-
400 BCE
Democritus's Theory of Atoms
Democritus, a philosopher in ancient Greece,
began the search for a description of matter.
He questioned whether matter could be
divided into smaller and smaller pieces
forever until eventually the smallest possible
piece would be obtained. He believed that
the smallest possible piece of matter was
indivisible. He named the smallest piece of
matter “atomos,” meaning “not to be cut.” -
Period: 300 BCE to
Aristotle's Atom Theory
In ancient Greece, the popular
philosopher Aristotle declared
that all matter was made of only
four elements: fire, air, water
and earth. He also believed that
matter had just four properties:
hot, cold, dry and wet. -
Dalton's Atom Theory
In the early 1800s, the English Chemist John
Dalton performed a number of experiments
that eventually led to the acceptance of the
idea of atoms. He formulated the first atomic
theory since the “death of chemistry” that
occurred during the prior 2000 years.
Dalton theorized that all matter is made of
atoms. Atoms are too small to see,
“uncuttable,” and indestructible. All atoms of
a given element are exactly alike and atoms
of different elements are different. -
Thompson's Atom Theory
In 1897, the English scientist named J.J.
Thomson provided the first hint that an
atom is made of even smaller particles.
He discovered the presence of a negative
particle in the atom – the electron.
He proposed a model of the atom that is
sometimes called the “Plum Pudding” model.
His theory was that atoms are made from a
positively charged substance with negatively
charged electrons scattered about, like
raisins in a pudding or chocolate chips in a
cookie. -
Einstein's Atom Theory
Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules (invisible in 1905). Furthermore, these molecules are always in random, ceaseless motion. -
Rutherford's Atom Theory
In 1908, the English physicist Ernest
Rutherford performed an experiment using
positively charged particles fired at gold foil.
Through his experiment, he proved that
atoms are not a “pudding” filled with a
positively charged material. He theorized
that atoms have a small, dense, positively
charged center, which he called the
“nucleus”. He said that nucleus is tiny
compared to the atom as a whole, because
the atom is mostly open space! -
Bohr's Atom Theory
In 1913, the Danish scientist Niels Bohr
proposed an improvement. He built on the
concept that the mass of an atom is
contained mostly in the nucleus. He also
theorized that electrons move in definite
orbits around the nucleus, much like planets
circle the sun. These orbits, or energy levels,
are located at certain distances from the
nucleus. -
Shrodinger's Atom Theory
Based on de Broglie's idea that particles could exhibit wavelike behavior, Austrian physicist Erwin Schrödinger theorized that the behavior of electrons within atoms could be explained by treating them mathematically as matter waves. -
Heisenberg's Atom Theory
Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly. -
Period: to
Modern Cloud Theory
According to today’s atomic
theory, electrons do not orbit
the nucleus in neat planet-like
orbits but move at high speeds
in an electron cloud around the
nucleus.
In the electron cloud, electrons
whirl around the nucleus
billions of times in one second.
They are not moving around in
random patterns; an electron’s
location depends upon how
much energy the electron has.