2. The Atom - Presentation Thurles Science...
Transcript of 2. The Atom - Presentation Thurles Science...
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2. The Atom
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2.1 Evidence for the existence
of the electron • The universe consists of energy & matter.
Matter occupies space and has mass.
• Law of conservation of mass states matter is neither created nor destroyed in the course of a chemical reaction
• All matter composed of particles (atoms, molecules or ions)
• Evidence is shown by expt involving the diffusion of ammonia gas and HCl gas in a glass tube
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Diffusion
Diffusion: is the movement of
gas from areas of high gas
concentration to areas of low
gas concentration
Definition!
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Diffusion of ammonia and HCl
gases
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2.1 Evidence for the existence
of the electron
• Atoms very minute particles
• e.g. H atom in its normal state has
radius of about 4 x 10-11m
• The atom is made of even smaller
particles
• The atom is considered to be the
basic unit from which all substances
are formed
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2.2 History of the
atom
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Greeks • Some ancient Greeks
noticed that
substances such as
iron, gold, lead, silver
were 'pure'
• They could not be
broken down into
simpler substances
by heat.
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400 BC
• The Greek philosophers
were the first to propose
that matter is made up of
tiny particles.
• They didn’t do any
experiments to back this
up.
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Greeks
The Greek word for ‘indivisible’ is
‘atomos’ from where we get the
word atom to describe these
indivisible particles
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• 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.
A Brief History of Chemistry
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
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The Greeks History of the Atom
• Not the history of atom, but the idea of the atom
• The Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air.
• Democritus and Leucippus Greek philosophers
~ ~
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Greek Model
• Greek philosopher
• Idea of ‘democracy’
• Idea of ‘atomos’
– Atomos = ‘indivisible’
– ‘Atom’ is derived
• No experiments to support
idea Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Democritus
“To understand the very large,
we must understand the very small.”
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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”.
Democritus
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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.
Some Early Ideas on Matter
O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,
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.
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Who was Right?
• Greek society was slave based
• Beneath famous to work with hands
• did not experiment
• Greeks settled disagreements by
argument
• Aristotle was more famous
• He won!
• His ideas carried through middle ages.
• Alchemists change lead to gold
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John Dalton
• 1808
• Proposed
an Atomic
Theory
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• Developed a theory – that matter
is composed of tiny, indivisible
particles called atoms that can
neither be created nor
destroyed i.e. no change in mass
‘law of conservation of mass’
John Dalton (Atomic theory)
.. the existence of atoms
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Conservation of Atoms
John Dalton
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
2 H2 + O2 2 H2O
4 atoms hydrogen
2 atoms oxygen
4 atoms hydrogen
2 atoms oxygen
H
H
O
O
O
O
H
H
H
H
H
H
H2
H2
O2
H2O
H2O
+
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Legos are Similar to Atoms
Lego's can be taken apart and built into many different things.
H
H
O
O
O
O
H
H
H
H
H
H H2
H2
O2
H2O
H2O
+
Atoms can be rearranged into different substances.
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45 g H2O ? g H2O
Conservation of Mass
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
High
voltage
Before reaction
electrodes
glass
chamber
5.0 g H2
80 g O2
300 g (mass
of chamber)
+ 385 g total
H2 O2
High
voltage
After reaction
0 g H2
40 g O2
300 g (mass
of chamber)
+ 385 g total
O2
H2O
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John Dalton proposed his atomic
theory.
1. All matter is made up of small
particles called atoms.
2. Atoms are indivisible – they cannot
be broken down into smaller
particles. False!
3. Atoms cannot be created or
destroyed. False!
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FACT
We now know that there are
particles within atoms –
subatomic particles e.g. protons,
neutrons and electrons.
Radioactive atoms can change
into other atoms.
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Dalton’s Symbols
John Dalton
1808
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Daltons’ Models of Atoms
Carbon dioxide, CO2
Water, H2O
Methane, CH4
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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.
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Discovery of the Electron
• In the 1800’s electricity was new,
exciting and the subject of a lot of
study.
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William Crookes
• 1875
• Crookes passed an
electric current through
a complete vacuum just
to see what would
happen.
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Crookes experiment
• Electricity seemed to leap across the
vacuum from the negative plate to the
positive plate.
• The radiation given off from the negative
cathode was called Cathode rays.
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Crookes Tube
William Crookes
Mask holder
Cathode
(-)
Anode
(+)
Crookes tube (Cathode ray tube)
Mask holder
Glow
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William Crookes
A discharge tube
• A long glass tube fitted with a metal electrode at each end
• When the gas inside is at a very low pressure, passing electricity through the tube results in invisible rays travelling in straight lines from the cathode (-ve) to the anode (+ve)
• Small objects placed in the path of the rays cast a sharp shadow in the fluorescence at the end of the tube
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Crooke’s Vacuum tube
Cathode (-)
Anode (+) High
Voltage
Vacuum tube means most of the air has been pumped out of it
Maltese Cross
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The Effect of an Obstruction on
Cathode Rays
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
High
voltage
cathode
source of
high voltage
yellow-green
fluorescence
shadow
Anode
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Crooke’s Vacuum tube
An electric current passing through air causes the air to glow
Cathode (-)
Anode (+)
Rays coming from the –ve electrode (cathode) cast a shadow
at the far end of the tube
Shadow of cross
on glass
Cathode rays Maltese cross
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• The cathode rays caused shadows and could spin light paddle wheels.
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Crooke’s experiment
• This proved that the cathode rays
carried energy, and that they
might be made of particles.
• This also indicates that the rays
(particles) moved from the
cathode to the anode.
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William Crookes
(cathode rays) (Hons)
• Showed some form of radiation passes from the -ve electrode (cathode) to the +ve electrode (anode) when a current passes through a vacuum tube
-ve cathode +ve anode
CNAP
• He called these cathode rays and was convinced they consisted of particles
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J J Thomson
• Was very interested in the Crooke’s discoveries.
• 1897 He used a cathode ray tube that had a positive and negative plate between which the rays had to travel.
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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 .
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A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
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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.
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CATHODE RAYS IN ELECTRIC
FIELDS
Thompson’s cathode ray tube
-
PROVIDES
ELECTRICAL FIELD
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Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
The Effect of an Electric Field on
Cathode Rays
High
voltage
cathode
source of
high voltage
positive
plate
negative
plate
anode
_
+
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Thomson’s Experiment
+ -
vacuum tube
metal disks
voltage
source
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Thomson’s Experiment
+ -
voltage
source OFF
ON
Passing an electric current makes a beam appear
to move from the negative to the positive end
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Thomson’s Experiment
+ -
voltage
source OFF
ON
+
-
By adding an electric field…
he found that the moving pieces were negative.
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• He found the cathode rays were
attracted towards the positive
plate.
• The rays had to be negative (as
opposites attract).
Thomson
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• 1897
• Thomson announced that cathode
rays consisted of negative charged
particles.
• Thomson had discovered the
ELECTRON!
• (He called them corpuscles)
Thomson 1897
Discovered the electron
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George Stoney
1891 UCG
• Proposed that the
smallest particle that
has a negative
charge should be
called an electron.
• So Thomson's
corpuscles were
called electrons.
An Irish scientist
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Electrons
Cathode rays are streams of
negatively charged particles
called electrons
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• But where had these tiny particles come
from?
• Since they were so small, Thomson
suggested that they could only have come
from inside atoms.
• So Dalton's idea of the indestructible atom
had to be revised.
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1808 AD
• John Dalton proposed his atomic theory.
1. All matter is made up of small particles
called atoms.
2. Atoms are indivisible – they cannot be
broken down into smaller particles.
False!
3. Atoms cannot be created or destroyed.
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• This discovery implied that Dalton was
wrong and the atom is not the smallest
particle of matter.
• It looked like the atom could be broken
down into smaller pieces – electrons.
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JJ Thomson
• He did another experiment.
• He placed an electro magnet around the
vacuum tube.
• He changed the strength of the magnetic
field to match the force of the electric field.
• Using the magnetic field he was able to
bring the beam of electrons back down to
its original position.
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Cathode Ray Experiment
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JJ Thomson
Electromagnet
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JJ Thomson (Hons)
• From the results of his expt:
• He was able to calculate the ratio of the
charge of an electron to its mass.
• This is the of the electron. e
m
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JJ Thomsom (hons)
• But he couldn’t determine the
individual value of either number
• He found that these –vely charged
particles were about 2000 times
lighter than H atoms and were to be
found in all matter
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JJ Thomsom
• Also proposed the ‘plum pudding’
model of the atom
• He visualised an atom as a sphere
of +ve charge into which –ve
electrons were embedded at
random
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Thomson Model of the Atom
• 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 -
-
-
-
-
-
-
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Plum-Pudding Model
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
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Robert Millikan (Hons)
• Oil Drop Experiment
• Determine the mass
& charge of the
electron
Actual apparatus used
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Robert Millikan
• 1911, Millikan used the ‘Oil Drop’ expt to measure accurately the charge on the electron (see book)
• This then allowed its mass to be calculated
• The mass is much smaller than that of any atom
• Confirming that the atom is made up of smaller particles
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Millikan’s oil drop expt
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Millikan’s oil drop expt
• He sprayed tiny drops of oil between two
charged plates.
• He used x-rays to cause the molecules in
the air to lose electrons.
• As the oil droplets passed through, they
picked up electrons from the air. i.e. the
oil droplets were now –vely charged
Molecules in air Oil droplets -
Electrons -
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Millikan’s oil drop expt
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Oil Drop Experiment
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oil droplets
oil droplet
under observation
Charged plate
Small hole
Charged plate
-
+
Telescope
oil atomizer
Robert Millikan
X-rays
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Millikan’s oil drop expt
• By attaching a battery to the
plates, he created an electric field
between the plates that would act
on the charged oil drops .
• i.e. the –ve oil droplets were
attracted to the +ve charged plate
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Millikan’s oil drop expt
• He adjusted the voltage
• till the electric field force would
just balance
• the force of gravity on an oil drop,
• and the oil drop would hang
suspended in mid-air.
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When the oil
droplet was
stationary, the
weight of the oil
droplet due to
gravity must have
been equal to the
electrical force
pulling the droplet
upwards
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Robert Millikan
• Using this information , He was able to
calculate:
1. The size of the charge on
the electron
2. The mass of the electron.
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JJ Thomson
• Since atom is neutral
• There must be a
positive charge to
neutralise the negative
electrons.
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JJ Thomson
• He developed the
idea that
• atoms are made of
negative electrons
embedded in a gel of
positive charge
• (a "plum pudding"
model).
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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?
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1909
• Scientists began to question
the simple view of atom.
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Ernest Rutherford
Discovered the nucleus & proton
• Worked in JJ
Thomson’s lab
• overturned Thomson's
atom model in 1911
• with his gold foil
experiment
• in which he
demonstrated that the
atom has a nucleus.
(The modern view of the atom was developed by Ernest Rutherford)
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Ernest Rutherford
• Two of Ernest Rutherford's
students, Hans Geiger and Ernest
Marsden, were doing an
experiment at Manchester
University with radiation.
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Ernest Rutherford
• They were using the dense,
positively charged particles
(called alpha particles)
• as 'bullets' to fire at a very thin
piece of gold foil.
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Rutherford ‘Scattering’
particle
source
Lead collimator Gold foil
a
q
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• They expected the
particles to pass straight
through the gold atoms.
GOLD ATOM
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Experiment method
• Bombarded a thin leaf of gold foil
with alpha particles (+vely
charged)
• Used a zinc sulfide screen to
detect scattered alpha particles
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Rutherford’s Goil Foil Experiment
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Experiment Results
Gold atom
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Ruherford’s experiment
• Most alpha particles were
observed to pass straight
through the gold foil.
• They had passed through
basically empty space!
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Rutherford’s experiment
• A few, were scattered at small
and large angles (90°), and
some even bounced back
toward the source.
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Interpreting the
Observed Deflections
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
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gold foil
deflected particle
undeflected
particles
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. beam of
alpha
particles .
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Density and the Atom
• Since most of the particles went through, the atom was mostly empty.
• 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
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Discovery of Nucleus
• Only a positively charged and
• relatively heavy particle,
• such as the proposed
nucleus, could account for
such strong repulsion.
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New model of Atom
• Tiny, dense, positively charged
core called a nucleus,
• around which the light, negative
electrons, circulate at some
distance, much like planets
revolving around the Sun.
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• Top: Expected results of Rutherford's gold foil experiment: alpha particles passing through the plum pudding model of the atom undisturbed.
• Bottom: Observed results: Some of the particles were deflected, and some by very large angles. Rutherford concluded that the positive charge of the atom must be concentrated into a very small location: the atomic nucleus.
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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
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Discovery of Proton
• Rutherford
continued to
bombard various
elements with
Alpha particles.
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Ernest Rutherford (Hons)
• With light atoms e.g. Oxygen and
Nitrogen
• The alpha particles broke up the
nucleus to release positive
charged particles.
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Rutherford
had discovered the
proton.
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James Chadwick
Discovered the Neutron
• Bombarded a
sample of Beryllium
with alpha
particles.
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Discovery of Neutron (Hons)
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James Chadwick
• Some type of radiation with no
charge came from the
Beryllium.
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James Chadwick
• The particles had the same
mass as protons but no
charge.
• He called them neutrons
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Models of the Atom
Dalton’s model
(1803)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Greek model
(400 B.C.)
+ - -
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+ +
e
e e
e
e
e e
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg
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2.3 Properties of sub-atomic
particles
Particle Relative
charge
Relative
mass
Location
Proton +1 1 Nucleus
Neutron 0 1 Nucleus
Electron -1 1/1838 ‘Shells’
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Recheck
• Evidence for the existence of small particles
• History of the atom
• Discovery of the electron
• Thomsom’s plum pudding model of the atom
• Millikan’s oil drop expt
• Discovery of the nucleus
• Discovery of the proton
• Discovery of te neutron
• Properties of sub-atomic particles