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The Atom
Chapter 5 and 14
Development of the Atom
The Hellenic Market
Fire
~
Water
Earth
Air
A Brief History of Chemistry
• In fourth century B.C., ancient Greeks proposed that
matter consisted of fundamental particles called
atoms.
• Over the next two millennia, major advances in
chemistry were achieved by alchemists. Their major
goal was to convert certain elements into others by a
process called transmutation.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
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
Democritus
DEMOCRITUS (400 BC) – First Atomic Hypothesis
Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.
Properties of atoms:
• indestructible.
• changeable, however, into different forms.
• an infinite number of kinds so there are an infinite number of elements.
• hard substances have rough, prickly atoms that stick together.
• liquids have round, smooth atoms that slide over one another.
• smell is caused by atoms interacting with the nose – rough atoms hurt.
• sleep is caused by atoms escaping the brain.
• death – too many escaped or didn’t return.
• the heart is the center of anger.
• the brain is the center of thought.
• the liver is the seat of desire.
“Nothing exists but atoms and space, all else is opinion”.
Four Element Theory
FIRE
• Plato was an atomist
• Thought all matter was
composed of 4 elements:
–
–
–
–
–
Earth (cool, heavy)
Water (wet)
Fire (hot)
Air (light)
Ether (close to heaven)
Hot
AIR
Dry
EARTH
‘MATTER’
Wet
Cold
WATER
Relation of the four elements and the four qualities
Blend these “elements” in different proportions to get all substances
Some Early Ideas on Matter
Anaxagoras
(Greek, born 500 B.C.)
–Suggested every substance had its own kind of “seeds” that clustered together to make
the substance, much as our atoms cluster to make molecules.
Empedocles
(Greek, born in Sicily, 490 B.C.)
–Suggested there were only four basic seeds – earth, air, fire, and water. The elementary
substances (atoms to us) combined in various ways to make everything.
Democritus
(Thracian, born 470 B.C.)
–Actually proposed the word atom (indivisible) because he believed that all matter
consisted of such tiny units with voids between, an idea quite similar to our own beliefs.
It was rejected by Aristotle and thus lost for 2000 years.
Aristotle
(Greek, born 384 B.C.)
–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements which
combined as shown in the diagram (previous page).
Hot + dry made fire; hot + wet made air, and so on.
O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,
Alchemy
• After that chemistry was
ruled by alchemy.
• They believed that that could
take any cheap metals and
turn them into gold.
• Alchemists were almost like
magicians.
– elixirs, physical immortality
Alchemy
Alchemical symbols for substances…
..
.
.. ..........
GOLD
SILVER
COPPER
IRON
SAND
transmutation: changing one substance into another
D
In ordinary chemistry, we cannot transmute elements.
Contributions
of alchemists:
Information about elements
- the elements mercury, sulfur, and antimony were discovered
- properties of some elements
Develop lab apparatus / procedures / experimental techniques
- alchemists learned how to prepare acids.
- developed several alloys
- new glassware
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
Timeline
Greeks
(Democritus ~450 BC)
Discontinuous
theory of matter
400 BC
Greeks
(Aristotle ~350 BC))
Continuous
theory of matter
ALCHEMY
300 AD
1000
Issac Newton
(1642 - 1727)
2000
American
Independence
(1776)
The Atomic Theory of Matter
• In 1803, Dalton proposed that elements consist of individual
particles called atoms.
• His atomic theory of matter contains four hypotheses:
1. All matter is composed of tiny particles called atoms.
2. All atoms of an element are identical in mass and
fundamental chemical properties.
3. A chemical compound is a substance that always
contains the same atoms in the same ratio.
4. In chemical reactions, atoms from one or more
compounds or elements redistribute or rearrange in
relation to other atoms to form one or more new
compounds. Atoms themselves do not undergo a
change of identity in chemical reactions.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
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.
Dalton’s Symbols
John Dalton
1808
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.
Discovery of the electron
1807 Davy suggested that electrical forces held
compound together.
1833 Faraday related atomic mass and the
electricity needed to free an element during
electrolysis experiments.
1891 Stoney proposed that electricity exists in units
he called electrons.
1897 Thomson first quantitatively measured the
properties of electrons.
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
Ernest Rutherford (1871-1937)
PAPE
R
• 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.
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
Thomson Model of the Atom
• J.J. Thomson discovered the electron and knew that electrons
could be emitted from matter (1897).
• William Thomson proposed that atoms consist of small,
negative electrons embedded in a massive, positive sphere.
• The electrons were like currants in a plum pudding.
• This is called the ‘plum pudding’ model of the atom.
-
-
electrons
-
-
-
-
The Rutherford Atom
-
-
p+
-
-
-
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
Bohr Atom
The Planetary Model of the Atom
Bohr Model of Atom
Increasing energy
of orbits
n=3
e-
n=2
e-
n=1
ee-
e-
e-
e-
e-
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
Bohr’s
Bohr’sModel
Model
Nucleus
Electron
Orbit
Energy Levels
Modern View
• The atom is mostly empty space
• Two regions
– Nucleus
• protons and neutrons
– Electron cloud
• region where you might find an electron
Dalton proposes the
indivisible unit of an
element is the atom.
Review
Models
of the
Atom
Thomson discovers
electrons, believed to
reside within a sphere of
uniform positive charge
(the “plum-pudding model).
Rutherford demonstrates
the existence of a positively
charged nucleus that
contains nearly all the
mass of an atom.
Bohr proposes fixed
circular orbits around
the nucleus for electrons.
In the current model of the atom,
electrons occupy regions of space
(orbitals) around the nucleus
determined by their energies.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.