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1 CE
Democritus
Democritus pictured the atom to look like the picture. Democritus' adopted his atomic theory from his mentor Leucippus. His theory states “The universe is composed of two elements: the atoms and the void in which they exist and move.” He believed that these atoms were an infinite number of minuscule, invisible amounts of matter that could not be destroyed, and were always moving. He believed that the atoms differed in shape, size and temperature. His hypothesis was created circa 465BC. -
2
Aristotle
Aristotle had a different theory than Democritus. He believed that everything on earth was made of small amounts of Earth, Fire, Water, and Air. People believed his theory, and as a result Democritus' Atomic Theory was overlooked for about 2,000 years.
This was around circa 335BC. -
Period: to
Antonie Lavoisier
Lavoisier was a French experimenter. Lavoisier's experiments helped him come up with the Law of Conservation of Matter, which states that the amount of matter in the universe has been, and will always be the same. This explains that atoms are not destroyed in a chemical reaction. He also discovered 11 elements, and helped create the metric system. -
Period: to
John Dalton
John Dalton theorized the Law of Multiple Proportions. He invented a system of symbols to represent elements and measured the equivalent mass of water using relative atomic weight and relative atomic ratios. He expanded Democritus' Atomic Theory. His theory states; All matter is made of indestructible atoms. Atoms of an element have the same mass and properties.Compounds, are formed by a chemical reaction of two or more different atoms, and are just a rearrangement of atoms. -
Period: to
Henri Becquerel
Henri Becquerel made a significant contribution to atomic theory, with his partial discovery of radioactivity, in 1896. While studying the effect of x-rays on photographic film, he discovered the some elements don't need sun to emit rays similar to X-rays and instead and give off their own rays. He received a joint Nobel prize with Marie Curie for his work. -
Period: to
J.J.Thomson
J.J. Thomson was a scientist, and inventor. Thomson discovered the electron, calculated it's charge to mass ratio and noticed the proton in his experiments. He proposed the "Plum Pudding model of the atom, and invented the mass spectrograph. His model of the atom showed that the majority of it was a big positive amount of mass, with small amounts of negatively charged particles to create neutrality. -
Period: to
Max Planck
He developed a theory to explain hot glowing matter, called the quantum theory. This contributed to understanding atomic, and subatomic particles. -
Period: to
Marie and Pierre Curie
The two scientists studied uranium and thorium's decay process. They called this process radioactivity. Their discovery lead then to discover two new elements, radium and polonium. In both 1903 and 1911, Marie received a Nobel prize. -
Period: to
Robert Millikan
Robert Millikan used an oil drop experiment around 1909 to determine the charge of an electron in units. This helped future scientists to calculate the electrons mass. He received a Nobel prize in 1923 for his work in physics. -
Period: to
Ernest Rutherford (1871-1937)
Rutherford discovered the nucleus with his gold foil experiment in 1911. He then proposed a new model of the atom, with the nucleus in the center, and electrons around it. He found that the nucleus had a positive charge and the electrons had a negative charge. Later in 1920, he proposed both the proton and the neutron. -
Period: to
Niels Bohr
He proposed a theory for the electron's place in the nucleus, in 1913, using quantum theory. He said that the electrons move around the nucleus in set orbits, and can jump from orbit to orbit. Each orbit has an energy level, and when an electron changes orbit, it emits quantum light. His theory gave way to the picture of the atom with the nucleus in the center, and the electrons around it on rings. -
Period: to
Erwin Schrödinger
He added to the Bohr model of the atom, by using an equation to predict the possibilities of where an electron would be. He thought of electrons as clouds, and introduced "wave mechanics". His model of the atom is known as the quantum model. -
Period: to
Henry Moseley
In 1913 Henry Moseley found the relationship between the atomic number on the periodic table, and the positive charge on the nucleus of the element's atoms. This caused the periodic table to be rearranged. The table was then organized by atomic number, not atomic mass, as had been prior to his discovery. -
Period: to
James Chadwick
In 1932, Chadwick shot alpha particles at atoms, and produced a new radiation. The radiation was made of particles with a neutral charge and the about the same mass as the proton. It was named the neutron. This caused the image of the atom to be altered to where the nucleus clearly showed both neutrons and protons, with the electrons orbiting around them on rings. This was an image used for quite some time. -
Henry Becquerel and Marie Curie: Radioactivity
While studying the effect of x-rays on photographic film, Becquerel discovered the some elements don't need sun to emit rays similar to X-rays and instead and give off their own rays. Marie furthered these studies and named the decaying process "radioactivity". -
J.J. Thomson: Discovery of the Electron
He discovered the electron in 1897. He found it by experimenting with "cathode rays" or "a beam of electrons emitted from the cathode of a high-vacuum tube." He then proposed his theory, and received a noble prize for his discovery in 1906. -
Period: to
Werner Heisenberg
He developed quantum mechanics based off of quantum theory. These mechanics are crucial for the development of the understanding of atomic and subatomic particles. He also proposed the Principle of Indeterminacy which states "you can not know both the position and velocity of a particle". -
Robert Millikan: Discovery of Electron Charge
Robert Millikan used an oil drop experiment around 1909 to determine the charge of an electron in units. -
Rutherford: Gold Foil Experiment
Rutherford discovered the nucleus with his gold foil experiment in 1911. He shot alpha particles at a sheet of gold foil and noticed that some went through, others bounced back. -
Henry Moseley: Relationship Between Atomic Number and Charge
In 1913 Henry Moseley found the relationship between the atomic number on the periodic table, and the positive charge on the nucleus of the element's atoms. -
Niels Bohr: Electron Theory
He proposed a theory for the electron's orbit in the nucleus, in 1913, using quantum theory. -
Erwin Schrödinger: Quantum Mechanical Model
Erwin Schrödinger developed a mathematical theory for predicting the position of the electron, by thinking of the electron as a cloud. He developed a new model of the atom called the Quantum Mechanical Model. -
James Chadwick: Discovery of the Neutron
In 1932, Chadwick shot alpha particles at atoms, and produced a new radiation. The radiation was made of particles with a neutral charge and the about the same mass as the proton. It was named it the neutron.
