-
Democritus, a Greek philosopher and one of the first atomic theorists, came up with his own Atomic Theory.
He believed that atoms were uniform, solid, hard, incompressible, indestructible and moved in infinite numbers through empty space until stopped. Democritus reasoned that if the stone were to be cut in half again and again; at some point, there would be a piece that would be so small it would be indivisible. He called these pieces "atomos," the Greek word for indivisible. -
Expanding on Demoncritus' work, John Dalton hypothesized that the law of conservation of mass and the law of definite proportions could be explained using the idea of atoms.
He proposed that all matter is made of tiny indivisible and indestructive building blocks called atoms, which he imagined as "solid, massy, hard, impenetrable, movable particle(s)". -
In 1897, JJ Thomas discovered the corpuscle (now known as an electron) using a cathode ray scope.
Thomson's experiments with cathode-ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. -
In 1900, German physicist Max Planck introduced the constant in his accurate formulation of the distribution of the radiation emitted by a blackbody.
A blackbody is a hypothetical perfect absorber and radiator of energy with no reflecting power.
10 years later, Niels Bohr and Albert Einstein used Planck's discovery to make a new model of the inside of an atom. In the Bohr model, when an electron moves from a higher energy orbit to a lower energy orbit, it gives off a photon. -
In 1904, Thompson proposed a model of the atom that consisted of more than one fundamental unit: the Plum Pudding model, in which negatively-charged electrons are embedded within a positively-charged "soup." Based on its appearance, which consisted of a “sea of uniform positive charge” with electrons distributed throughout, Thompson’s model was later dubbed the “Plum Pudding Model”. -
In 1906, JJ Thomson was awarded the Nobel Prize "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases". -
In 1909, Rutherford, Hans Geiger and Ernest Marsden shot alpha particles– which Rutherford discovered before– at a piece of thin gold foil. When the particles came into contact with a screen covering the foil, there is a flash. Some of the particles passed directly through the foil, but others either bounced back or were deflected. He concluded that most of the atom is composed of empty space and something small, dense, and positive inside the atom repels the positively charged alpha particles. -
In 1911, Rutherford announces his atomic model, in which he describes the atom as "having a tiny, dense, and positively charged core" called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus, around which the lighter, negative components called electrons circulate at a distance, like planets orbiting the sun. He also suggested that J. J. Thompson's "pudding model" of the atom was incorrect. -
From 1911 to 1913, James Chadwick studied under Ernest Rutherford in the Physical Laboratory in Manchester, working on radioactivity problems.
-
The discovery of radioactivity in 1896 was followed by numerous scientific studies and theories.
Among them was Frederick Soddy, who proposed that atoms could have the same chemical properties but different properties when it came to radioactivity, making them variants of known elements rather than new ones.
In 1913, Soddy theorised that an element can have different variations (which he called isotopes) but still be the same element. It turned out that these isotopes had different weights. -
In 1913, Niels Bohr introduced his Atomic Hydrogen Model. It was based on three assumptions: (1) an electron moves around the nucleus in a circular orbit, (2) an electron’s angular momentum in the orbit is quantized, and (3) the change in an electron’s energy as it makes a quantum jump from one orbit to another is always accompanied by the emission or absorption of a photon. Bohr's model was later proved wrong because it didn't work for atoms with more than one electron (Hydrogen or He+). -
In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and if electrons jump to a lower-energy orbit, the difference is sent out as radiation. The key difference between the Bohr and quantum models is that the Bohr model states that electrons behave as particles whereas quantum model explains that the electron has both particle and wave behavior. -
Ernest Rutherford observed that his scintillation detectors detected hydrogen nuclei when a beam of alpha particles was fired into the air. Upon further investigation, he found that these hydrogen nuclei came from nitrogen atoms in the air. He then shot beams of alpha particles into pure nitrogen gas, observing that a greater number of hydrogen nuclei were present afterwards. He concluded that the hydrogen nuclei came from the nitrogen atom, and the hydrogen nucleus is a part of all other atoms. -
In 1932, Chadwick made a fundamental discovery: he proved the existence of neutrons – elementary particles devoid of any electrical charge.
In contrast with Alpha rays, these neutrons had no charge, meaning they easily overcame any electric barrier and were capable of splitting the nuclei of even the heaviest elements.
This prepared the way for the fission of uranium 235 and the creation of the atomic bomb. -
James Chadwick was awarded the Nobel Prize for Physics in 1935 "for the discovery of the neutron." -
From 1943 to 1945, James Chadwick was the head of the British Mission to the Manhattan Project because he discovered the neutron and had expertise in the field.
Bombarding heavy elements with neutrons can succeed in penetrating and splitting nuclei, generating an enormous amount of energy. This is called nuclear fission and is how atomic bombs are made.
In 1945, the first atomic bomb was dropped. James Chadwick was the head of the British mission to aid the Manhattan project at the time. -
For his help with the Manhattan Project and other contributions to science such as discovering the neutron, James Chadwick was knighted in 1945.
You are not authorized to access this page.