History of atom

Aristotle

Continuous Theory of Matter: Matter could be divided and subdivided into smaller and smaller pieces without limit

440 BC

Aristotle thought that matter was continuous,infinite,present in every form,and always around us.

440 BC

Democritus

3.) There is empty space between atoms

400 BC

2.) Atoms are indestructible and constantly in motion

400 BC

Discontinuous Theory of Matter: expanded the work of his mentor Luecippus,who believed matter was actually finite and not limitless

400 BC

1.) Everything was composed of “atoms” which are physically invisible

400 BC

John Dalton

Law of Multiple Proportions:

1803

John Dalton thought matter always combined in fixed ratios based on weight

1803

If two elements form more than one compounds between them,then the ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small whole numbers

1803

3.Atoms cannot be subdivided,created,or destroyed

1804

1. All matter is composed of extremely small particles called atoms

1804

2. Atoms of a given element are identical in size,mass,and other properties.Atoms of different elements differ in size,mass,and other properties

1804

Dalton’s Atoms Theory:

1804

4.Atoms of different elements can combine in simple whole number ratios to form chemical compounds

1804

Thompson

Presented his work

1897

Thomson revised the model of the atom into what became known as the plum pudding model

1897

was able to show that cathode rays could be deflected by a magnetic field

1897

He hypothesized that the atom was comprised of negatively charged particles in a field of positive charge (positively charged particles had not yet been discovered)

1897

Thomson’s interpretation of this effect was that cathode rays must consist of charged particles that have mass

1897

Millikan

Millikan was also able to calculate the mass of an electron

1909

Millikan calculated the charge of an electron to be 1.5924 × 10−19 coulombs

1909

Robert Millikan and Harvey Fletcher devised what is known as the oil drop experiment to determine the charge of a single electron

1909

Rutherford

Gold Foil Experiment:

1900

utherford used a radioactive source to direct alpha particles toward a very thin sheet of gold foil. Surrounding the foil was a screen that fluoresced when struck by the alpha particles.

1900

Rutherford concluded that the nucleus must be made up of these positively charged alpha particles, which he named protons

1900

alpha (α) particles, which were positively charged, and beta (β) particles, which were negatively charged.

1900

He discovered that uranium released two different types of particles:

1900

He proposed that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons

1900

Bohr

proposed what became known as a planetary model of the atom

1913

Chadwick

he concluded that the nucleus also contains a particle which has equal mass to the proton, but unlike the proton, is electrically neutral

1932

Discovered the neutron

1932