Download the electron! - Parkway C-2

Why is learning about the atom
important….?
• The atom was considered to be an
“imaginary” thing, something that people
laughed at!
• Roosevelt trusted Einstein, and funded his
secret research known as the “Manhattan
Project”
• In a few short years, the idea of an invisible
atom had been developed into a citydestroying weapon - the atomic bomb!
In Hiroshima, of a resident civilian population of 250 000 it was estimated that
45 000 died on the first day and a further 19 000 during the subsequent four
months. In Nagasaki, out of a population of 174 000, 22 000 died on the first day
and another 17 000 within four months. Over 90% of each city was destroyed.
Only two atomic bombs have ever
been used in wartime history The U.S. used them both against
Japan to end World War II….
The first video shows one of the
actual bombs being
detonated….The second, the first
testing of a “Hydrogen” Bomb…
Experiments in
Atomic Structure
•Democritus, a Greek philosopher, was the first the believe that matter was
“atomos”, or indivisible!
•This idea was lost for centuries, and then brought forth again by John Dalton
in 1803, and remained accepted for a number of years!
•These men, and most scientists, had believed that atoms were solid, marblelike structures
•Are atoms solid…?
•Atoms have parts, as we know!
•Many experiments were done with basic electricity that led to these parts
being discovered!
The Early 1800’s!
• Lots of introductory work with electricity
• Matter is electrical in nature!
• There are two types of “charge” in the world:
•
•
•
•
•
+ charged objects
- charged objects
Ben Franklin is the first
to assign these charges!
Franklin didn’t know that these
charges were part of the atom,
or part of matter!
But technology was limited until
the late 1800’s, early 1900’s….
• With technology comes
experimentation!
• With these experiments, what part of the
atom do you think was the first to be
discovered…..?
…………..THE ELECTRON!
Why do you think it was the
first to be discovered……?
Cathode Ray Tube, or Crooke’s
Tube
•
Evacuated tube
containing two metal
electrodes or plates
connected to a battery
• William Crookes designed
this tube in 1879
• The cathode ray inside
glows green
• Crookes was convinced
this beam consisted of
charged particles
So what are
Cathode Rays….?
• J.J. Thomson, in 1897,
answered this question!
• Thomson used a Crooke’s
tube and two charged
plates above and below
the beam
• Which way do you think
the beam bent….?
• The Beam was attracted
to + plate, and was
deflected from – plate!
• From this, he concluded
that cathode rays were
composed of negatively
charged particles!, called
electrons!
• He also saw that a magnet caused the beam to
move as well, concluding that magnetism and
electricity were related!
• This allowed him to calculate ratio of electron mass
to charge, which won him the 1906 Nobel Prize
What is each beam made of….?
What causes the beam to move to
each point….?
The cathode ray tube was the
precursor to the modern day…..
TELEVISION!
Black and White…..
Color…….
Millikan Oil-Drop
Experiment
• Robert Millikan in 1909 discovered the charge and mass of
an electron
• He placed electrically charged oil drops in an electric field
• The oil drops were sprayed into the upper chamber
• Some drops gained – charge (static electricity)
e- eee-
• Then Millikan adjusted
voltage on plates in
bottom chamber
• The - particles attracted to
the upper plate to offset
pull of gravity, and this
suspended the drop in
space
• From this he calculated
the charge of an electron,
knowing the charge on the
plates
• The smallest change
between two drops was
taken to be charge of an
electron
ee-
e-
e-
• Using Thomson’s mass to
charge ratio, Millikan could
calculate the mass of an
electron, too!
• The mass of an electron was
calculated to be 9.1X10-28 g really tiny compared to a proton
•
The
charge
of
an
electron
or neutron!
was calculated to be -1.60 x
10-19 C, which we simply
call -1
• Why would we care about
the exact mass and charge of
an electron….?
Now that we have found
negative particles….
• Eugen Goldstein in 1886 used a Crooke’s tube with holes in the
cathode and discovered positive particles
• He shot a cathode beam (beam of electrons) at hydrogen atoms
ee-
eee-
+
+
+
+
+
• The electrons from the atoms went with the beam of
electrons to the + plate
• He also observed + particles moving backwards
towards the + plate and going through the holes!
• Different numbers of + particles were produced
when different gases were used…..Why?
• Each element has a different number of protons!
Who puts all of this information
together into “The Big Picture”?
• J.J. Thomson theorized that these
negative and positive particles were
actually part of matter - part of the
atom!
• We are all made of charge!
• He develops the first working model
of the atom, known as the plum
pudding model!
• This rejected Dalton’s idea that atoms
were solid particles like marbles that
weren’t made of smaller pieces
Why was any of this
monumental…?
• People didn’t believe in atoms, and didn’t
understand what things like electricity and light
were!
• Thomson showed that electricity was nothing
more than a flow of little particles called
electrons!
• And electrons, along with protons, make up every
atom, in every person, plant, building and object
in the universe
• We are made of charged particles - the same
particles that we use for electricity
So atoms are made of + and - particles. How are
some atoms “radioactive”? What is
“radioactivity”?
• Antoine Becquerel, in 1895, put a chunk of Uranium in a
desk drawer with some photographic paper
• When he came back after the weekend, the paper had a
picture of the Uranium chunk on it!
• Becquerel knew the Uranium atoms must be giving
“something” off. He called this something “radioactivity”
or “radiation”.
• The same year, William Roentgen had a similar experience
- his Uranium actually caused photographic paper to
develop in a nearby lab - through a wall!
• He called his rays, “X-Rays”. They were immediately
used for medical purposes!
• What was this “radiation”, though? How did it relate to
what Thomson had discovered?
Ernest Rutherford was determined to figure out
what this “radiation” was!
• Ernest Rutherford, a physicist from New
Zealand, took a chunk of uranium, and
set out to determine what it “gave off”!
• He placed the Uranium in a lead box
• He took two charged plates, and placed
them above and below the beam of
“radiation” that the Uranium was giving
off
• It split into three beams!!!
Rutherford’s Radiation Experiment
What are the beams made of….?
• One beam went to the
positive plate
• One beam went to the
negative plate
• One beam went
straight through,
unaffected
• There were three types
of radiation, not one
that were given off!
• What were these
beams made of……?
The beam that goes to the
negative plate must be….
• Positively charged!
• Rutherford called this
alpha (radiation, or A
radiation
• They are actually Helium
Nuclei
• 2 protons, 2 neutrons
• Charge of +2
• Mass of 4
• They move very…..
• Slow!
• Why…..?
• They are very heavy!
• Not much energy!
The beam that goes to the
positive plate must be….
• Negatively charged!
• Rutherford called this
beam beta ( radiation, or
B radiation
• They are actually…..
• Electrons!
• Charge of -1
• Mass of 0
• They move very…..
• Fast
• Why…..?
• They are very light!
• Move fast, but no energy
because no mass!
e e- e- e- e- e - e- e- e-
The beam that goes straight
through is……
Not charged at all!
•
• Rutherford called this
gamma (radiation, or C
radiation
• What was this beam made
of….?
• Can’t be neutrons - they are
part of the alpha radiation!
• It is made of…..
• Pure energy or light!
• No mass!
• Moves at the speed of….
• Light!
• Can be deadly, depending on
what kind of light it is!
Why do atoms give off radiation,
then?
• Atoms, we know, contain
protons, neutrons, and
electrons
• Up until this point, only
protons and electrons were
known!
• Why did some atoms give
off radiation?
• To answer this question,
we have to look back at
the structure of the atom!
•People still accepted Thomson’s
model of the atom!
•His model was wrong though!
Why is Thomson’s
model wrong….?
• Rutherford decides to conduct another experiment, using this
new found “radiation”!
• He shoots alpha () particles at ultra thin gold foil
• Most of the particles went straight through, and did
NOTHING.
• A very few were deflected, and some even bounced straight
back!
• Rutherford described this as a bowling ball being sent at a
piece of paper, full speed, and bouncing back!
• Rutherford’s graduate student, Earnest Marsden, actually
conducted the experiment!
The Gold Foil Experiment….
What is happening? Why are the alpha particles bouncing
off?
Gold Foil Experiment
What is going on….?
•
•
•
•
•
•
•
The alpha particles are bouncing off of something!
What charge does an alpha particle have?
What could it be bouncing off of?
They must be bouncing off of something that is….
Positive!
And big!
Rutherford theorized that all of the protons
weren’t scattered about, but were all concentrated
into a dense center, which he called the nucleus!
• The electrons must be orbiting around this
nucleus! Why?????
Why did he think that ALL the
protons were in the center?
•
•
•
•
•
•
Alpha particles are huge!
They contain 2 protons and 2 neutrons!
They have a positive charge, because neutrons have no charge!
Like charges….
REPEL
But one proton wouldn’t have enough force to push away an alpha
particle coming in!
• He knew that there must be MANY protons in the center blocking the
alpha particles!
Alpha Particle
p+
p+
n
n
Nucleus
p+ won’t stop
Alpha Particle
p+
p+
n
n
p+
Nucleus
many p+ will stop
p+ p+
p+ p+
p+
Rutherford’s “nuclear” model!
e-
eNucleus
p+ p+
p+ p+ p+
p+ p+
e-
e-
eee-
What is the relative size of the
atom….?
• An atomic model the size of
Busch Stadium and parking
would contain a pea sized
nucleus containing 95.95% of
the atoms mass.
• The pea at the pitcher’s mound
would be the nucleus, and an
ant crawling on the parking lot
outside would be an electron!
• The atom is mostly…..
• Empty space! Nothing!
So why do some atoms give off
radiation????
•
•
•
•
•
•
Rutherford looked at his model of the atom for answers!
All of the protons are jammed into a tiny little nucleus!
And they all have what charge….?
The same charge! Positive!
Which means…..
They are constantly trying to push each other out! Like
charges REPEL!
Help us!
We are
• What keeps them in there…..?
crammed
in here!
Nucleus
p+
p+
p+ p+
p+ p+
p+ p+ p+
……..The neutron does!
•
•
•
•
•
•
•
•
The neutron was the last particle to be discovered!
Why was it the last particle to be discovered….?
IT HAS NO CHARGE!
It was discovered when James Chadwick noticed that Beryllium atoms
were giving off some “unknown particle” when hit with alpha radiation!
These particles were neutrons!
Why are the neutrons even in the nucleus?
They have a special force, known as “STRONG NUCLEAR FORCE”,
that holds the protons together, because the protons are always pushing
each other out!
I call it “NEUTRON GLUE”!
I discovered the
neutron, and I
am very serious
about what I
do!
Nucleus
p+
n
p+
Chadwick’s experiment…..
•These particles were coming
from the nucleus!
•These particles, though, were not
affected by charge!
•These particles were heavy
enough to move protons!
•These particles couldn’t be light,
though!
•Light has no mass, and can only
move electrons - not protons!
Mass
Spectrometers….
How the neutron
was discovered…
Problems after element 82…..
•
•
•
•
•
•
•
After lead, element 82, there are too many protons in the nucleus, and no
number of neutrons can hold in the protons
The atom has gotten too big, and begins ejecting stuff to release the pressure!
This stuff is called radiation, or radioactivity!
Radioactivity is when an atom has gotten too big and it gives off stuff to
relieve its pressure
Most atoms normally have more neutrons than protons anyway to keep the
protons in place - atoms need added reinforcement!
So, any atom after lead is constantly giving off radiation to relieve its pressure!
What kind of “stuff” could an atom give off if it has gotten too big?
92 e-
URANIUM
ATOM THAT IS
TOO LARGE

2 p+
2n
92 p+

126 n

e-
ENERGY
In Summary,
Type of
radiation
Alpha (
Mass (amu)
4
Animation
charge
Speed and
Energy
+2
Slow and no
energy
Beta (B
0
-1
Gamma (
0
0
Fast, but no
mass so little
energy
No mass, but
speed of light!
Can be
dangerous!
In summary…..
•
•
•
•
•
We know that the nucleus contains…
Protons and neutrons!
And the electrons are…..
Orbiting around the outside!
Atoms that have too many protons in the
nucleus (more than 82) will give off…
• Radiation! To relieve the pressure!