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Atomic Theory
PS 9.26
PS 9.27
What is the nature of matter?
• The ancient Greeks
pondered that
• Empedocles in 460
B.C.suggested that all
matter was made up of
four elements, earth,
air, fire and water.
• Democritus and Leucipus in 370
B.C. suggested that if you cut a
piece of matter into smaller and
smaller pieces, eventually, you
will find that you will come to a
particle which you cannot divide
any smaller.
• He called these particles atomos,
which means indivisible. The
word atom is derived from this.
Continuous Theory
• Aristotle in 320 B.C. disagreed
with Democritus
• He thought that you could keep
cutting a piece of material
• The argument went back and
forth for over 2000 years.
A New Atomic Theory
• In 1803, the next great
development in Atomic
Theory came to be.
• John Dalton
developed a theory
which has come to be
known as the “Billiard
Ball Model”.
Reasons for Dalton’s Theory
• The Law of Definite Proportions.
If matter exists as atoms, the
atoms always combine in the
same ratios in any given
Principles of Billiard Ball Model
All elements are composed of tiny
indivisible particles called atoms
Atoms of the same element are
identical. Atoms of any one element
are different from those of any other
Atoms only combine in small, whole
number ratios to form compounds
Atoms cannot be created or
destroyed or changed.
Discoveries of the 19th Century
• Michael Faraday made an important
discovery in 1825.
• One of his discoveries involved
hooking a high voltage source up to a
glass tube from which he removed
most of the air.
• He observed an eerie glow in the
tube. Other scientists investigated.
Building on Faraday’s Work
• Geissler developed a better
vacuum pump in 1875, enabling
scientists to remove even more of
the air from the tubes.
• They studied the glow in the tube
and concluded that the glow was
caused by rays coming from the
• They called the rays Cathode
• William Crooks developed tubes
which led to some major
discoveries in 1873.
• One tube that he created looked
like this.
• The cathode
rays struck a
metal piece
shaped like a
• The cathode
rays left a
shadow, proving
that the cathode
rays travelled in
straight lines
unless acted
upon by other
• He also discovered that when he
put a small metal paddle wheel in a
cathode ray tube that the cathode
rays caused it to spin.
Here is a closeup of the
paddle wheel
• He concluded from this that the
cathode rays must have mass.
• Other experiments showed that
the cathode rays were deflected
by electrical fields and magnetic
• From this, scientists deduced that
the cathode rays were, in fact,
tiny positively charged particles,
streaming from the cathode.
• JJ Thomson studied
these particles.
• On April 30, 1897, Joseph
John (J.J.) Thomson
announced that cathode
rays were negatively
charged particles which
he called 'corpuscles.'
• We now call them
electrons. He devised
experiments to study the
properties of these
Thomson developed a new
theory called the Raisin Bun
Model (Or Plum Pudding)
• Thomson described the atom
as being like a raisin bun.
The electrons were little, (-)
raisins embedded in a (+)
• The total (-) charge and total
(+) charge must balance out
• The next development in atomic
theory came about as a result of
an amazing discovery in 1895.
• In that year, working with
specialized cathode ray tubes, a
scientist by the name of Wilhelm
Roentgen discovered a
marvelous new ray which he
called X-rays.
• A French scientist named
Becquerel decided to see if he
could find any substances which
would spontaneously give off Xrays.
• One day, in 1896, he
inadvertently left uranium sitting
on a covered photographic plate.
When he exposed the plate, he
discovered a strong image on the
Henri Becquerel
• Pierre and Marie
Curie worked to
discover other
elements which gave
off these rays 1898.
• They called these
rays radioactivity.
• While working at McGill in 1911,
Ernest Rutherford discovered
that there were three types of
radioactivity, which he called
alpha, beta and gamma. Alpha
released a helium atom. Beta
releases electrons. Gamma rays
releases photons.
Gold Foil Experiment
• He set up an
apparatus using a
source of alpha
• The alpha particles
were directed
through a narrow slit
at a gold foil.
The Nucleus
• Rutherford devised a new theory
for the atom.
• He said that the atom consisted
mostly of empty space. At the
center of this empty space is a
small, hard, dense positivelycharged central particle which he
called a nucleus.
The Nuclear Model
A scientist named Moseley in 1914,
showed that each element has a
unique number of protons. A
hydrogen nuclei has one proton, a
helium nuclei has two and so on. The
number of protons in an atom’s
nucleus is known as its atomic #.
It has been discovered that the amount
of charge is equal and opposite to the
amount of charge on an electron.
Therefore, any neutral atom must
have equal numbers of protons and e-
• Neils Bohr in 1922, discovered
that the electrons in an atom were
not just randomly placed around
the nucleus but instead were in
orbits. This was the beginning of
the quantum theory.
• James Chadwick in 1932
discovered a neutral subatomic
particle that had almost the exact
same mass as the proton and
therefore explained why an
atom’s atomic mass did not match
the weight of all the protons and