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The Birth of Atomic Theory
By: Mr. Hunter
Atoms

What is an atom?

How small do you think it is?

What is made up of atoms?
Atoms
Democritis

Called the smallest piece of matter an
atom, which means “uncuttable”
Atoms

Element: a substance that cannot be
broken down into simpler substances;
first discovered in the 19th century

The smallest unit of carbon dioxide
can be split apart into one atom of
carbon and two atoms of oxygen. Is
carbon dioxide an element? Explain.
By the end of the 19th
century



1. Scientists assumed that atoms were
solid and indestructible, like tiny, hard
balls.
2. Scientists realized that each
element was made of a different kind
of atom.
3. Scientists assumed that atoms
contained no electrical charges.
Electrical Charges

When scientists exposed gases to
electric fields, the motion of the atoms
in the gases did not change. This
seemed to confirm that atoms
contained no charged particles. Why
was this an incorrect idea?
Atoms

J.J. Thompson discovered a small,
negatively charged particles and
named them electrons.
Atoms

JJ Thompson proposed the plum
pudding model
Atoms

Rutherford discovered that uranium
emitted _____ and _____.
Apha Particles

Far more massive

Positively charged
Alpha Particles

Predict what will happen if an electron
is brought near an alpha particle.

Predict what will happen if two alpha
particles are brought near each other.
Rutherford

Rutherford aimed alpha particles at
gold foil.

He hypothesized that all of the alpha
particles would go through the thin foil
and that most of the alpha particles
would bend slightly as they interacted
with the positively charged clouds of
the gold atoms.
What were the results of
Rutherford’s experiment?



1. Alpha particles went through the
gold foil.
2. The paths of only a few of the alpha
particles were bent by their passage
through the foil.
3. A few of the alpha particles bounced
off the foil and came right back.
Rutherford’s results

How did his results contradict the plum
pudding model of the atom?

Name one new finding that
Rutherford’s model had to explain.
Rutherford

Rutheford’s experiment violated the
plum pudding model.

How could atoms contain negative
particles but still be neutral?

How could particles move through
substances made of atoms?
Rutherford

Rutherford’s new model of the atom
had to explain what could cause a fastmoving alpha particle to bounce back
off a thin sheet of gold foil.
Modern Atomic Theory

What is the modern atomic theory?

Atoms are made of nuclei. Nuclei are
made up of positively-charged protons,
neutrally- charged neutrons, and
surrounded by negatively-charged
electrons.
Theories vs. Laws

Scientists use theories to make sense
of a set of facts.

Any theory is open to questioning and
testing.

Science is open to improving itself.
What is a law?


A scientific law is a description of a
natural phenomenon or principle that
invariably holds true under specific
conditions and will occur under certain
circumstances.
Ex.: Law of conservation of mass