Transcript Democritus: >ancient Greek philosopher, not scientist >first to

Atom

Definition: the
smallest particle
of any element
that retains the
properties of
that element.
Practice quiz question:
What is the smallest particle of
magnesium that retains the
properties of magnesium?
Interesting side note…


Atoms are fantastically durable.
Because they last so long, atoms
really get around!
Every atom you possess has
almost certainly passed through
several stars and been part of
millions of organisms on its way
to becoming you.
Interesting side note…

It has been suggested
that up to a billion of
each person’s atoms
once belonged to
Shakespeare. Another
billion came from
Beethoven. You
probably have some
atoms from any
historical figure you
can think of!
Nowadays, with the use of tunneling
electron microscopes, we can sort of
“see” atoms…
image of copper atoms

This is an image of silicon atoms
arranged on a face of a crystal. It is
impossible to "see" atoms this way
using ordinary light. The image was
made by a Scanning Tunneling
Microscope, a device that "feels" the
cloud of electrons that form the outer
surface of atoms, like a phonograph
needle feels the grooves in a record.
These electron microscopes
were invented in the mid-1900’s.
How did scientists figure out
atomic structure without being
able to look at any atoms?
The History of Atomic Theory
But first…
What is a theory in science?
A theory is a well-tested explanation of
what happens in nature.
In layman’s terms, if something is said
to be “just a theory,” it usually means
that it is a mere guess, or is
unproven. But in scientific terms, a
theory implies that something has
been well-tested and verified many
times by multiple groups of scientists.
And while we’re at it…
A scientific law is a statement of
something that seems to be true in
the natural world.
Example: The Law of Gravity. Newton
could use this law to predict the
behavior of a dropped object, but he
couldn't explain why it happened.
Einstein’s Theory of Relativity
explains gravity as a curve in the
fabric of space-time:
Remember!
A law states or describes what
happens in nature.
A theory explains what happens.
Both are well-tested by
experiments.
Memory hint: “explanation starts
with an “e” and “theory” has an
“e”.
True or False?
The difference between a
scientific law and a scientific
theory is that a law has been
proven, but a theory has not
been proven.
The History of Atomic Theory
Democritus




ancient Greek philosopher, not scientist
first to suggest that matter is made of
tiny particles called “atomos”
“atomos” = Greek for “indivisible”
ideas were rejected by Aristotle, who was
very influential, and therefore forgotten
for two thousand years.
Democritus’ atomic model:
atoms were small, hard particles that
were different shapes and sizes
John Dalton:
First to propose an atomic theory
based on science:
1. Each element is composed of small
particles called atoms.
2. All atoms of a given element are identical*;
the atoms of different elements are different.
3. Atoms are not created or destroyed in
chemical reactions.
4. Compounds are formed when atoms of
more than one element combine.
John Dalton:
First to propose an atomic theory
based on science:
1. Each element is composed of small particles called
atoms.
2. All atoms of a given element are identical*; the atoms of
different elements are different.
3. Atoms are not created or destroyed in chemical reactions.
4. Compounds are formed when atoms of more than one
element combine.
* We now know this part is not true!
Dalton’s atomic model:
a tiny sphere that is indivisible
1.
All matter is composed of
extremely small particles
called atoms
2.
Atoms of a given
element are identical in
size, mass, and other
properties; atoms of
different elements differ
in size, mass, & other
properties
Atomic Theory
ELEMENT
2
ELEMENT
3
ELEMENT
4
3.
4.
Atoms cannot be
created or
destroyed
Atoms of different
elements combine
to form compounds
5.
In chemical rxns,
atoms are
combined,
separated, or
rearranged
Atomic Theory
+
+
J.J. Thomson:

First to suggest that there were particles smaller
than the atom.

Discovered the electron.

Developed the “plum pudding model.”
Thomson’s “plum-pudding” model:
the atom is composed of electrons surrounded by a
cloud of positive charge to balance the electron's
negative charge
Ernest Rutherford



gold foil experiment
discovered the nucleus, and realized
that it was very dense and positively
charged
suggested that most of the atom is
empty space
Gold Foil Experiment:
Rutherford shot tiny positively-charged
alpha particles through a thin sheet of
gold foil
What Rutherford expected:
What really happened:
Gold Foil Experiment:
Some of the alpha particles bounced
back. “It was as if you fired a 15-inch
cannon ball at a piece of tissue paper
and it came back and hit you.”
What Rutherford expected:
What really happened:
Such huge deflections could
mean only one thing: some of
the alpha particles had run into
massive concentrations of
positive charge and, since like
charges repel, had been hurled
straight back by them.
Rutherford’s atomic model:
All of the positive charge is crammed inside a tiny,
massive nucleus about ten thousand times smaller
than the atom as a whole. The atom is mostly empty
space.
Animation of Rutherford’s
experiment

http://www.mhhe.com/physsci/chem
istry/essentialchemistry/flash/ruther
14.swf
James Chadwick


Scientists were looking for
missing mass – the protons
and electrons in the atom didn’t
weigh enough to account for the
mass of the atom
proved the existence of
neutrons
Chadwick’s model of the atom:
Protons and neutrons are in the nucleus, which only takes up one
millionth of a billionth of the volume of the atom. The electrons are
in the electron cloud, which constitutes almost all of the volume of
the atom.
In fact, if an atom were expanded
to the size of a cathedral,
the nucleus would be about the size of a housefly!
Subatomic Particles to Know
Subatomi
c
Particle
Proton
Location
Mass
Charge
Nucleus
1 amu
+1
Neutron
Nucleus
1 amu
0
Electron
Electron
Cloud
Practically -1
zero