-
340
Aristotle
In contrary to Democritus' theory, Aristotle thought that there were only 5 elements in existence. The air which was light, earth which was cool and heavy, water which was wet, fire which was hot , and Aether which was known to be a divine substance that made up the stars and the planets. Even though he was right in saying there are elements he caused the delay to the emergence of the atomic theory. -
400
Democritus
His atomic theory states that everything is physically made op of atoms. He stated that atoms are indivisible (or in a word: indestructible) and can never be broken down into smaller pieces. He added that in between atoms are empty spaces, the more empty space the heavier the atom gets.
His atomic model included a sphere that can't be seen. This invisible sphere has no nucleus or electrons and varies in size, shape, and temperature. -
Period: 400 to
The Understanding of the Atom throughout history
-
Antoine Lavoisier
Antoine was a highly skilled experimentalist who had discovered oxygen and hydrogen were combined to make water. Lavoisier also discovered what is known as the law of conservation of mass, which means mass can't be created or destroyed. This also means that the mass of the reactants is equal to the mass of the product. -
John Dalton
Dalton stated that tiny particles called atoms form elements. Different elements would have atoms of different sizes and mass. He thought that every atom was unique, as they couldn't be created, divided, or destroyed by any type of chemical process. The later discovery of nuclear fusion and nuclear fission altered this viewpoint, though these were nuclear and not chemical reactions. -
Henri Becquerel
Henri Becquerel made the discovery of nuclear radiation in 1896. This occurred when he was testing the relation between phosphorescence and x-rays (a newly discovered form of raidation at the time). From his experiment, he discovered that phosphorescence wasn't the case, but that the rock containing uranium was giving off some type of radiation. -
Joseph John Thomson
J.J. Thomson thought that John Dalton's model was outdated and needed to be updated. He discovered that electrons are present im atoms. He proved this by using a cathode ray tube. The model that Thomson made from his studies was called the "plum pudding" model. This model was mostly positive matter (pudding) and electrons were scattered throughout (plums). This was an accurate discovery because it identified a new part of the atom that is still used today. -
Marie and Pierre Curie
After Becquerel's discovery, Marie and her husband began experimenting to determine the true nature of this "radioactivity."These experiments laid the groundwork for a new era of physics and chemistry. Eventually this would lead to the discovery of the neutron. Later, in 1903, Marie and Pierre Curie and Henri Becquerel went on to win a Nobel Prize for their work in radioactivity. -
Max Planck
Max Planck is said to have created the quantum theory. He alsi discovered a fundamental constant that's known as Planck's constant, and is, for example used to calculate a photon's energy. A year later, he discovered the law of heat radiation, which later became known as Planck's law of black body radiation. His law became the basis of quantum theory which was later used by Niels Bohr -
Robert Millikan
Millikan determined the unit charge of the electron with his oil drop experiment.This experiment has made it possible to obtain the calculation of the mass of the electron and the positively charged atoms.(e = 1.60 x 10-19 coulombs) -
Ernest Rutherford
Rutherford proposed the nuclear atom as a result of his gol foil experiment(1911). He also proposed that the positive charge and the mass of the atom took up a small volume in the center of the atom (nucleus) and most of the volume of the atom was taken up by empty space occupied by the electrons. In addition to this dicovery, Rutherford also proposed the existence of the third atomic particle, the neutron in 1920 -
Niels Bohr
Bohr proposed the quantum theory of an atom, which was based on the quantum theory of radiation.He depicts the atom as a tiny, spherical body which consists nucleus at center and negatively charged particles (electrons) revolving around nucleus in a certain path known as orbit. He proposed some new postulate with same basis concepts of Rutherford theory. -
Henry Mosely
Mosely discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. This idea rearranged the placement of the elements in the periodic table by their atomic number instead of their atomic mass in terms of progression in the elements. This new table is currently used today to show the progression of elements, but this table also created holes for elements that would soon be discovered. -
Erwin Schrodinger
Erwin Schrodinger added the final part to electron arrangement around the nuclei of atoms. He suggested that electrons behave in a wave-like manner rather than just as particles. Schrodinger also thought that the exact location within an orbit could not be precisely calculated. -
Werner Heisenberg
Established the field of quantum mechanics and is best known for his uncertainty principle. The principle posits limits to the accuracy of knowledge about atomic behavior. Heisenberg was a former student of Bohr's and had actually questioned the mathematics of Bohr. -
James Chadwick
Chadwick's major contribution to atomic theory was the discovery of the neutron. The neutron was first discovered in the atomic nucleus. The neutron got its name due to a lack of electric charge.Unlike positively charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier, which means they can penetrate and split the nuclei of even the heaviest elements.
