s8pe-03-ts-ca
12/6/05
11:44 AM
MAZER
Page 301
1808
John Dalton Says: “Bring Back
the Atom”
English chemist John Dalton revives the
ancient Greek idea that all matter is
made of atoms. Dalton claims that each
element has its own type of atom and
that the atoms combine in fixed ratios
with one another.
1808
Humphrey Davy Shocks Chemistry
English chemist Humphrey Davy applies
an electric current to different materials.
He discovers that many materials once
thought to be elements break apart into
even simpler materials. Davy succeeds in
isolating the elements sodium, calcium,
strontium, and barium.
1800
1820
1840
1897
It’s Smaller Than the Atom!
English physicist Joseph John
Thomson discovers the electron—the
first subatomic particle to be identified. Thomson concludes that these
tiny particles have a negative charge.
Thomson will later propose that
atoms are made of a great many of
these negative particles floating in a
sea of positive charge. Thomson
suggests that each atom resembles a
dish of pudding with raisins in it.
The electrons are the raisins and the
pudding the positive charge in which
they float.
1860
1880
TECHNOLOGY
Chemistry and Electric Charge
In 1800 Italian physicist Alessandro Volta announced
that he had produced an electric current from a
pile, or battery, of alternating zinc and silver discs.
Volta’s invention was important for the study of
atoms and elements in two ways. First, the fact that
the contact of two different metals could produce
an electric current suggested that electric charge
must be part of matter. Second, the powerful
electric current produced by the batteries enabled
chemists to break apart many other substances,
showing that there were more elements than
previously thought.
Timelines in Science 301
PDF