Marie Skłodowska-Curie was a Polish-born French physicist and
chemist who is famous for her pioneering research on radioactivity. She and her husband are considered to have laid the
cornerstone of the nuclear age with their research on radioactivity. Marie was
fascinated with the work of Henri Becquerel, a French physicist who discovered in 1896 that uranium casts
off rays similar to the X-rays discovered by Wilhelm Röntgen. Marie Curie began
studying uranium in late 1897 and theorized, according to a 1904 article she
wrote for Century magazine, "that the emission of rays by the compounds of
uranium is a property of the metal itself—that it is an atomic property of the
element uranium independent of its chemical or physical state." Curie took
Becquerel's work a few steps further, conducting her own experiments on uranium
rays. She discovered that the rays remained constant, no matter the condition
or form of the uranium. The rays, she theorized, came from the element's atomic
structure. This revolutionary idea created the field of atomic physics and the Curies coined the word radioactivity to
describe the phenomena.
Pierre Curie, known
for his work on radioactivity as well as on ferromagnetism, para-magnetism,
and diamagnetism;
notably Curie's law and Curie point. Pierre
and Marie further explored radioactivity by working to separate the substances
in uranium ores and then using the electrometer to
make radiation measurements to ‘trace’ the minute amount of unknown radioactive
element among the fractions that resulted. Working with the mineral pitchblende, the
pair discovered a new radioactive element in 1898. They named the element polonium, after
Marie's native country of Poland. On December 21, 1898, the Curies detected the
presence of another radioactive material in the pitchblende. They presented
this finding to the French
Academy of Sciences on December 26, proposing that the new element be
called radium. The
Curies then went to work isolating polonium and radium from naturally occurring
compounds to prove that they were new elements. In 1902, the Curies announced
that they had produced a decigram of pure radium, demonstrating its existence
as a unique chemical element. While it took three years for them to isolate
radium, they were never able to isolate polonium. Along with the discovery of
two new elements and finding techniques for isolating radioactive isotopes,
Curie oversaw the world's first studies into the treatment of neoplasms, using
radioactive isotopes. With Henri Becquerel and her husband, Pierre Curie, she
was awarded the 1903 Nobel Prize for Physics. She was
the sole winner of the 1911 Nobel
Prize for Chemistry. She was the first woman to win a Nobel Prize, and she is the
only woman to win the award in two different fields.
While working
with Marie to extract pure substances from ores, an undertaking that really
required industrial resources but that they achieved in relatively primitive
conditions, Pierre himself concentrated on the physical study (including
luminous and chemical effects) of the new radiations. Through the action of
magnetic fields on the rays given out by the radium, he proved the existence of
particles electrically positive, negative, and neutral; these Ernest Rutherford was afterward to call alpha, beta, and gamma rays. Pierre
then studied these radiations by calorimetry and also observed the physiological effects of radium,
thus opening the way to radium therapy. Among Pierre Curie's discoveries were
that ferromagnetic substances exhibited a critical temperature transition,
above which the substances lost their ferromagnetic behavior - this is known as
the "Curie point." He was elected to
the Academy of Sciences (1905), having in 1903 jointly with Marie received the
Royal Society's prestigious Davy Medal and jointly with her and Becquerel the
Nobel Prize for Physics. He was run over by a carriage in the rue Dauphine in Paris in 1906 and died instantly. His complete works
were published in 1908.
New
Zealand-born chemist and physicist Ernest Rutherford is considered to be "the father of nuclear physics." Rutherford is best known for devising the names alpha, beta, and gamma to classify various forms of radioactive "rays"
which were poorly understood at his time (alpha and beta rays are particle
beams, while gamma rays are a form of high-energy electromagnetic
radiation).
Rutherford deflected alpha rays with both electric and magnetic fields in 1903.
Working with Frederick Soddy, Rutherford explained
that radioactivity is due to the transmutation of elements, now
known to involve nuclear reactions. He also observed that the intensity of radioactivity of a
radioactive element decreases over a unique and regular amount of time until a
point of stability, and he named the halving time the "half-life." In 1901 and 1902 he worked with Frederick Soddy to prove
that atoms of one radioactive element would spontaneously turn into another, by
expelling a piece of the atom at high velocity. In 1906 at the University of
Manchester, Rutherford oversaw an experiment conducted by his students Hans Geiger(known for the Geiger counter) and Ernest Marsden. In the Geiger–Marsden
experiment,
a beam of alpha particles, generated by the radioactive decay of radon, was directed normally
onto a sheet of very thin gold foil in an evacuated chamber. Under the
prevailing plum pudding model, the alpha particles
should all have passed through the foil and hit the detector screen, or have
been deflected by, at most, a few degrees. However, the actual results
surprised Rutherford. Although many of the alpha particles did pass through as
expected, many others were deflected at small angles while others were
reflected back to the alpha source. They observed that a very small percentage
of particles were deflected through angles much larger than 90 degrees. The
gold foil experiment showed large deflections for a small fraction of incident
particles. Rutherford realized that, because some of the alpha particles were
deflected or reflected, the atom had a concentrated centre of positive charge
and of relatively large mass - Rutherford later termed this positive center the
"atomic nucleus". The alpha
particles had either hit the positive centre directly or passed by it close
enough to be affected by its positive charge. Since many other particles passed
through the gold foil, the positive centre would have to be a relatively small
size compared to the rest of the atom - meaning that the atom is mostly open
space. From his results, Rutherford developed a model of the atom that was
similar to the solar system, known as Rutherford model. Like planets, electrons orbited a central, sun-like nucleus.
For his work with radiation and the atomic nucleus, Rutherford received the
1908 Nobel Prize in Chemistry.
In 1903, Mikhail Tsvet invented chromatography, an important analytic technique. In 1904, Hantaro Nagaoka proposed
an early nuclear model of the atom, where electrons orbit a dense massive
nucleus. In 1905, Fritz Haber and Carl Bosch developed the Haber process for making ammonia, a milestone in industrial chemistry with deep consequences in
agriculture. The Haber process, or Haber-Bosch process, combined nitrogen and hydrogen to form ammonia in industrial quantities for production of
fertilizer and munitions. The food production for half the world's current
population depends on this method for producing fertilizer. Haber, along
with Max Born, proposed the Born–Haber cycle as a method for
evaluating the lattice energy of an ionic solid. Haber has also been described
as the "father of chemical warfare" for his work developing and deploying chlorine and other
poisonous gases during World War I.
In 1905, Albert Einstein explained Brownian motion in a way that definitively proved atomic theory. Leo Baekeland invented bakelite,
one of the first commercially successful plastics. In 1909, American
physicist Robert Andrews Millikan - who had studied in
Europe under Walther Nernst and Max Planck - measured the charge of individual electrons with
unprecedented accuracy through the oil drop experiment, in which he measured the
electric charges on tiny falling water (and later oil) droplets. His study
established that any particular droplet's electrical charge is a multiple of a
definite, fundamental value — the electron's charge — and thus a confirmation
that all electrons have the same charge and mass. Beginning in 1912, he spent
several years investigating and finally proving Albert Einstein's proposed
linear relationship between energy and frequency, and providing the first
direct photoelectric support for Planck's constant. In 1923 Millikan was
awarded the Nobel Prize for Physics.
In 1909, S. P. L. Sørensen invented the pH concept and develops
methods for measuring acidity. In 1911, Antonius Van den Broekproposed the idea that the
elements on the periodic table are more properly organized by positive nuclear
charge rather than atomic weight. In 1911, the first Solvay Conference was held in Brussels, bringing together most of the most
prominent scientists of the day. In 1912, William Henry Bragg and William Lawrence Bragg proposed Bragg's law and established the field of X-ray crystallography, an important tool for
elucidating the crystal structure of substances. In 1912, Peter Debye develops the concept of molecular dipole to describe
asymmetric charge distribution in some molecules.
In 1913, Niels Bohr, a Danish physicist, introduced the concepts of quantum mechanics to atomic structure by proposing what is now known as
the Bohr model of the atom, where
electrons exist only in strictly defined circular orbits around the nucleus similar
to rungs on a ladder. The Bohr Model is a planetary model in which the
negatively charged electrons orbit a small, positively charged nucleus similar
to the planets orbiting the Sun (except that the orbits are not planar) - the
gravitational force of the solar system is mathematically akin to the
attractive Coulomb (electrical) force between the positively charged nucleus
and the negatively charged electrons.
In the Bohr
model, however, electrons orbit the nucleus in orbits that have a set size and energy
- the energy levels are said to be quantized, which means that only
certain orbits with certain radii are allowed; orbits in between simply don't
exist. The energy of the orbit is related to its size - that is, the lowest
energy is found in the smallest orbit. Bohr also postulated that
electromagnetic radiation is absorbed or emitted when an electron moves from
one orbit to another. Because only certain electron orbits are permitted, the
emission of light accompanying a jump of an electron from an excited energy
state to ground state produces a unique emission spectrum for each element.
Niels Bohr also
worked on the principle of complementarity, which states that an
electron can be interpreted in two mutually exclusive and valid ways. Electrons
can be interpreted as wave or particle models. His hypothesis was that an
incoming particle would strike the nucleus and create an excited compound
nucleus. This formed the basis of his liquid drop model and later provided a
theory base for the explanation of nuclear fission.
In 1913, Henry Moseley, working from Van den Broek's earlier idea, introduces concept
of atomic number to fix inadequacies of Mendeleev's periodic table, which had
been based on atomic weight. The peak of Frederick Soddy's career in
radiochemistry was in 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms
which have different atomic weights but which are indistinguishable chemically.
He is remembered for proving the existence of isotopes of certain radioactive
elements, and is also credited, along with others, with the discovery of the
element protactiniumin 1917. In 1913, J. J. Thomson
expanded on the work of Wien by showing that charged subatomic particles can be
separated by their mass-to-charge ratio, a technique known as mass spectrometry.
American
physical chemist Gilbert N. Lewis laid the foundation of valence bond theory; he was instrumental in
developing a bonding theory based on the number of electrons in the outermost
"valence" shell of the atom. In 1902, while Lewis was trying to
explain valence to his students, he depicted atoms as constructed of a concentric
series of cubes with electrons at each corner. This "cubic atom"
explained the eight groups in the periodic table and represented his idea that
chemical bonds are formed by electron transference to give each atom a complete
set of eight outer electrons (an "octet").
Lewis's theory
of chemical bonding continued to evolve and, in 1916, he published his seminal
article "The Atom of the Molecule", which suggested that a chemical
bond is a pair of electrons shared by two atoms. Lewis's model equated the
classical chemical bond with the sharing of
a pair of electrons between the two bonded atoms. Lewis introduced the
"electron dot diagrams" in this paper to symbolize the electronic structures
of atoms and molecules. Now known as Lewis structures, they are discussed in virtually every introductory chemistry
book.
Shortly after
publication of his 1916 paper, Lewis became involved with military research. He
did not return to the subject of chemical bonding until 1923, when he
masterfully summarized his model in a short monograph entitled Valence and the
Structure of Atoms and Molecules. His renewal of interest in this subject was
largely stimulated by the activities of the American chemist and General
Electric researcher Irving Langmuir, who between 1919 and 1921 popularized and elaborated Lewis's
model. Langmuir subsequently introduced the term covalent bond. In 1921, Otto Stern and Walther Gerlach establish concept of quantum mechanical spin in subatomic
particles.
For cases where
no sharing was involved, Lewis in 1923 developed the electron pair theory
of acids and base: Lewis redefined an acid as any atom or molecule with an
incomplete octet that was thus capable of accepting electrons from another
atom; bases were, of course, electron donors. His theory is known as the
concept of Lewis acids and bases. In 1923, G. N. Lewis
and Merle Randall published Thermodynamics
and the Free Energy of Chemical Substances, first modern treatise on
chemical thermodynamics.
The 1920s saw a rapid adoption and application of Lewis's model
of the electron-pair bond in the fields of organic and coordination chemistry.
In organic chemistry, this was primarily due to the efforts of the British
chemists Arthur Lapworth, Robert Robinson, Thomas Lowry, and Christopher Ingold; while in coordination
chemistry, Lewis's bonding model was promoted through the efforts of the
American chemist Maurice Huggins and the British chemist Nevil Sidgwick.
In 1924, French
quantum physicist Louis de Broglie published his thesis, in which he introduced a
revolutionary theory of electron waves based on wave–particle duality in his thesis. In
his time, the wave and particle interpretations of light and matter were seen as being at odds with one another, but de
Broglie suggested that these seemingly different characteristics were instead
the same behavior observed from different perspectives — that particles can
behave like waves, and waves (radiation) can behave like particles. Broglie's
proposal offered an explanation of the restriction motion of electrons within the atom. The first publications of Broglie's idea
of "matter waves" had drawn little attention from other physicists,
but a copy of his doctoral thesis chanced to reach Einstein, whose response was
enthusiastic. Einstein stressed the importance of Broglie's work both
explicitly and by building further on it.
In 1925,
Austrian-born physicist Wolfgang Pauli developed the Pauli exclusion
principle,
which states that no two electrons around a single nucleus in an atom can
occupy the same quantum state simultaneously, as described by four quantum numbers. Pauli made major contributions to quantum mechanics and
quantum field theory - he was awarded the 1945 Nobel Prize for Physics for his
discovery of the Pauli exclusion principle - as well as solid-state physics,
and he successfully hypothesized the existence of the neutrino. In addition to his original work, he wrote masterful syntheses
of several areas of physical theory that are considered classics of scientific
literature.
In 1926 at the
age of 39, Austrian theoretical physicist Erwin Schrödinger produced the papers
that gave the foundations of quantum wave mechanics. In those papers he
described his partial differential equation that is the basic equation of
quantum mechanics and bears the same relation to the mechanics of the atom
as Newton's equations of
motion bear
to planetary astronomy. Adopting a proposal made by Louis de Broglie in 1924
that particles of matter have a dual nature and in some situations act like
waves, Schrödinger introduced a theory describing the behavior of such a system
by a wave equation that is now known as the Schrödinger equation. The solutions to
Schrödinger's equation, unlike the solutions to Newton's equations, are wave
functions that can only be related to the probable occurrence of physical
events. The readily visualized sequence of events of the planetary orbits of
Newton is, in quantum mechanics, replaced by the more abstract notion of probability. (This aspect of the quantum theory made Schrödinger and
several other physicists profoundly unhappy, and he devoted much of his later
life to formulating philosophical objections to the generally accepted
interpretation of the theory that he had done so much to create.)
German
theoretical physicist Werner Heisenberg was one of the key creators of quantum mechanics. In 1925,
Heisenberg discovered a way to formulate quantum mechanics in terms of
matrices. For that discovery, he was awarded the Nobel Prize for Physics for
1932. In 1927 he published his uncertainty principle, upon which he built his
philosophy and for which he is best known. Heisenberg was able to demonstrate
that if you were studying an electron in an atom you could say where it was
(the electron's location) or where it was going (the electron's velocity), but
it was impossible to express both at the same time. He also made important
contributions to the theories of the hydrodynamics of turbulent flows, the atomic nucleus, ferromagnetism, cosmic rays, and subatomic particles, and he was instrumental
in planning the first West German nuclear reactor at Karlsruhe, together with a research reactor in Munich, in 1957. Considerable controversy surrounds his
work on atomic research during World War II.
Some view the
birth of quantum chemistry in the discovery of the Schrödinger equation and its application
to the hydrogen atom in 1926. However,
the 1927 article of Walter Heitler and Fritz London is often recognised as the first milestone in the history
of quantum chemistry. This is the first application of quantum mechanics to the diatomic hydrogen molecule, and thus to the phenomenon of the chemical bond. In the following years much progress was accomplished by Edward Teller, Robert S. Mulliken, Max Born, J. Robert Oppenheimer, Linus Pauling, Erich Hückel, Douglas Hartree, Vladimir Aleksandrovich
Fock, to
cite a few.
Still,
skepticism remained as to the general power of quantum mechanics applied to
complex chemical systems. The situation around 1930 is described by Paul Dirac:
The underlying
physical laws necessary for the mathematical theory of a large part of physics
and the whole of chemistry are thus completely known, and the difficulty is only
that the exact application of these laws leads to equations much too
complicated to be soluble. It therefore becomes desirable that approximate
practical methods of applying quantum mechanics should be developed, which can
lead to an explanation of the main features of complex atomic systems without
too much computation.
Hence the
quantum mechanical methods developed in the 1930s and 1940s are often referred
to as theoretical molecular or atomic physics to underline the fact that they were more the application
of quantum mechanics to chemistry and spectroscopy than answers to chemically relevant questions. In 1951, a
milestone article in quantum chemistry is the seminal paper of Clemens C. J. Roothaan on Roothaan equations. It opened the avenue
to the solution of the self-consistent fieldequations for small
molecules like hydrogen or nitrogen. Those computations were performed with the help of tables of
integrals which were computed on the most advanced computers of the time.
In the 1940s
many physicists turned from molecular or atomic physics to nuclear physics (like J. Robert Oppenheimer or Edward Teller). Glenn T. Seaborg was an American nuclear chemist best known for his work on
isolating and identifying transuranium elements (those heavier
than uranium). He shared the 1951 Nobel Prize for Chemistry with Edwin Mattison McMillan for their independent discoveries of
transuranium elements. Seaborgium was named in his honour, making
him the only person, along Albert Einstein and Yuri Oganessian, for whom a chemical element was named during his lifetime.
By the mid 20th
century, in principle, the integration of physics and chemistry was extensive,
with chemical properties explained as the result of the electronic structure of the atom; Linus Pauling's book on The Nature of the Chemical Bond used
the principles of quantum mechanics to deduce bond angles in ever-more complicated molecules. However, though some
principles deduced from quantum mechanics were able to predict qualitatively
some chemical features for biologically relevant molecules, they were, till the
end of the 20th century, more a collection of rules, observations, and recipes
than rigorous ab initio quantitative
methods.
This heuristic
approach triumphed in 1953 when James Watson and Francis Crick deduced the double helical structure of DNA by constructing
models constrained by and informed by the knowledge of the chemistry of the
constituent parts and the X-ray diffraction patterns obtained by Rosalind Franklin. This discovery lead to an explosion of research into
the biochemistry of life.
In the same
year, the Miller–Urey experiment demonstrated that
basic constituents of protein, simple amino acids, could themselves be built up from simpler molecules in a simulation of primordial processes on Earth. Though many questions remain about the true
nature of the origin of life, this was the first attempt by chemists to study
hypothetical processes in the laboratory under controlled conditions.
In 1983 Kary Mullis devised
a method for the in-vitro amplification of DNA, known as the polymerase chain
reaction (PCR),
which revolutionized the chemical processes used in the laboratory to
manipulate it. PCR could be used to synthesize specific pieces of DNA and made
possible the sequencing of DNA of organisms, which
culminated in the huge human genome project.
An important
piece in the double helix puzzle was solved by one of Pauling's students Matthew Meselson and Frank Stahl, the result of their collaboration (Meselson–Stahl
experiment)
has been called as "the most beautiful experiment in biology".
They used a
centrifugation technique that sorted molecules according to differences in
weight. Because nitrogen atoms are a component of DNA, they were labelled and
therefore tracked in replication in bacteria.
In 1970, John Pople developed the Gaussian program greatly
easing computational chemistry calculations.[93] In 1971, Yves Chauvinoffered an explanation of the reaction mechanism of olefin metathesis reactions. In 1975, Karl Barry Sharpless and his group
discovered a stereoselective oxidation reactions
including Sharpless epoxidation, Sharpless asymmetric dihydroxylation,
and Sharpless oxyamination.[100][101][102] In 1985, Harold Kroto, Robert Curl and Richard Smalley discovered fullerenes, a class of large carbon molecules superficially resembling
the geodesic dome designed by
architect R. Buckminster Fuller. In 1991, Sumio Iijima used electron microscopy to discover a type
of cylindrical fullerene known as a carbon nanotube, though earlier work had been done in the field as early as 1951.
This material is an important component in the field of nanotechnology. In 1994, Robert A. Holton and his group achieved the first total synthesis of
Taxol. In
1995, Eric Cornell and Carl Wieman produced the first Bose–Einstein
condensate,
a substance that displays quantum mechanical properties on the macroscopic
scale.
Classically,
before the 20th century, chemistry was defined as the science of the nature of
matter and its transformations. It was therefore clearly distinct from physics
which was not concerned with such dramatic transformation of matter. Moreover,
in contrast to physics, chemistry was not using much of mathematics. Even some
were particularly reluctant to use mathematics within chemistry. For
example, Auguste Comte wrote in 1830:
Every attempt
to employ mathematical methods in the study of chemical questions must be
considered profoundly irrational and contrary to the spirit of chemistry.... if
mathematical analysis should ever hold a prominent place in chemistry -- an
aberration which is happily almost impossible -- it would occasion a rapid and
widespread degeneration of that science.
However, in the
second part of the 19th century, the situation changed and August Kekulé wrote in 1867:
I rather expect
that we shall someday find a mathematico-mechanical explanation for what we now
call atoms which will render an account of their properties.
As
understanding of the nature of matter has evolved, so too has the
self-understanding of the science of chemistry by its practitioners. This
continuing historical process of evaluation includes the categories, terms,
aims and scope of chemistry. Additionally, the development of the social
institutions and networks which support chemical enquiry are highly significant
factors that enable the production, dissemination and application of chemical
knowledge. (See Philosophy of chemistry)
The later part
of the nineteenth century saw a huge increase in the exploitation of petroleum extracted from the earth for the production of a host of
chemicals and largely replaced the use of whale oil, coal tar and naval stores used previously. Large-scale production and refinement of petroleum provided feedstocks for liquid fuelssuch as gasoline and diesel, solvents, lubricants, asphalt, waxes, and for the production
of many of the common materials of the modern world, such as synthetic fibers, plastics, paints, detergents, pharmaceuticals, adhesives and ammonia as fertilizer and for other uses. Many of these required new catalysts and the utilization of chemical engineering for their
cost-effective production.
In the
mid-twentieth century, control of the electronic structure of semiconductor materials was made precise by the creation of large ingots
of extremely pure single crystals of silicon and germanium. Accurate control of their chemical composition by doping with
other elements made the production of the solid state transistor in 1951 and made possible the production of tiny integrated circuits for use in
electronic devices, especially computers.
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