Transcript teachers.sd43.bc.ca

HISTORY OF STRUCTURE OF THE ATOM
Early models of the atom
• Century 5th BC, Democritus used the word “atomos.” He said that
the differences between substances were the direct result of
differences in the size & shape of tiny, uniform, uncuttable
particles
• Aristotle & other C 4th BC scholars rejected Democritus’s idea
• The Greeks said there were 4 elements:
• Air, fire, earth, water
• The properties were : hot, dry, moist, cold
ALCHEMISTS – MIDDLE AGES
Eurpoean alchemists tried to change lead into gold
• Alchemists experimental work
involved separation of metals
from ores and many types of
distillations
• Ideas were based on philosophy
DALTON’S ATOMIC THEORY
In 1908 Dalton re-introduced the idea of the atom & supported his theory by
experimentation. His theory summarized:
• Elements are made up of extremely small
particles called atoms
• Atoms of an element are all identical &
different types of atoms have different
properties
• Each compound is unique & consists of a
particular combination of specific types of
atoms put together in a distinctive way
• Chemical reactions involve the
reshuffling of atoms in a compound to
make new compounds. The new
compounds are made from the same
atoms which were present in the
original compounds
• Dalton’s model of the atom was an
indivisible, uniformly dense,
solid sphere that entered into
chemical reactions but was left
unchanged by the reaction
• Dalton’s theory lead to laws like the
Law of Conservation of Mass
(Total mass of reactants = total mass of
products)
DALTON’S MODEL OF THE ATOM
Dalton describe atoms as uniformly dense solid spheres
SIR WILLIAM CROOKES’ CRT EXPERIMENTS
Crooks used a Cathode Ray Tube & found the atom to be electrical
• The CRT is a tube containing a gas
at low pressure with two electrodes
implanted which are connected to
an electric power source
• The negative electrode is a cathode
& the positive electrode is the
anode
• With power on, a green glowing
beam was emitted from the
negative electrode
• Crooks concluded that the beam
was composed of particles & that
the beam
must be
negative, since it came from the
negative electrode
CATHODE RAY TUBE. TV’S & COMPUTER
SCREENS ARE CRTS
JJ THOMSOM USED CRT
• 1897 Thomson conducted further experiments using cathode ray tubes
• Thomson used a CRT with a fluorescent
screen at the end. The screen would glow
whenever stuck by a beam of charged
particles.
• Thomson positioned a magnet close to the
CRT
• He found that the cathode rays were
deflected in the same direction by the
magnetic field as negatively charged particles
would be
PLUM PUDDING MODEL
• Later these particles were identified
as ELECTRONS
• Thomson’s model of the atom was
called the PLUM PUDDING model
because he used a sphere with
subatomic particles embedded:
negatively charged electrons &
positively charged protons
dispersed to give an overall neutral
charge
• By 1920 Thomson ionized
hydrogen into H+ & e-. The
positive proton in the nucleus was
known
JJ THOMSON’S PLUM PUDDING MODEL OF THE
ATOM
BECQUEREL DISCOVERED RADIOACTIVITY BY
ACCIDENT
1896 Becquerel investigated the nature of X rays, which he knew had the ability to
fog a photographic plate even when the plate was covered by black paper
•
He coated a some black paper with a uranium compound & put on a
photographic plate. When exposed to sunlight the plate showed the outline
of the compound from the paper.
•
He put the experimental plates away in a dark closet, to wait for another
sunny day to continue experimenting.
•
He decided to develop the plates, even without sun exposure. The outline of
the uranium compound was there.
•
He concluded that the uranium itself must have given off some kind of rays
on its own without the outside influence of sunlight
EARNEST RUTHERFORD DISCOVERED 3 TYPES
OF RADIOACTIVE EMISSIONS
Gold foil experiments were designed to bombard thin metal foil (gold) with alpha
particles. When some particles were deflected, instead of passing straight through, he
concluded that they must have an electrical charge
• Rutherford realized that the centre of the atom must have a positive charge. He
called the centre the nucleus
• It was the positive nucleus of atoms in the gold foil that cause the positive alpha
particles to deflect
• An alpha
particle is a high speed positively charged particle that has a mass = to
a helium nucleus
• A beta particle is a high speed electron
• Gamma rays have no mass, but are high energy rays, with more energy than Xrays.
• All three types of radioactive emissions come out of the unstable nucleus of elements
that are radioactive, like uranium
RUTHERFORD’S GOLD FOIL EXPERIMENT
Rutherford expected the result in the top red picture. Instead he found deflection,
showing that some of the + charged alpha particles hit some of the nuclei of the gold
atoms & were deflected.
ROBERT MILLIKAN1909
Millikan successfully measured the charge on an electron using a falling,
negatively charged oil drop
• Oil drops were sprayed into a chamber & exposed to X rays. The chamber has a
positively charged upper plate & a negatively charged bottom plate
• A falling, negatively charged oil drop is made to rise between the plates
because of exposure to X rays, which ionized some of the drops. There were
positive and negative ions
• He calculated the charge on the drop by knowing the rate at which the drop
was rising, the strength of the electrical field, & the weight of the oil drop
• Millikan used his experiments & Thomson’s to calculate the
mass of the electron to be 9.11 X 10 (-28th ) grams
MILLIKAN’S OIL DROP EXPERIMENT
The ionized oil drops that were negatively charged actually were repelled by the
negative bottom plate & rose up.
MARIE & PIERRE CURRIE
• Curries followed up Becquerel’s discovery of radioactivity & discovered the two
radioactive elements Radium & Polonium
• Curries coined the term “radioactive decay” for unstable elements that emit
radiation from their nuclei
• Their daughter Irene Joliot-Currie & her husband Frederic did further research
• They bombarded beryllium (Be) with alpha particle (He nucleus +2 charge) & a beam
of high penetrating power was formed
• JAMES CHADWICK demonstrated that the beam was made of particles with the same
mass of Protons, and it was NOT affected by electric or magnetic fields
• Chadwick was credited with discovering the neutron.
named so because they did not possess a charge (neutral)
Neutrons were
NEILS BOHR
1913 Bohr expanded on Rutherford’s model of the atom
• Bohr’s model has a defined nucleus in the centre with positive protons & neutral
neutrons, surrounded by electrons in “energy
levels” or “orbits”
• In a neutral atom the # of protons (+) = the # of electron (-)
• The “rules” for filling the energy levels with electrons: first orbit has room for 2
electrons, second & third orbit has room for 8 electrons. “Stable octet” is the term
used to describe a full shell. In models, we fill orbits with electrons this way: 2, 8, 8,
18, 18, 32… Then it becomes more complicated
HISTORY OF ATOMIC MODELS
Sketch the atomic models.
We will use Bohr models to draw on flat paper, but be
aware that all atoms are 3D, like the most current model depicts