Transcript Atomic Structure - Broadneck High School

Development of the Atom
The Greeks
History of the Atom
• Not the history of atom, but
the idea of the atom
• In 400 B.C the Greeks tried to
understand matter
(chemicals) and broke them
down into earth, wind, fire,
and air.
~
~
• Democritus and Leucippus
Greek philosophers
Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
• Greek philosopher
• Idea of ‘democracy’
• Idea of ‘atomos’
– Atomos = ‘indivisible’
– ‘Atom’ is derived
• No experiments to support
idea
• Continuous vs. discontinuous
theory of matter
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Timeline
Greeks
(Democritus ~450 BC)
Discontinuous
theory of matter
ALCHEMY
400 BC
300 AD
Issac Newton
(1642 - 1727)
1000
2000
Greeks
(Aristotle ~350 BC))
Continuous
theory of matter
American
Independence
(1776)
Early Ideas on Elements
Robert Boyle stated...
– A substance was an
element unless it could
be broken down to two
or more simpler
substances.
– Air therefore could not
be an element because
it could be broken down
in to many pure
substances.
Robert Boyle
Modern Chemistry
• Beginnings of modern chemistry were seen in the
sixteenth and seventeenth centuries, where great
advances were made in metallurgy, the extraction of
metals from ores.
• In the seventeenth century, Boyle described the
relationship between the pressure and volume of air and
defined an element as a substance that cannot be
broken down into two or more simpler substances by
chemical means.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Modern Chemistry
 During the eighteenth century, Priestley discovered
oxygen gas and the process of combustion where
carbon-containing materials burn vigorously in an
oxygen atmosphere.
Priestley
 In the late eighteenth century, Lavoisier discovered
respiration and wrote the first modern chemistry text.
His most important contribution was the
law of conservation of mass, which states
that in any chemical reaction, the mass of
the substances that react equals the mass
of the products that are formed. He is known
Lavoisier
as the father of modern chemistry.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Foundations of Atomic Theory
Law of Conservation of Mass (Lavoisier)
Mass is neither destroyed nor created during ordinary chemical
reactions.
Law of Definite Proportions (Proust)
The fact that a chemical compound contains the same elements
in exactly the same proportions by mass regardless of the size
of the sample or source of the compound.
Law of Multiple Proportions (Dalton)
If two or more different compounds are composed of the
same two elements, then the ratio of the masses of the
second element combined with a certain mass of the first
elements is always a ratio of small whole numbers.
Dalton Model of the Atom
Late 1700’s - John Dalton- England
Teacher- summarized results of his experiments
and those of others
Combined ideas of elements with that of atoms
in Dalton’s Atomic Theory
Daltons Atomic Theory
• Dalton stated that
elements consisted of
tiny particles called atoms
• He also called the
elements pure
substances because all
atoms of an element
were identical and that in
particular they had the
same mass.
Dalton’s Atomic Theory
1. All matter consists of tiny particles.
Dalton, like the Greeks, called these particles “atoms”.
2. Atoms of one element can neither be subdivided nor changed into
atoms of any other element.
3. Atoms can neither be created nor destroyed.
4. All atoms of the same element are identical in mass, size, and
other properties.
5. Atoms of one element differ in mass and other properties from
atoms of other elements.
6. In compounds, atoms of different elements combine in simple, whole
number ratios.
Conservation of Atoms
2 H2 + O2
2 H2O
John Dalton
H
H
H2
O
H
O2
+
H2
H
O
H2O
O
H 2O
H
H
O
H
H
4 atoms hydrogen
2 atoms oxygen
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
4 atoms hydrogen
2 atoms oxygen
Legos are Similar to Atoms
H
H2
H
H
O
+
H2
H
H
O2
H
O
H 2O
H
O
O
H
H 2O
Lego's can be taken apart and built into many different things.
Atoms can be rearranged into different substances.
Daltons’ Models of Atoms
Carbon dioxide, CO2
Water, H2O
Methane, CH4
Structure of Atoms
• Scientist began to wonder what an atom
was like.
• Was it solid throughout with no internal
structure or was it made up of smaller,
subatomic particles?
• It was not until the late 1800’s that
evidence became available that atoms
were composed of smaller parts.
History: On The Human Side
1834
1895
1896
1896
1897
1898
1899
1900
Michael Faraday - electrolysis experiments
suggested electrical nature of matter
Wilhelm Roentgen - discovered X-rays when
cathode rays strike anode
Henri Becquerel - discovered "uranic rays" and
radioactivity
Marie (Marya Sklodowska) and Pierre Curie discovered that radiation is a property of the
atom, and not due to chemical reaction.
(Marie named this property radioactivity.)
Joseph J. Thomson - discovered the electron
through Crookes tube experiments
Marie and Piere Curie - discovered the
radioactive elements polonium and radium
Ernest Rutherford - discovered alpha and beta
particles
Paul Villard - discovered gamma rays
1919
1932
1934
1938
1940
1941
1942
1903
1910
1911
Ernest Rutherford and Frederick Soddy established laws of radioactive decay and
transformation
Frederick Soddy - proposed the isotope concept
to explain the existence of more than one atomic
weight of radioelements
Ernest Rutherford - used alpha particles to
explore gold foil; discovered the nucleus and the
proton; proposed the nuclear theory of the atom
1944
1964
Ernest Rutherford - announced the first artificial
transmutation of atoms
James Chadwick - discovered the neutron by
alpha particle bombardment of Beryllium
Frederick Joliet and Irene Joliet Curie - produced
the first artificial radioisotope
Otto Hahn, Fritz Strassmann, Lise Meitner, and
Otto Frisch - discovered nuclear fission of
uranium-235 by neutron bombardment
Edwin M McMillan and Philip Abelson discovered the first transuranium element,
neptunium, by neutron irradiation of uranium in a
cyclotron
Glenn T. Seaborg, Edwin M. McMillan, Joseph
W. Kennedy and Arthur C. Wahl - announced
discovery of plutonium from beta particle
emission of neptunium
Enrico Fermi - produced the first nuclear fission
chain-reaction
Glenn T. Seaborg - proposed a new format for
the periodic table to show that a new actinide series of 14
elements would fall below and be analogous to the 14
lanthanide-series elements.
Murray Gell-Mann hypothesized that quarks are the
fundamental particles that make up all known subatomic
particles except leptons.
A Cathode Ray Tube
Source of
Electrical
Potential
Stream of negative
particles (electrons)
Metal Plate
Gas-filled
glass tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Metal plate
Background Information
•
•
•
•
Cathode Rays
Form when high voltage is applied across
electrodes in a partially evacuated tube.
Originate at the cathode (negative electrode)
and move to the anode (positive electrode)
Carry energy and can do work
Travel in straight lines in the absence of an
external field
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Cathode Ray Experiment
1897 Experimentation
• Using a cathode ray tube, Thomson was
able to deflect cathode rays with an
electrical field.
• The rays bent towards the positive pole,
indicating that they are negatively
charged.
The Effect of an Electric Field on
Cathode Rays
source of
high voltage
High
voltage
cathode
negative
plate
_
+
anode
positive
plate
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Cathode Ray Experiment
Displacement
Volts
Anodes / collimators
Cathode
+
Deflection
region
Drift region
Conclusions
• He compared the value with the mass/ charge ratio for the
lightest charged particle.
• By comparison, Thomson estimated that the cathode ray
particle weighed 1/1000 as much as hydrogen, the lightest
atom.
• He concluded that atoms do contain subatomic particles - atoms
are divisible into smaller particles.
• This conclusion contradicted Dalton’s postulate and was not
widely accepted by fellow physicists and chemists of his day.
• Since any electrode material produces an identical ray, cathode
ray particles are present in all types of matter - a universal
negatively charged subatomic particle later named the electron
J.J. Thomson
• He proved that atoms of
any element can be
made to emit tiny
negative particles.
• From this he concluded
that ALL atoms must
contain these negative
particles.
• He knew that atoms did
not have a net negative
charge and so there must
be something balancing
the negative charge.
J.J. Thomson
Plum-Pudding Model
• In 1910 proposed
the Plum Pudding
model
– Negative electrons
were embedded into
a positively charged
spherical cloud.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Other pieces
• Proton - positively charged pieces
– 1840 times heavier than the electron
• Neutron - no charge but the same mass as
a proton.
• How were these pieces discovered?
• Where are the pieces?
Rutherford Model of the Atom
(The modern view of the atom was developed by Ernest Rutherford)
Rutherford
Ernest Rutherford (1871-1937)
PAPER
• Learned physics in
J.J. Thomson’ lab.
• Noticed that ‘alpha’
particles were
sometime deflected
by something in the
air.
• Gold-foil experiment
Animation by Raymond Chang – All rights reserved.
Rutherford ‘Scattering’
• In 1909 Rutherford undertook a series of experiments
• He fired a (alpha) particles at a very thin sample of gold foil
• According to the Thomson model the a particles would only
be slightly deflected
• Rutherford discovered that they were deflected through large
angles and could even be reflected straight back to the source
Lead collimator
Gold foil
a particle
source
q
Rutherford’s Apparatus
beam of alpha particles
radioactive
substance
fluorescent screen
circular - ZnS coated
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
Lead
block
Polonium
Florescent
Screen
Gold Foil
California WEB
What He Expected
• The alpha particles to pass through
without changing direction (very much)
• Because
• The positive charges were spread out
evenly. Alone they were not enough to
stop the alpha particles
California WEB
What he expected…
California WEB
What he expected…
Because he thought the mass was
evenly distributed in the atom.
-
-
-
Single Gold atom
Because, he thought
the mass was evenly
distributed in the atom
---
--
-
--
--
--
--
--
-
--
--
--
--
-
---
--
--
--
-Thin layer of Gold atoms in foil
---
--
What he got…
richocheting
alpha particles
What he got…
richocheting
alpha particles
The Predicted Result:
expected
path
expected
marks on screen
Observed Result:
mark on
screen
likely alpha
particle path
Density and the Atom
• Since most of the particles went through,
the atom was mostly empty space.
• Because the alpha rays were deflected so
much, the positive pieces it was striking
were heavy.
• Small volume and big mass = big density
• This small dense positive area is the
nucleus
California WEB
Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
+
+
-
-
+
+
-
+
+
-
+
-
+
-
-
Plum-pudding atom
Nuclear atom
Thomson’s model
Rutherford’s model
Results of foil experiment if plumpudding had been correct.
Electrons scattered
throughout
-
+
-
positive
charges
+
+
+
+
-
-
+
+
+
-
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
-
Interpreting the Observed
Deflections
deflected particle
.
.
.
.
.
.
beam of
alpha
particles
.
.
.
.
.
.
.
.
.
.
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
.
undeflected
particles
Rutherford’s
Gold-Leaf
Experiment
Conclusions:
Atom is mostly empty space
Nucleus has (+) charge
Electrons float around nucleus
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
Evidence for Particles
In 1886, Goldstein, using equipment similar to cathode ray tube,
discovered particles with charge equal and opposite to that of
electron, but much larger mass.
Rutherford later (1911) found these particles to be identical to
hydrogen atoms minus one electron
- named these particles protons
Chadwick (1932) discovered particles with similar mass to proton
but zero charge.
- discovered neutrons
Bohr Atom
The Planetary Model of the Atom
Niels Bohr
Niels Bohr
• In the Bohr Model (1913)
the neutrons and protons
occupy a dense central
region called the nucleus,
and the electrons orbit
the nucleus much like
planets orbiting the Sun.
• They are not confined to
a planar orbit like the
planets are.
Bohr Model
Neils Bohr
Planetary
model
After Rutherford’s discovery, Bohr
proposed that electrons travel in definite
orbits around the nucleus.
Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels
• Bohr’s contributions to the understanding of
atomic structure:
1. Electrons can occupy only certain regions of space,
called orbits.
2. Orbits closer to the nucleus are more stable —
they are at lower energy levels.
3. Electrons can move from one orbit to another by
absorbing or emitting energy, giving rise to
characteristic spectra.
• Bohr’s model could not explain
the spectra of atoms heavier
than hydrogen.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Bohr Model of Atom
Increasing energy
of orbits
n=3
e-
n=2
e-
n=1
ee-
e-
e-
ee-
e-
e-
eA photon is emitted
with energy E = hf
The Bohr model of the atom, like many ideas in
the history of science, was at first prompted by
and later partially disproved by experimentation.
http://en.wikipedia.org/wiki/Category:Chemistry
Cartoon courtesy of NearingZero.net
Quantum Mechanical Model
Niels Bohr &
Albert Einstein
Modern atomic theory describes the
electronic structure of the atom as the
probability of finding electrons within certain
regions of space (orbitals).
Modern View
• The atom is mostly empty space
• Two regions
– Nucleus
• protons and neutrons
– Electron cloud
• region where you might find an electron
Models of the Atom
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg
e
e
+
e
+
e
+
+
e
+e
+
e
e
+ e + e
Greek model
Dalton’s
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
Bohr’s model
(1913)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
-
- +
Rutherford’s model
(1909)
Charge-cloud model
(present)
Models of the Atom
e
e
+
e
-
+
e
+
+
e
+e
+e
e
+ e + e
Dalton’s
model
Greek model
(1803)
(400 B.C.)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1800
-
Thomson’s plum-pudding
model (1897)
- +
Rutherford’s model
(1909)
1897 J.J. Thomson, a British
1911 New Zealander
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1805 ..................... 1895
1900
1905
1910
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
1915
Bohr’s model
(1913)
1926 Erwin Schrödinger
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
1920
1925
Charge-cloud model
(present)
1930
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1935
1940
1945
1924 Frenchman Louis
1932 James
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
Dalton (1803)
Thomson (1904)
(positive and negative charges)
+
+
+
+
+
Rutherford (1911)
(the nucleus)
+
..
.
. .
.. .
.
.
.
.
.
.
..
..
Schrödinger (1926)
(electron cloud model – orbitals)
From the time of Dalton to Schrödinger, our model
of the atom has undergone many modifications.
Ralph A. Burns, Fundamentals of Chemistry 1999, page 137
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Bohr (1913)
(energy levels - orbits)
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Home Work:
Historical
Atomic
Model