Atomic Structure

Yazan Fahmawi Sept. 30, 1995
T3 IBS Chemistry
Ms. Redman
Historical Development of
Atomic Structure
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 the same
era of the development of the atom lived a man, just across the North Sea from these three
learned individuals, in Denmark. Neils Henrik Bohr, born in 1885, was also a considerable
man when it came to nuclear and atomic physics. He moved to Cambridge University in 1911,
working under J. J. Thomson, but soon moved to Manchester to work under Rutherford's
supervision. He won the Nobel Prize in physics in 1922 for his theory on atomic structure
(also known as the Quantum Theory), which was published in papers between 1913 and 1915.
He based his work around Rutherford's conception of the atom. This theory, that suggests
that electrons only emit electromagnetic energy when they jump from one quantum level to
another, contributed tremendously to future developing of theoretical atomic physics. His
work helped lead to the notion that electrons exist in shells and that the electrons in
the outermost shell certify an atom's chemical properties. He later illustrated that
uranium-235 is the singular isotope of uranium that undergoes nuclear fission. The Bohrs
moved to England, and then to the US, where Bohr went to work for the government at Los
Alamos, New Mexico, along with James Chadwick, until the first bomb's detonation in 1945.
He disapproved complete secrecy of the nuclear bomb, and believed that its consequences
would revolutionize the modern world. He wanted some sorts of international law to watch
over the use of nuclear devices. In 1945 Bohr returned to the University of Copenhagen in
Denmark, where he began developing peaceful uses for atomic energy, such as power plants
using nuclear resources as opposed to fossil fuels such as coal, oil, and natural gases.
Bohr died in Copenhagen on November 18, 1962. In Austria in 1887 a man by the name of
Erwin Schrödinger was born. He became a physicist best known for his mathematical studies
of the wave mechanics of orbiting electrons. His most famous and important contribution to
the understanding of atomic structure is a meticulous and precise mathematical description
of the standing waves orbiting electrons follow. His theory was published in 1926, and
along with a German physicist's theory of matrix mechanics, their theories became the
basis of quantum mechanics. Schrödinger shared the 1933 Nobel Prize in physics with the
British physicist Paul A. M. Dirac for his contribution to the development of quantum
mechanics. Through the centuries that have passed, minds have been boggled, countless
questions have been answered, and many great minds conceived, however, there is no doubt
that there is still much to discover about the atom, such as sub-atomic, elementary
particles.
A whole new generation of great scientists is still to come, to explore and unlock the
universe's secrets.