Development of Atomic Theory

Introduction to Atoms
Beta Pod Science
Interesting Facts
 Democritus: was a Greek philosopher and leading advocate
of the theory that all phenomena in nature could be
understood in terms of the movements of particles called
atoms (from the Greek word atmos, meaning “indivisible”).
 The views of Democritus sharply contrasted those of
Aristotle and others, who held to the theory that all matter
could be reduced to a combination of four elements: earth,
water, air, and fire.
From Greek to Modern Atomic Theory
 Democritus and other Greek philosophers laid the
groundwork for the modern atomic theory, but it was not
until the 16th and 17th centuries that interest in atoms and
atomic structure was renewed. During that time, the work
of Sir Isaac Newton, Robert Boyle, and Pierre Gassendi
helped further the development of the atomic theory.
 In the 19th Century, experiments by John Dalton, Amedeo
Avogadro, James Clerk Maxwell, and Rudolf Clausius began
to reveal the nature and structure of atoms.
From Greek to Modern Atomic Theory
 Sir Joseph John Thomson’s discovery of electrons in 1897
and his later research on protons and gases indicated that
atoms were not the smallest indivisible units of matter, as
previously thought. Thomson showed that subatomic
particles with either a negative or positive charge form at
least part of the structure of an atom. Thomson won the
Nobel Prize in physics in 1906.
 While Thomson was director of the Cavendish Laboratory at
Cambridge University in Cambridge England, one of his
graduate students was Ernest Rutherford. Rutherford went
on to win the Nobel Prize in chemistry in 1908 for his work
on radioactivity.
The beginning of Atomic Theory
 Democritus thought you could keep cutting something in half
until at some point it couldn’t be cut anymore…the atom.
 The atom comes from the Greek word “atomos” meaning
“not able to be divided.”
 Atom: the smallest particle into which an element can be
divided and still be the same substance.
John Dalton
 By the 1700’s, scientists knew elements combined in certain
proportions based on mass to form compounds. John Dalton
wanted to know why.
 In 1803 he published his theory stating:
 All substances are made of atoms. Atoms are small particles
that cannot be created, divided, or destroyed.
 Atoms of the same element are exactly alike, and atoms of
different elements are different.
 Atoms join with other atoms to make new substances.
 He was mostly right… but not completely.
J.J Thomson
 J.J. was the first to find a mistake in Dalton’s theory. He
found that there were even smaller particles inside the atom.
Which means that atoms can be divided into even smaller
Thomson used a “cathode-ray tube” to discover electrons.
Electrons: Negatively charged particles.
Thompson discovered electrons
Thomson thought these particles were arranged like a round
scoop of chocolate chip ice cream where the electrons would
be like the chocolate chips arranged throughout the scoop.
Ernest Rutherford
 Rutherford tested Thomson’s theory. He aimed a beam of
positively charged atoms at a thin sheet of gold foil.
 Rutherford put a special coating behind the foil which glowed
when hit by the positively charged particles. This way he could see
where the positively charged particles went after hitting the gold.
 Rutherford expected the particles to pass straight through the gold
showing that the particles were indeed like a mass of “chocolate
chip icecream” but he found that some deflected and hit other
areas of the foil.
 This event proved that Thomson’s theory of how atoms were
arranged was wrong. Rutherford realized that atoms must be
considered mostly empty space with a tiny part made of highly
dense matter.
The Nucleus
 Rutherford revised the atomic theory.
 Rutherford proposed that in the center of the atom is a
 Nucleus: a tiny, extremely dense, positively charged area of
an atom made of protons and neutrons.
 Rutherford calculated that a nucleus was 100,000 times
smaller than the diameter of the gold atom.
Bohr’s Electron Levels
 In 1913, Niels Bohr, a Danish scientist who worked with
Rutherford, studied the way atoms react to light.
 Bohr’s results concluded that electrons move around the
nucleus in certain paths, or energy levels.
 Bohr’s theory compared the levels to a ladder… you can
stand on the rungs of a ladder, but not between the rungs.
 Bohr’s theory wasn’t completely correct.
Modern Atomic Theory
 Erwin Schrodinger and Werner Heisenberg further explained
the nature of electrons in the atom.
 They found that electrons do not travel in specific paths as
Bohr suggested. In fact, they cannot even be predicted.
 Electrons only have areas where they are “likely” to be found.
 Electron Clouds: Regions where electrons are “likely” to be
 The Electron Cloud model was presented by Erwin
Shroedinger. The cloud-like region is an estimate where the
electron is likely to be found. It can't pinpoint where an
electron is, but can identify where it ought ought to be. The
electron leaves a dot where it was. This creates a cloud and
forms the electron cloud model.

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