-
400 BCE
Democtitus
Democritus 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.”
To Democritus, atoms were small, hard
particles that were all made of the same
material, but were formed into different
shapes and sizes. -
Period: 300 BCE to
Aristotle
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.
This started the "Death of Chemistry" -
Dalton
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. -
Tomson
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. -
Rutherford
The physicist Ernest Rutherford performed an experiment using positively charged particles fired at gold
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 an atom is mostly open space
He concluded that the negatively charged
particles are scattered outside the nucleus -
Bohr
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. -
Modern Cloud Theory (Schrodinger, Heisenberg, Einstein and many other scientists)
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.
