7 protons.
The mass number (A) is the total number of protons and neutrons.
⮚Protons and neutrons are collectively referred to as nucleons.
Mass number
(number of protons + neutrons)
Elemental symbol
Atomic number
(number of protons)
Atomic Number, Mass Number, and Isotopes
Most elements have two or more isotopes, atoms that have the
same atomic number (Z) but different mass numbers (A).
1 proton
0 neutrons
1 proton
1 neutron
1 proton
2 neutrons
Isotopes of the same element exhibit similar chemical properties,
forming the same types of compounds and displaying similar
reactivities.
Ions
Atoms are neutral—meaning that the number of protons
is equal to the number of electrons
If an atom loses or gains electrons the atom is
No longer neutral but has a charge.
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
Nuclear Symbol of Ions
A C
X
X = element symbol
A = atomic number
Z = mass number
C = charge of Ion
Z
Number of Protons = Z
Numbers of Neutrons = A – Z
Number of electrons = Z - C
Average Atomic Mass
Atomic mass is the mass of an atom in atomic mass units (amu).
1 amu = 1/12 the mass of a carbon-12 atom
The average atomic mass on the periodic table represents the
average mass of the naturally occurring mixture of isotopes.
Isotope
Isotopic mass (amu)
Natural
abundance (%)
12C
12.00000
98.93
13C
13.003355
1.07
Average mass (C) = (0.9893)(12.00000 amu) + (0.0107)(13.003355 amu)
= 12.01 amu