History of Atomic Structure
Transcript History of Atomic Structure
How long have people been interested in
understanding matter and its structure?
A. Thousands of years
B. Hundreds of years
C. A few years
History of Atomic
When: 460-370 b.c.e.
What: Democritus believed that matter was made
of small particles he named “atoms”. Said they
were solid, homogeneous, and couldn’t be divided
• Atoms of different elements have different sizes,
shapes and properties
• Why: Democritus used observation and inference
to explain the existence of everything.
• When: 334-322 B.C.E.
• Aristotle believed in 4 elements: Earth, Air,
Fire, and Water.
• No such thing as empty space
Who: John Dalton
What: Described atoms as tiny particles that could
not be divided or destroyed. Thought each element
was made of its own kind of atom that were exactly
the same in size and mass.
• Atoms of elements can combine to form
• Why: Building on the ideas of Democritus in ancient
Sir William Crookes
• Cathode Ray tube
• Found that electricity
had a negative charge
• Late 1800’s
Discovery of Electrons
Who: J. J. Thompson
What: Thompson discovered that electrons
were smaller particles of an atom and were
negatively charged. Plum pudding model
• Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
J. J. Thompson
Found the charge and mass of electrons
Atomic Structure I
Who: Ernest Rutherford
What: Conducted an experiment to isolate
the positive particles in an atom. Decided
that the atoms were mostly empty space, but
had a dense central core. Gold foil
• Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
Atomic Structure II
Who: Niels Bohr
What: Proposed that electrons traveled in
fixed paths around the nucleus. Scientists
still use the Bohr model to show the number
of electrons in each orbit around the nucleus.
• Why: Bohr was trying to show why the
negative electrons were not sucked into the
nucleus of the atom.
Electron Cloud Model
• Electrons travel around the nucleus in
• Scientists cannot predict where they will be
at any given moment.
• Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
Electron Cloud Model
Atomic Structure Timeline
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