Rutherford’s atomic theory was that an atom had a central positive nucleus with negative electrons orbiting
it. He developed this theory with his gold foil experiment.
Ernest Rutherford’s gold foil experiment involved a particle emitter, a round detecting screen with a slit in it
and a slip of gold foil in the middle. The detecting screen had zinc sulfide in it to allow Rutherford to detect
the presence of particles after they passed through the filtering gold foil. Through this experiment,
Rutherford determined that the vast majority of the particles he fired at the gold foil passed right through it.
Only about one in 8,000 was deflected away into the surrounding detecting screen. As a result, Rutherford
created a theory that stated that most of an atom was empty space. This made the most sense, since it
explained why so few particles were hitting the gold foil. The angle of deflection from the particles also
showed that there was most likely a strong positively charged nucleus in the middle of the atom with
negatively charged particles circling around it. Rutherford linked this motion to the orbit of planets around
the sun. The Bohr atomic model later replaced the Rutherford model.
30 August 1871 – 19 October 1937
Fun Fact
The element 'rutherfordium' was named in Rutherford’s honor.
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's
theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical
reactions. Also, there are different kinds of atoms (differing by their masses) within an element that are
known as "isotopes", but isotopes of an element have the same chemical properties.
Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory.
Dalton's theory quickly became the theoretical foundation in chemistry.
J.J. Thomson Biographical Data
Tomson was born December 18, 1856, Cheetham Hill, near Manchester, England. He died August 30, 1940,
Cambridge, Cambridgeshire, England. Thomson is buried in Westminster Abbey, near Sir Isaac Newton. J.J.
Thomson is credited with the discovery of the electron, the negatively-charged particle in the atom. He is
known for the Thomson atomic theory.
Many scientists studied the electric discharge of a cathode ray tube. It was Thomson's interpretation that
was important. He took the deflection of the rays by the magnets and charged plates as evidence of 'bodies
much smaller than atoms'. Thomson calculated these bodies had a large charge to mass ratio and he
estimated the value of the charge itself. In 1904, Thomson proposed a model of the atom as a sphere of
positive matter with electrons positioned based on electrostatic forces. So, he not only discovered the
electron but determined it was a fundamental part of an atom.
Thomson Atomic Theory
Thomson's discovery of the electron completely changed the way people viewed atoms. Up until the end of
the 19th century, atoms were thought to be tiny solid spheres. In 1903, Thomson proposed a model of the
atom consisting of positive and negative charges, present in equal amounts so that an atom would be
electrically neutral. He proposed the atom was a sphere, but the positive and negative charges were
embedded within it. Thomson's model came to be called the "plum pudding model" or "chocolate chip
cookie model". Modern scientists understand atoms consist of a nucleus of positively-charged protons and
neutral neutrons, with negatively-charged electrons orbiting the nucleus. Yet, Thomson's model is
important because it introduced the notion that an atom consisted of charged particles.
Interesting Facts About J.J. Thomson
Prior to Thomson's discovery of electrons, scientists believed the atom was the smallest fundamental unit of
matter.
Thomson called the particle he discovered 'corpuscles' rather than electrons.
Thomson's master's work, Treatise on the motion of vortex rings, provides a mathematical description of
William Thomson's vortex theory of atoms. He was awarded the Adams Prize in 1884.
Thomson discovered the natural radioactivity of potassium in 1905.
In 1906, Thomson demonstrated a hydrogen atom had only a single electron.
Thomson's father intended for J.J. to be an engineer, but the family did not have the funds to support the
apprenticeship. So, Joseph John attended Owens College in Manchester, and then Trinity College in
Cambridge, where he became a mathematical physicist.
In 1890, Thomson married one of his students, Rose Elisabeth Paget. They had a son and a daughter. The
son, Sir George Paget Thomson, received the Nobel Prize in Physics in 1937.
Thomson also investigated the nature of positively-charged particles. These experiments led to the
development of the mass spectrograph.
Thomson was closely aligned with chemists of the time. His atomic theory helped explain atomic bonding
and the structure of molecules. Thomson published an important monograph in 1913 urging the use of the
mass spectrograph in chemical analysis.
Many consider J.J. Thomson's greatest contribution to science to be his role as a teacher. Seven of his
research assistants, as well as his own son, went on to win the Nobel Prize in Physics. One of his best-known
students was Ernest Rutherford, who succeeded Thomson as Cavendish Professor of Physics.
Erwin Schrödinger’s contributions to the atomic model came after the Bohr model of the atom , in relation
to Schrödinger’s work in quantum physics and to the Schrödinger equation :
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger
used mathematical equations to describe the likelihood of finding an electron in a certain position. This
atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the
quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of
the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud.
Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the
electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of
sub-energy levels.
Some more details and clarifications about this topic :
Schrödinger combined the equations for the behavior of waves with the de Broglie equation to generate a
mathematical model for the distribution of electrons in an atom. The advantage of this model is that it
consists of mathematical equations known as wave functions that satisfy the requirements placed on the
behavior of electrons. The disadvantage is that it is difficult to imagine a physical model of electrons as
waves.
The Schrödinger model assumes that the electron is a wave and tries to describe the regions in space, or
orbitals, where electrons are most likely to be found. Instead of trying to tell us where the electron is at any
time, the Schrödinger model describes the probability that an electron can be found in a given region of
space at a given time. This model no longer tells us where the electron is; it only tells us where it might be.
The Bohr model was a one-dimensional model that used one quantum number to describe the distribution
of electrons in the atom. The only information that was important was the size of the orbit, which was
described by the n quantum number. Schrödinger's model allowed the electron to occupy
three-dimensional space. It therefore required three coordinates, or three quantum numbers, to describe
the orbitals in which electrons can be found.
The three coordinates that come from Schrödinger's wave equations are the principal (n), angular (l), and
magnetic (m) quantum numbers. These quantum numbers describe the size, shape, and orientation in space
of the orbitals on an atom.
Chadwick's Atomic Model
Neutrons were found to have the same mass as protons which accounted for more of the mass of the atom
and allowed the known mass of an atom and the known mass of its particles to match. The common
understanding of an atom was now a nucleus containing positively charged protons and neutral neutrons
with the rest of the atom being space in which negatively charged electrons, with a mass so small compared
to that of the nucleus "orbit" the nucleus on energy levels corresponding to the amount of energy the
electrons hold.[5]
The Chadwick model of an atom
James Chadwick, the Nucleus, and the Neutron
In 1923, aged 32, Chadwick became Rutherford’s Assistant Director of Research in the Cavendish Laboratory
where he continued to study the atomic nucleus.
In those days, most researchers believed there were electrons within the nucleus as well as outside it. For
example, the nucleus of a carbon atom was thought to contain 12 protons and 6 electrons, giving it an
electric charge of +6. Orbiting the nucleus were supposed to be another 6 electrons causing the atom’s
overall electric charge to be 0.
Rutherford, Chadwick, and some others believed in the possibility that particles with no charge could be in
the nucleus.
In his spare time, through the 1920s, Chadwick made a variety of attempts in the laboratory to find these
neutral particles, but without success. He was, however, increasingly convinced in the existence of a neutral
particle – the neutron. He couldn’t, however, get the evidence he needed to prove its existence.
Then, at the beginning of 1932, Chadwick learned of work that Frederic and Irene Joliot-Curie had just done
in Paris. The Joliot-Curies believed they had managed to eject protons from a sample of wax using gamma
rays. This did not make sense to Chadwick, who thought gamma rays were not powerful enough to do this.
However, the evidence that protons had been hit with sufficient energy to eject them was convincing.
The gamma ray source had been the radioactive element polonium. Chadwick drew the conclusion that the
protons had actually been hit by the particle he was looking for: the neutron.
Feverishly, he began working in the Cavendish laboratory. Using polonium as a source of (what he believed
were) neutrons, he bombarded wax. Protons were released by the wax and Chadwick made measurements
of the protons’ behavior.
The protons behaved in exactly the manner they ought to if they had been hit by electrically neutral
particles with a mass similar to the proton. Chadwick had discovered the neutron.
Within two weeks he had written to the prestigious science journal Nature to announce the Possible
Existence of a Neutron.
Chadwick did not think he had discovered a new elementary particle. He believed the neutron was a
complex particle consisting of a proton and an electron. The German physicist Werner Heisenberg showed
that the neutron could not be an electron-proton pair, and was actually a new elementary particle.
In 1935, James Chadwick received the Nobel Prize in Physics for his discovery of the neutron.
World War 2 and the Atomic Bomb
In 1939, the first year of World War 2, Chadwick was asked by the British Government about building an
atomic bomb. He said it was possible, but would not be easy.
Preliminary research began in a number of universities. Working conditions in Chadwick’s laboratory were
arduous. The neighborhood in Liverpool was frequently attacked in air-raids by the German Air Force.
Despite the bombing, by spring 1941, Chadwick’s research group had discovered that the critical mass of
uranium-235 for a nuclear detonation was about 8 kilograms.Chadwick was present when the US and UK
governments agreed that the bomb could be used against Japan. He then attended the Trinity nuclear test
on July 16, 1945, when the world’s first atomic bomb was detonated.
In 1945, the British Government knighted him for his wartime contribution, and he became Sir James
Chadwick. The U.S. Government awarded him the Medal of Merit in 1946.