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460 BCE
Democritus
He was a Greek philosopher and his ideas were based on reasoning rather than science. He knew that if a stone was divided in half it would have the same proprietors as the whole and if the stone were to continually be cut up into smaller and smaller pieces then there would be such a small piece that it would be invisible -
100 BCE
Alchemist
Their goal was to treat different metals and ores to change the structure of the item to become more valuable. Although they failed to turn common items into gold, they did discover a scientific process that would allow for the eventual discovery of the atom. -
Joseph Louis Proust
Joseph Proust first proposed the idea that compounds have defined chemical formulas. After a number of experiments, he observed that no matter how much different elements would react with oxygen, they would always react in defined proportions. -
Antoine Lavoisier
He began to burn substances and showed that they combined with air to make new materials that weighed more than their original substance. Based on these observations, Lavoisier established the Law of Conservation of mass. -
Law of Conservation of Mass
The Law of Conservation of Mass dates back to Antoine Lavioser’s 1789 discovery that mass is not created or destroyed in chemical reactions. If all of the reactants and products in a chemical reaction are accounted for, then the total mass will be the same in a closed system. -
Law of Definite Proportions
Joseph Proust created the Law of Definite Proportions when conducting an experiment in which he observes that no matter how different elements reacted with oxygen, they always reacted in definite proportions -
Dalton's Atomic Theory
John Dalton made the first modern atomic theory in 1803. His theory of the atoms was based on four main concepts; all matter is composed of indivisible particles called atoms, all atoms of a given element are identical, chemical reactions involve the combination of atoms, not the destruction of atoms, and when elements react to form compounds, they react in defined, whole-number ratios. -
John Dalton
Dalton used previous information and combined all of them together to develop an atomic theory. His theory was based on four assumptions and was generally accepted because it explained the laws of conservation of mass. Dalton however incorrectly imagined that atoms hooked together to form molecules. -
Cathode Ray Tube
Faraday noticed that when passing a current through a tube, an arc of electricity was observed. The arc began at the negative plate which is known as the cathode. He observed a luminescence that started in the telling and traveled to the positive end. -
Dmitri Mendeleev
He was a Russian chemist who developed the periodic classification of the elements. He found that when all the known chemical elements were arranged in order of increasing atomic weight it resulted in a table that displayed a recurring pattern of properties within groups of elements. -
J.J Thomson
he showed that electrons have a negative charge and come from matter and created the Plum Pudding model. He also proposed that negatively charged electrons were mixed with smeared-out positive charges. -
Plum Pudding Atomic Model
The Plum Pudding atomic model was an entirely new model of the atom with the electron now discovered. Thomson proposed this model and that the negatively charged electrons which were described as the dried fruit in the pudding were randomly spread out in what he called “a sphere of uniform positive electrification” or the body of the pudding. -
Gold Foil Experiment
It was thought that the particles would pass straight through due to the previous information under the Thomson Plum Pudding model. Despite what the Plum Pudding model predicted, the particles acted like they encountered a hard object. Rutherford suggested that atoms are made up of largely empty space and that there was a dense positively charged area in the atoms that caused the ricocheting of the particles. -
Ernest Rutherford
Rutherford performed a series of experiments that contained alpha particles fired at a thin piece of gold foil. This experiment became known as the gold foil experiment and led to the development of a model of the atom with a dense, positively charged area of the atom at the center known as the nucleus. -
Nuclear Model
Rutherford overturned Thomson's model in 1911 with his gold foil experiment which led to the idea that there was a positively charged area in an atom and caused the backscattering of alpha particles. -
Planetary Model
This model was proposed by Rutherford who rejected Thomson’s Plum Pudding model. He proposed a model that was similar to the planet Saturn with rings of electrons revolving around a positive center. -
Oil Drop Experiment
This experiment was done by Robert Millikan in 1913 and was done by spraying tiny oil droplets from an atomizer into a sealed chamber. The drops then fell down due to gravity in between two electrical plates and became charged when interacting with the air that had been ionized by x-rays. He concluded that the differing charges were because of the different numbers of electrons. -
Robert Millikan
He conducted an experiment that allowed the calculation of the specific value of the negative charge of the electron. This was done by an oil drop experiment where tiny oil drops were sprayed from an atomizer into a sealed chamber, and when the oil drops fell down into the space between two electrical plates and became charged. -
Neils Bohr
Bohr went to England to work with J.J. Thomson at the Cavendish Laboratory but did not impress him with his experiments. He then received an invitation to work with Ernest Rutherford and adapted Rutherford’s nuclear structure to Max Plank’s quantum theory to create the Bohr model in 1913. This began the most widely accepted model of the atom. -
Henry Moseley
During his first research, he did most things with radioactivity and beta radiation in radium. He studied the X-ray spectra of the elements and conducted a series of experiments to find a relationship between the frequencies of corresponding lines in the x-ray spectra. -
Werner Heisenberg
He was a German physicist who in 1925 discovered the way to formulate quantum mechanics in terms of matrices. In 1927 he created the uncertainty principle that states the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. -
Erwin Schrodinger
Schrodinger used Bohr's atom model a step further by using a mathematical equation to describe the likelihood of finding an electron in a certain position. This atomic model became known as the quantum mechanical model of the atom. -
Quantum Mechanical Model
Describe the possibility of placing electrons within an atom by describing the principal energy level, energy level, orbital arbitrary level, and spin of. This model is based on quantum theory, which states that matter has wave-like properties. -
James Chadwick
Chadwick was working for Ernest Rutherford who proposed the idea of a neutral atomic particle that had a mass. Chadwick further developed an experiment by using polonium as the source of alpha particles.