atomic theory

democritus

400 BCE

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.”

aristole

300 BCE

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

1803

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.

thomson

1897

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.

einstien

1905

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).

rutherford

1908

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”.

bohr

1913

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.

shrodinger

1926

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.

heinsberg

1927

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.

Modern Cloud Theory

2000

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.