Download Investigating Atoms and Atomic Theory

A History of Atomic Theory
Atomic Models
What is a model ?
MODEL: detailed, 3-D representation of an object,
(typically on smaller scale than original)
model airplane
model car
clothing model
Let’s Take a Trip Through Time!
It Started with the Greeks
DEMOCRITUS
• Lived in Greece 2500
years ago (460-370 BC)
• “Father of modern
science”
Demacritus said: “All matter is
made of atoms that are tiny,
indestructible and indivisible.”
Democritus
Greek banknote with Democritus and drawing of atom
Where did Democritus get his ideas for
small, indestructible, indivisible atoms?
Atoms are small…
new rings
old ring
EVIDENCE: Old gold rings wear away slowly,
getting thinner and thinner, but you never see
gold atoms on your finger, so gold atoms
must be very small!
Atoms are indestructible…
EVIDENCE: Earth has been around long
time. Even though mountains get washed
away, rocks continue to exist. New plants
grow where old plants die.
Atoms are indivisible…
EVIDENCE: None
This was HYPOTHESIS made by Democritus
to explain nature as he saw it.
What did Democritus think
atoms looked like?
Appearance of atoms assumed based on
behavior:
• Liquids pour
• Solids are hard and rigid
Liquids pour. Why?
Liquids require a container
If liquid atoms were like little balls they
would roll out when you tip the container
Greek idea of liquid atom
Solids are rigid. Why?
cocklebur
plant
Velcro® fastener
cockleburs stick on
clothing and each other
Velcro hooks and loops
little hooks on cockleburs
Greek idea of solid atom
Aristotle had other ideas
Greek banknote and coin picturing Aristotle
ARISTOTLE: Famous Greek philosopher, born 384 BC.
• student of Plato (another famous philosopher)
• teacher of Alexander the Great (who later conquered
the world)
Aristotle’s idea of matter
Aristotle did not believe
Democritus’s idea of atoms
was correct
Aristotle believed all matter
made from four elements:
Earth
Air
Fire
Water
Aristotle’s idea of matter
Aristotle was more famous than
Democritus, so people believed
him, even though he was
wrong!
Democritus’s idea of atomos
(atoms) was lost for nearly
2000 years until John Dalton
brought it back in 1803
Atom idea lost for ~2000 years
John Dalton, New Atom,1803
Born in England, 1766
Studied chemistry, physics,
and color blindness
Brought back Democritus’s
idea of an indivisible atom
Color blindness
Can you see a number in this gray box?
If not, you may be color blind.
(More males are color blind than females)
Dalton’s Theory of Atoms
Five parts to Dalton’s modern atomic theory:
# 1:
Elements are made of extremely
small particles called atoms
# 2:
Atoms of given element are
identical in size, mass, and other
properties;
atoms of different elements are
different in size, mass, and other
properties
(found out not exactly correct)
#3:
Atoms cannot be subdivided,
created, or destroyed
(later proved wrong)
So Dalton’s atoms are kind of like
billiard balls
Atoms combine in
whole-number ratios
#4:
Atoms of different elements
combine in simple wholenumber ratios to form chemical
compounds
(H2O
2:1 ratio H:O)
#5:
In chemical reactions, atoms
are combined, separated or
rearranged
2H2O + 2Na  2NaOH + H2
In a reaction, atoms are rearranged
+

+
+
Dalton’s Chemical Symbols
We use different symbols today
Matter is electrical!
Michael Faraday
(English chemist, born 1791)
In 1834 learned that matter
would interact with electricity
Realized that electricity had
to be made up of particles
that could be counted, but
did not know what they were
(were electrons)
J.J. Thomson
English physicist (1856-1940)
won Nobel Prize in 1906
Discovered the ELECTRON
in 1897:
- studied cathode rays using
Crooke’s tube
- showed atoms were
divisible
Crooke’s tube
cathode |
| cross-shaped anode
– cross-shaped shadow
Cathode rays (stream of electrons) move from metal
cathode (on left) to the cross-shaped anode on right
and casts a cross-shaped shadow on glass on right.
So … Electrons are particles with negative charge
Cathode Ray Tube Experiment
Thought cathode rays were streams of particles
smaller than atoms
electric or
magnetic fields
deflect beam of
charged particles
What do we mean by charge?
ELECTRICAL CHARGE: property of matter that
means it has either more or fewer electrons than
protons
batteries have (+) and (-) ends
ELECTRONS are negatively charged (-1)
PROTONS are positively charged (+1)
MATTER IS NEUTRAL (no charge) because
(+) charges are equal to (-) charges
(cancel each other out)
More About Charge…
Opposite charges attract, alike charges repel
(push away.
JJ Thomson’s discovery of
(-) charged electrons proved
that atoms were divisible!
Thomson’s “Plum pudding” atom
– electron
ENGLISH PLUM PUDDING
– raisin
RULE: Matter is NEUTRAL so… for every one
electron [(-) charge] there must also be one
positive charge to equal the zero charge
(“neutral”) of matter
we move from
Dalton’s Billard Ball Model
to
Thomson’s “Plum pudding” model
Thomson’s Plum Pudding Model:
(+) charge
evenly spread out
while (-) charge
is in bits –
like chocolate
chips in cookies
Ernest Rutherford
1871-1937
• Discovered the proton in 1920
• Won Nobel Prize in
chemistry in 1908
Discovered that most of
mass of atom is in nucleus
Rutherford’s Experiment – 1911
Rutherford’s exp’t: animation
To test plum pudding model of
atom, Rutherford sent tiny
radioactive particles called a
(alpha) particles, like bullets,
towards thin sheet of gold foil.
Rutherford gold foil experiment
Rutherford expected a
particles to pass straight
through, like this…
most a particles went
straight through –
didn’t bump into anything
so most of atom is empty
space
But even though most a particles went straight
through, a few were deflected, like this so the
particles must have hit something:
 really heavy
 (+) charged
The gold foil experiment
scattering results meant the
Plum pudding model of the
atom was INCORRECT, so
Rutherford had to think of new
model that fit what he saw so…
proposed Nuclear Model
Rutherford concluded:
• all atoms have positive (+) nucleus that contains most of
mass of atom
• atom is mostly empty space (except very tiny (–) electrons)
electrons (–)
/
nucleus
/ [protons (+)]
The
NUCLEAR
atom model
Rutherford proposed
Nuclear atom
Rutherford did
not speculate
how electrons
arranged around
nucleus
Problem with Rutherford’s Model
To prevent (-) electrons from
being attracted to (+) nucleus,
electrons have to orbit nucleus
like Earth orbits sun
But…
electrons need energy to orbit nucleus, and there’s
no source of energy for this so every Rutherford
atom would “die” yet Real atoms do not die
So how big is the nucleus
compared to the entire atom?

If atom as big as football stadium, nucleus
smaller than flea on 50-yard line!

If atom big as period at end of sentence in
textbook, it would have mass of 70 cars!
So how big is an atom?

Most atoms are 1-2 angstroms across

An angstrom = 1 X 10-10 m

6 Billion Cu atoms in a line = less than 1
meter!

Can we “see” an atom?
James Chadwick
1891-1974



Worked with
Rutherford
Discovered Neutron
1932
Nobel Prize in
Physics 1935
Niels Bohr 1885 - 1962
Created quantized atom
model, 1915
Worked on Manhattan
Project
Won Nobel Prize in 1922
In Bohr’s model, electrons can only move between
energy levels in the atom, emitting energy when
they jump from higher to lower levels, absorbing
energy when they jump from lower to higher levels.
Bohr addressed the issue of electrons
in the atom
 Nucleus surrounded by electrons
orbiting at different energy levels
 Electrons have definite orbits
Quantum Mechanical Theory
Electron in a Hydrogen atom
Bohr’s new atomic model had quantized
energy levels, meaning the electrons could
only move by jumping between levels
(numbered n = 1, n = 2, n = 3, etc.)
electrons absorb energy when they jump out, and
emit (send out) energy when they jump in
Bohr’s Planetary Model
•
Electrons travel only in specific orbits
•
Each orbit has definite energy
•
Inner orbit (n=1): least energy
•
Outer orbit (n=7): most energy
•
Atoms emit radiation when e- jumps from outer
orbit to inner orbit
•
Outermost orbits determine atom’s chemical
properties
Erwin Schrödinger Wave Model
Austrian scientist (1887-1961)
• won the Nobel Prize in physics
in 1933
• calculated wave model of
hydrogen atom in 1926
• also called cloud model,
quantum mechanical model,
modern model of the atom
• atom model we still use today
In Schrödinger’s wave model of atom, electron
behaves as energy wave as well as a matter particle
(Light also behaves as particle and wave)
Einstein had predicted that energy and matter were
related in his equation E = mc2
If we could see an electron it
might look like this “cloud”
Modern/Wave/Cloud Model
 Electron’s
energy is quantized (has only
certain values)
 Electrons in probability zones called
“orbitals”, not orbits
- location cannot be pinpointed
 Electrons are particles & waves at same
time
 Electrons move around nucleus at speed
of light
Orbitals
A Missing Particle – The Neutron
James Chadwick discovered the
last major piece of the atom in
1932
• discovered the neutron, which
has no charge (“neutral”)
• won the Nobel Prize in physics
in 1935
neutron weighs about as much as
a proton while electron is much
smaller
(1891-1974)
The Complete Modern Atom
An atom is:
• mostly empty space
• nucleus has most of
mass of an atom
• nucleus contains protons
& neutrons
• electrons are in energy
levels around nucleus
• electrons jump between
levels, emitting and
absorbing energy as jump
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
The development of
atomic theory
represents the work
of many scientists
over many years
Next Atomic Theory ?
Which one of you will develop
a better theory and win the
Nobel Prize?