The document traces the history of atomic theory from ancient Greece to modern times. It discusses the ideas of early philosophers like Democritus who hypothesized atoms as indivisible particles. Experimental discoveries throughout the 18th-19th centuries led to identifying elements through spectroscopy and discovering over 55 elements near volcanoes. Niels Bohr incorporated these findings into his 1913 model of the atom with electrons orbiting in discrete shells. John Dalton further refined atomic theory in 1808 by postulating that atoms are uniform, distinct, and combine to form compounds. Later, J.J. Thomson discovered the electron and Ernest Rutherford deduced the nuclear structure of atoms through deflection experiments.

1. A History
of the
Atomic
Theory
Monday, August 20, 2012

2. In 500 BC Ancient Greece
• Democritus philosophized that
everything in nature is ultimately made
of only one substance. He gave it the
name “Atomos”. That means
something that cannot be divided. So, if
you divide matter down to its smallest
thing, then you get something really
small and hard to break. He was almost
right about it, when considering that
most people imagine infinite types of
atoms – we now agree on only
90 naturally occurring elements.
Monday, August 20, 2012

3. 1802 William Wollaston
• Wanted a better system for classifying
gunpowder to be used during the
Napoleonic Wars. He Invented a
method of burning the powder and
letting the light of the burning material
pass through a prism. This rainbow of
color would shine against a backdrop,
and small pieces of spectrum would be
missing. This method of identifying
elements by their absorption, and
equally common,
emission spectra
are, both even today,
the most preferred
techniques.
Monday, August 20, 2012

4. Early Alchemists
Tried to make elements
turn in to Gold. They
discovered that the best place
to find pure elements was
close to volcanoes, or
geologically active areas. By
1830, 55 elements had been
discovered, most of which
were metals.
Monday, August 20, 2012

5. Niels Bohr
Uses Wollaston’s element
classification with spectral lines to
create the Bohr model of the atom.
He called the different “pieces” of missing
colors in the rainbow – Quanta. He theorized
that these lines were caused by electrons
orbiting the atoms absorbing discrete packets
of energy. Since these always occurred at exact
spots in the spectrum, it meant that the
electrons must always “live” at exact distances
or energies away from the nucleus. This
parceling of energy rather than mixing
randomly into the entire spectrum of possibility
is the basis for quantum theory.
Monday, August 20, 2012

6. Monday, August 20, 2012

7. Modern Atomic Theory – by John
Dalton, an English Teacher 1808
• Every element is made of atoms.
• All atoms of any element are the same.
• Atoms of different elements have different
properties (like weight and size).
• Atoms of different elements combine to form
compounds.
• Atoms are not made, destroyed, or changed (in
chemical reactions).
• The numbers of atoms remain the same (in
chemical reactions).
Monday, August 20, 2012

8. Joseph John Thomson
By the end of the 19th century
most scientists had agreed on the
atomic theory. Thomson
reasoned that the blue glow
at the cathode end of a tube of gas
energized with electricity, is made
by small negative pieces of atoms
that have almost no mass –
“Electrons”
Monday, August 20, 2012

9. Ernest Rutherford
• In 1911 Rutherford performs an experiment
where gold film, almost 1 atom thick, is
bombarded with alpha particles. He finds that the
particles mostly go straight through the film without
bouncing off, but they bounce back about 1 out of
10,000 times. Sometimes they come straight back. He
reasoned the electrons weren't massive enough to
cause a large straight deflection like that, so they must
be positively charged to give the deflection extra
energy, and the nucleus must be 1/10,000th the size
of the whole atom, since it happened 1/10,000 times.
Monday, August 20, 2012

10. Double-Click to the right to watch movie on
Rutherford’s Gold Foil Experiment
Monday, August 20, 2012

11. If the nucleus of an atom were a
pencil eraser placed in the middle
of the field,
then the electrons would be
orbiting the nucleus about the
distance of the furthest row of
bleachers in a stadium.
Monday, August 20, 2012

12. • Atoms – smallest piece of matter that can be
derived chemically.
• Nucleus – the center of the atom. Consists of
protons (+ charge), and neutrons (neutral
charge).
• Electrons – almost weightless, surround the
nucleus (- charge)
• Molecules – smallest piece of matter that exists
in nature (physically)
• Element – All of the same kind of atoms
• Compound – Different elements put together to
make a molecule. (chemically different than its
elements)
• Mixture – Components mixed and can be
separated physically.
Monday, August 20, 2012