Atomic Theory History Timeline

By Ellie, Laura and Gabby

400BCThe Ancient Greeks

• In about 400 BC the Greek philosopher Democritus developed the theory that all matter was made of tiny units.  The different properties of matter were caused by interactions between

the tiny units. Matter made of smooth units formed liquids, because the units could move

around freely.   Matter made of units that stuck together formed metals and other solids.

1803John Dalton

• John Dalton proposed that an element was composed of atoms that were identical, had the

same mass and also had invisible characteristic of that element and no other (billiard ball model). He also proposed that compounds were atoms from

different elements combined together.

1896Periodic Table Creation

• The Russian chemist Dmitri Ivanovich Mendeleyev developed the periodic table in 1869. By his time scientists had discovered 60 elements and from

the information that he and chemists had he could determine the weight of the elements.

1897 Joseph John Thomson

• Joseph John Thomson was the discoverer of the electron. He came to the conclusion that all matter, whatever its source, contains particles of the same kind that are much, much smaller than the atoms of

which they form a part. They are now called electrons, although he originally called them

corpuscles. Through his discovery of the electron he showed that atoms were in fact not indivisible. He proposed the ‘plum pudding model’ which was the thought that atoms consisted of a soft positively charged jelly-like sphere into which negatively

charged electrons were embedded which was where the name of the plum pudding model came from as

it was said they were like plums in a pudding.

1907Ernest Rutherford

• In 1907, Ernest Rutherford, Hans Geiger and Ernest Marsden carried out the Geiger-Marsden

experiment, an attempt to examine the structure of the atom. The surprising results of this

experiment demonstrated the existence of the atomic nucleus and became an integral part of

the Rutherford model of the atom. The Rutherford model of the atom was simplified in a well-known

symbol showing electrons circling around the nucleus like planets orbiting the sun. This symbol

became popular and has been used by various organizations around the world as a symbol for

atoms and atomic energy in general. The element 'rutherfordium' was named in Rutherford’s honour.

1915 Niels Bohr

• Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call

Bohr's Model the Rutherford-Bohr Model. The modern model of the atom is based on quantum

mechanics. The Bohr Model contains some errors, but it is important because it describes most of

the accepted features of atomic theory without all of the high-level math of the modern version.

Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of

atomic hydrogen.

1932 James Chadwick

• Chadwick identified a particle in the nucleus that had almost the same mass as a proton but no

charge. It was called a neutron.

PresentHiggs Boson particle

• The Higgs Boson particle is a subatomic particle believed to be the key to the formation of stars,

planets and eventually life after the Big Bang 13.7 billion years ago. The Higgs is the last missing piece of the Standard Model, the theory that describes the

basic building blocks of the universe. Scientists believe that in the first billionth of a second after the Big Bang, the universe was a huge spin of particles

racing around at the speed of light without any mass. It was through their contact with the Higgs field that

they gained mass and eventually formed the universe. After a search going for nearly half a

century, physicists said they had found a new sub-atomic particle consistent with the Higgs boson,

which is believed to present mass. 

Bibliography• http://www.barcodesinc.com/articles/timeline-on-a

tomic-structure.htm

• http://www.cartage.org.lb/en/themes/sciences/physics/atomicphysics/atomicstructure/atomictimeline/atomictimeline.htm

• http://www.angelfire.com/mb2/atoms/

• http://patentpending.blogs.com/patent_pending_blog/2005/04/the_ancient_gre.html

• http://www.sciencekids.co.nz/sciencefacts/scientists/ernestrutherford.html