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 city-destroying 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, marble-like 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 -

e -

e - e -

• 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

e-e-

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 or neutron! • The charge of an electron

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

e-

e-e- +

+ ++

+

e-e-

• 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!

• 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!

Why do atoms give off radiation, then?

•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!

p+p+ p+

n n

p+ won’t stop

p+ p+p+ p+

p+

many p+ will stopp+ p+

n n

p+

Alpha Particle

Alpha Particle

Nucleus

Nucleus

Rutherford’s “nuclear” model!

p+ p+p+ p+

p+

Nucleus

p+p+

e-e-

e-

e-

e-e-

e-

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!• What keeps them in there…..?

p+ p+p+ p+

p+

Nucleus

p+p+

p+

p+

Help us! We are crammed in here!

……..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”!

p+

p+

n

NucleusI discovered theneutron, and I am very seriousabout what Ido!

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 p+

126 n

92 e-

URANIUM ATOM THAT IS TOO LARGE

2 p+2 n

e-

ENERGY

In Summary,

Type of radiation Mass (amu) charge Speed and

Energy

Alpha ( 4 +2 Slow and no energy

Beta ( 0 -1Fast, but no

mass so little energy

Gamma 0 0

No mass, but speed of light!

Can be dangerous!

Animation

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!