Download History of atom

History of atom
The atom is a basic unit of matter consisting of a dense, central nucleus
surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a
mix of positively charged protons and electrically neutral neutrons.
The name atom comes from the Greek ´´átomos´´, which means uncuttable, or
indivisible, something that cannot be divided further.The concept of an atom as an
indivisible component of matter was first proposed by early Indian and Greek
philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this
idea by showing that certain substances could not be further broken down by chemical
methods. During the late 19th and early 20th centuries, physicists discovered subatomic
components and structure inside the atom, thereby demonstrating that the 'atom' was
divisible. The principles of quantum mechanics were used to successfully model the
atom.
In 1803, English instructor and natural philosopher John Dalton used the concept of
atoms to explain why elements always react in a ratio of small whole numbers - the law of
multiple proportions - and why certain gases dissolve better in water than others. He
proposed that each element consists of atoms of a single, unique type, and that these
atoms can join together to form chemical compounds. Dalton is considered the originator
of modern atomic theory.
Additional validation of particle theory (and by extension atomic theory) occurred
in 1827 when botanist Robert Brown used a microscope to look at dust grains floating in
water and discovered that they moved about erratically - a phenomenon that became
known as "Brownian motion". J. Desaulx suggested in 1877 that the phenomenon was
caused by the thermal motion of water molecules, and in 1905 Albert Einstein produced
the first mathematical analysis of the motion. French physicist Jean Perrin used Einstein's
work to experimentally determine the mass and dimensions of atoms, thereby
conclusively verifying Dalton's atomic theory.
The physicist J. J. Thomson, through his work on cathode rays in 1897, discovered
the electron. These subatomic particles had the same properties, regardless of the type
of atom whence they came. This universal component of all atoms destroyed the concept
of atoms as being indivisible units. Thomson postulated that the low mass, negativelycharged electrons were distributed throughout the atom, possibly rotating in rings, with
their charge balanced by the presence of a uniform sea of positive charge. This later
became known as the plum pudding model.
In 1909, Hans Geiger and Ernest Marsden, under the direction of physicist Ernest
Rutherford, bombarded a sheet of gold foil with alpha rays - by then known to be
positively charged helium atoms - and discovered that a small percentage of these
particles were deflected through much larger angles than was predicted using Thomson's
proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive
charge of a heavy gold atom and most of its mass was concentrated in a nucleus at the
center of the atom - the Rutherford model.