Transcript From his information, Rutherford proposed that an atom had a small

The
History of
the Atom
Aristotle
 Aristotle
was the first scientist that we
have record of questioning what stuff
was made of.
 What
did he think?
The search for finding out about
matter began with the Greek
philosopher Democritus more than
2000 years ago.
THE QUESTION
Could matter be divided into
smaller and smaller pieces
forever, or was there a limit to
the number of times a piece of
matter could be divided?
The Answer:
Democritus concluded that matter could not
be divided into smaller and smaller pieces
forever. Eventually the smallest piece
would be obtained that could not be
divided. This piece is called the “atom”
coming from the Greek word “atomos”
which means not to be cut, or indivisible.
The hypothesis:
The people that shared this thought with
Democritus hypothesized that atoms were
all small, hard particles that were all made
of the same material, but were of different
sizes and shapes.
Is that
hypothesis
correct?
In 1803, John Dalton from England
combined his and other observations
to propose the atomic theory.
The Atomic
Theory
** All elements are composed of atoms. Atoms are
indivisible and indestructible particles.
**Atoms of the same element are exactly the alike.
**Atoms of different elements are different.
**Compounds are formed by joining atoms of 2 or
more elements.
John Dalton’s model

Billiard Ball Model
atoms are neither created or
destroyed in a chemical reaction
Law of conservation of Mass
Laws of Proportions
 Definite:
 Multiple:
water is always 2:1
rearrange and the same
elements in a 2:2 ration= a new
substance!!
The Ancestor of the TV
Tube.
At the end of the 19th century
cathode rays were being
experimented with. These are
produced in glass tubes under a
vacuum and the tube it is in is
called a cathode ray tube (CRT).
In 1897 J.J. Thompson observed
that the cathode rays are made
up of very small, negatively
charged particles that are a
fundamental part of an atom.
**What did J.J. Thompson just discover?
Thompson discovered the electron. He knew that
something had to be in the atom to balance the negative
charge, but was unable to find it.
With his information he created a new model of the
atom and called it the plum pudding model.
Millican

Discovered the MASS of the electron to be
1/2000 (1/1837) of a Hydrogen atom.
 All atoms have electrons (acted the same in
the cathode ray)
 2 inferences made from here:


Since atoms are electrically neutral-ther are = #s
of protons and electrons
Since electrons mass is so small, most atom’s
mass must be in the neucleus.
The Next New Find
In 1908 Ernest Rutherford fired a stream
of positively charged particles at gold
foil and found that most of them passed
right through while others hit something
and bounced away.
What would cause something to repel
a positive charge?
Another positive charge.
From his information, Rutherford proposed that an
atom had a small, dense, positively charged center
that he named the nucleus.
The Rutherford Model
In 1913 an improvement
to the Rutherford model
was made by Niels Bohr.
According to this model,
the electrons move in
definite orbits around the
nucleus, much like
planets in the solar
system. These orbits, or
energy levels, are
located at certain
distances from the
nucleus.
Niels Bohr
The current atomic
model has electrons
in a probable
location based on
how much energy
the electron has.
An electron cloud is
the space in which
the electrons are
likely to be found.
This cloud is broken
into energy levels.
Electrons with lower
energy levels are
found close to the
nucleus.