Historical Development of Atomic Structure


Yazan Fahmawi
Sept. 30, 1995
T3 IBS Chemistry
Ms. Redman

The idea behind the "atom" goes back to the Ancient Greek society, where
scientists believed that all matter was made of smaller, more fundamental
particles called elements. They called these particles atoms, meaning "not
divisible." Then came the chemists and physicists of the 16th and 17th centuries
who discovered various formulae of various salts and water, hence discovering
the idea of a molecule.

Then, in 1766 was born a man named John Dalton born in England. He is known as
the father of atomic theory because he is the one who made it quantitative,
meaning he discovered many masses of various elements and, in relation,
discovered the different proportions which molecules are formed in (i.e. for
every water molecule, one atom of oxygen and two molecules of hydrogen are
needed). He also discovered the noble, or inert gases, and their failure to
react with other substances. In 1869 a Russian chemist, best known for his
development of the periodic law of the properties of the chemical elements
(which states that elements show a regular pattern ("periodicity") when they are
arranged according to their atomic masses), published his first attempt to
classify the known elements. His name was Mendeleyev, and he was a renowned
teacher. Because no good textbook in chemistry was available at the time, he
wrote the two-volume Principles of Chemistry (1868-1870), which later became a
classic. During the writing of this book, Mendeleyev tried to classify the
elements according to their chemical properties. In 1871 he published an
improved version of the periodic table, in which he left gaps for elements that
were not yet known. His chart and theories gained acceptance by the scientific
world when three elements he "predicted"—gallium, germanium, and scandium—were
subsequently discovered In 1856 another important figure in atomic theory was
born: Sir Joseph John Thomson. In 1906, after teaching at the University of
Cambridge and Trinity University in England, he won the Nobel Prize in physics
for his work on the conduction of electricity through gases. He discovered what
an electron is using cathode rays. An electron is the smallest particle in an
atom, whose mass is negligible compared to the rest of the atom, and whose
charge is negative. Though scientists did not know it at the time, electrons
were located in an electron cloud rotating around the nucleus, or center of the
atom.

Another prominent figure in nuclear physics is a man called Ernest Rutherford,
born in 1871. He also was a professor at the University of Cambridge, the
University of Manchester (both of which are in England), and at McGill College
in Montreal, Canada. His importance comes after the discovery of radioactivity
in 1896 by a French scientist named Becquerel. Rutherford identified the three
main components of radioactivity: alpha, beta, and gamma particles. He also
found the alpha particle to be a positively charged helium atom. Also,
Rutherford was the first one to discover the true structure of an atom, it
having a central, heavy nucleus with an electron cloud surrounding it. It was
Rutherford that, through experiments such as passing alpha particles through a
thin gold foil and watching some repel, discovered the second constituent of the
atom (also the first component of the nucleus): the proton. The proton has a
relative atomic mass of one and has a positive charge. Rutherford also went down
in history as the first man to artificially cause a nuclear reaction when, in
1919, he bombarded nitrogen gas with radioactive alpha particles, which resulted
in atoms of an oxygen isotope and protons. A unit of radioactivity, the
rutherford, was named in his honor. A colleague of Rutherford\'s at Cambridge
University was a man named James Chadwick discovered the third fundamental
particle that makes up the atom: the neutron. This discovery led immediately to
the discovery of nuclear fission and the atom bomb The neutron has a relative
atomic mass of one, and has no positive or negative charge (i.e. it is neutral).
It is found in the nucleus of atoms, along with the proton.Chadwick was one of
the first British scientists to stress the development of a possible atom bomb.
His name was strongly associated with the British atomic bomb effort, especially
during World War II. During the last two years of W.W.II (1943-1945) Chadwick
moved to New Mexico, where he spent much of his time researching at the Los
Alamos Scientific Laboratory, a site chosen by the US government for nuclear
weapon research. The first atomic bomb was developed here with the help of James
Chadwick. Chadwick earned the Nobel Prize for physics in 1935. In