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400 BCE
Democritus
*Democritus was a Greek philosopher who lived between 470-380 B.C. He developed the concept of the 'atom', Greek for 'indivisible'.
* Democritus believed that everything in the universe was made up of atoms, which were microscopic and indestructible. -
John Dalton
*Smooth
*Hard balls that could be broken into smaller pieces
*All elements consist of atoms that can't be divided
*All atoms of the same elements are exactly alike and have the same mass. Atoms of different and have different masses.
*An atom of one element can't be changed into an atom of a different element by a chemical reaction. -
JJ Thomson
*Now, the atoms are made of even smaller parts.
*In 1897, JJ Thomson discovered that atoms contain negatively charged particles called electrons.
*Thomson reasoned that atoms must also contain some sort of positive charge. -
Ernest Rutherford
*The particles would be only slightly deflected because the positive charge was thought to be spread out in the gold atoms.
*He observed that most of the particles passed straight thought the foil with little deflection.
*He called the the positively charged particles in an atoms's nucleus protons.
*Rutherford's Gold Foil Experiment proved the existence of a small massive center to atoms, which would later be known as the nucleus of an atom. -
Neils Bohr
*In 1913, Bohr suggested that electrons are found only in specific orbits around the nucleus.
*Each possible electron orbit has a fixed energy. -
Marie Curie
*Thus, she deduced that radioactivity does not depend on how atoms are arranged into molecules, but rather that it originates within the atoms themselves.
*For their joint research into radioactivity, Marie and Pierre Curie were awarded the 1903 Nobel Prize in Physics. -
Albert Einstein
*Einstein treated matter and energy as exchangeable. Albert Einstein became famous for the theory of relativity, which laid the basis for the release of atomic energy.
*In 1905 Albert Einstein formulates Special Theory of Relativity.
*He advanced the theory of relativity when he was only 26 years old. -
James Chadwick
*Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons.
*In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced.
*This particle became known as the neutron.
