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430 BCE
Democritus
Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped.He argued that matter was subdivided into indivisible and immutable particles that created the appearance of change when they joined and separated from others. -
Period: 430 BCE to
Atomic Theory
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350 BCE
Alchemists
The Alchemists. broke down the chemical composition of the 4 basic elements of its period, Fire, Earth, Wind, and water eventually evolved into The Periodic table we use today. Alchemy Contributed to the Atomic Theory, by laying down the foundation of the Modern Day Periodic Table of the elements. -
Law of Conservation of Mass
The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. -
Antoine Lavoisier
Lavoisier believed that matter was neither created nor destroyed in chemical reactions, and in his experiments he sought to demonstrate that this belief was not violated. -
Law of Definite Proportions
The law of definite proportions, states that every chemical compound contains fixed and constant proportions (by mass) of its constituent elements. -
Joseph Louis Proust
Proust's law of proportions is that chemical substances only truly combine to form a small number of compounds, each of which is characterized by components that combine in fixed proportions by weight. He suggested that all matter was composed of tiny indivisible particles, which he called atoms. -
Dalton's Atomic Theory
Dalton developed his atomic theory as a way of trying to answer certain questions about the atmosphere.Dalton viewed matter as composed of spherical particles and believed that these particles or atoms contained a shield of heat around them. -
John Dalton
Dalton established the idea that the atoms of different elements are distinguished by differences in their weight. -
Cathode Ray Tube
Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. -
Dmitri Mendeleev
Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. -
J.J. Thomsan
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup." -
Plum Pudding Atomic Model
In the Plum Pudding Atomic Model, Thomson proposed that the negatively charged electrons (analogous to the raisins) were randomly spread out among what he called "a sphere of uniform positive electrification" (analogous with the dough or body of the pudding). -
Gold Foil Experiment
A piece of gold foil was hit with alpha particles, which have a positive charge. Most alpha particles went right through. This showed that the gold atoms were mostly empty space. -
Nuclear Model
Ernest Rutherford proposed the nuclear model is any of the several theoretical descriptions of the structure and function of atomic nuclei. Each of the models is based on a plausible analogy that correlates a large amount of information and enables predictions of the properties of nuclei. -
Ernest Rutherford
The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. He conducted the Gold Foil Experiment. -
Planetary Model
It proposed that the atom is structured like the Saturnian ring system or the solar system. The planetary model suggested that in the atom, small, electrically negative electrons orbit a relatively massive, positively charged nucleus. -
Robert Millikan
Robert Millikan conducted an ingenious experiment that allowed for the specific value of the negative charge of the electron to be calculated. In his famous oil drop experiment, Millikan and co-workers sprayed tiny oil droplets from an atomizer into a sealed chamber. -
Oild Drop Experiment
Robert Millikan and his co-workers sprayed tiny oil droplets from an atomizer into a sealed chamber. The oil drops fell downward, under the influence of gravity, into a space between two electrical plates. There they became charged, by interacting with air that had been ionized by X-rays. -
Niels Bohr
Niels Bohr received the Nobel Prize in physics in 1922 for the quantum mechanical model of the atom that he had developed a decade earlier, the most significant step forward in scientific understanding of atomic structure since English physicist John Dalton first proposed the modern atomic theory in 1803. -
Henry Moseley
Henry Moseley was an English physicist who experimentally demonstrated that the major properties of an element are determined by the atomic number, not by the atomic weight. -
Erwin Schrodinger
Erwin Schrodinger made the quantum mechanical model of the atom. -
Quantum Mechanical Model
The Quantum Mechanical Model was made by Erwin Schrodinger. The Quantum Mechanical model treats electrons as waves. -
James Chadwick
Chadwick showed, by using a cloud chamber filled with nitrogen, that the radiation caused the nitrogen atoms to recoil with such energy as could be imparted only by collisions with uncharged particles having approximately the mass of protons. Chadwick had proven the existence of the neutron and received the Nobel Prize in physics in 1935. -
Werner Heisenburg
Wener Heisenberg embarked on a research program to create a quantum field theory, uniting quantum mechanics with relativity theory to comprehend the interaction of particles and fields.