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Democritus
Democritus was an Ancient Greek philosopher born in Abdera, Thrace, Greece. A pupil of Leucippus, he was an influential pre-Socratic philosopher who formulated an atomic theory for the universe. He was the first person to propse the idea that matter was not infinitely divisible. He believed matter was made up of tiny individual particls call "atomos", from which the English word atom is derived. He believed that atoms coul not be created, destroyed, or further divided. -
Period: 300 to
Atomic Timeline Observations Timespan
The Atomic Theory has been created, observed, and altered through time. This timeline will help you know how it all came to be. -
Antoine Lavoisier
Antoine-Laurent de Lavoisier; 26 August 1743 – 8 May 1794; French pronunciation: was a French nobleman, chemist, central to the 18th-century Chemical Revolution and a large influence on both the histories of chemistry and biology. He is widely considered to be the "Father of Modern Chemistry." Lavoisier is most noted for his discovery of the role oxygen plays in combustion. He also created the Law of Conservation of Mass. -
Law of Conservation of Mass
Created and discovered by French scientist, Antoine Lavoiser, which states that mass is neither created nor destroyed during a chemical reaction-- it is conserved. The law of conservation of mass is one of the most fundamental concepts of chemistry. -
John Dalton
The work done in the 19th century by John Dalton (1766-1844), a schoolteacher in England, marks the beginning of the development of modern atomic theory. Dalton revived and revised Democritus's ideas based upon the results of scientific research he conducted. He created "Dalton's Atomic Theory." -
Dalton's Atomic Theory
All matter is composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element. Atoms cannot be created, divided into smaller particles, or destroyed. Different atoms combine in simple whole-number ratios to form compounds. In a chemical reaction, atoms are sparated, combine, or rearranged. -
Dmitri Mendeleev
Dmitri Mendeleev studied science at St. Petersburg and graduated in 1856. Mendeleev was appointed to a professorship and in succeeded to the Chair in the University. The Russian chemists greatest accomplishment was the stating of the Periodic Law and the development of the Periodic Table. -
Cathode Ray Tube
JJ Thomson did experiments with the cathode ray tubes, and it showed that when an electric current passed through a gas, the ray would bend away from any negative fields. He used different metals and gases and conluded that the stream of particles must be hegatively charged. This helped the discovery of the electron. -
JJ Thomson
JJ Thomson (1856-1940), an English physicist, was the one that discovered the electron through the cathode ray tube experiment. He determined the mass to charge ratio of the particles, and made up the Plum Pudding model. -
Plum Pudding Atomic Model
JJ Tomson's plum pudding atomic model proposed that negatively charged electrons were distributed througout a uniform postitive charge. -
Robert Millikan
Robert Millikan (1856-1953), an American physicist, determine the charge of an electron. He also discovered the mass of an electron as well --> 9.1 x 10 ^ -28 g or 1/1840 mass of a hydrogen atom. -
Rutherford Model
Rutherford's nuclear model of the atom explains the results of the gold foil experiment. Most of an atom consists of electrons moving rapidly through empty space. The electrons move through available spces surounding the nucleus and are held within the atom by their attraction to the positively charged nucleus. -
Henry Moseley
(1887-1915): A British chemist, Henry Moseley studied under Rutherford and brilliantly developed the application of X-ray spectra to study atomic structure; Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers. Tragically for the development of science, Moseley was killed in action at Gallipoli in 1915. -
Niels Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths. -
Bohr Planetary
Bohr published a theory about the structure of the atom based on an earlier theory of Rutherford's. Rutherford had shown that the atom consisted of a positively charged nucleus, with negatively charged electrons in orbit around it. Bohr expanded upon this theory by proposing that electrons travel only in certain successively larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom's chemical properties. -
Gold Foil Experiment
In the gold foil experiment, Ernest Rutherford bombarded a thin gold foil with alpha particles. Almost all alpha particles passed through the foil, but some were deflected, a very small number of it were reflected backwards. He concluded that most of the atom was empty space, a postive charge was concentrated in a tiny volume called the nucleus, electrons revolved around the nucleus, and the nucleus contained most of the mass. -
Ernest Rutherford
Ernest Rutherford (1871-1937) discovered protons, positively charged particles, using the gold foil experiment. He determined that: The atom was made up of mostly empty space through which the electrons moved. There was a tiny, dense region called the nucleus that holds all the positively charged particles and most of its mass. -
Quantum Mechanical Model
Quantum-mechanical theory was developed by several unusually gifted scientists including Albert Einstein, Neils Bohr, Louis de Broglie, Max Planck, Werner Heisenberg, P. A. M. Dirac, and Erwin Schrödinger. These scientists did not necessarily feel comfortable with their own theory. The model is based off of an atom with a tiny nucleus with probability waves instead of sharp orbits to describe the distribution of electrons, which have fuzzy positions but definite energy levels. -
Erwin Schrodinger
In Niels Bohr's theory of the atom, the electrons absorb and emit radiation of fixed wavelengths when jumping between the fixed orbits around a nucleus. The theory gave a good description of the spectrum from the hydrogen atom, but must be further developed for more complicated atoms and molecules. Assuming that matter, e.g. electrons, could be regarded both as particles and as waves, Erwin Schrödinger formulated in 1926 a wave-equation that accurately gave the energy levels of atoms. -
James Chadwick
James Chadwick discovereds that the neutron has no charge, and itis found in the nucleus. James Chadwick could show that in the reaction, a neutral particle with a mass about that of a proton, was emitted. Ernest Rutherford had earlier proposed that such a neutral particle might exist in nuclei. The particle was discovered and was named neutron. -
Electron Cloud Model
Erwin Schrodinger, an Austrian physicist came up with the electron cloud model. The electron cloud model is an atom model wherein electrons are no longer depicted as particles moving around the nucleus in a fixed orbit. Instead, we shouldn’t know exactly where they are, and hence describe their probable location around the nucleus only as an arbitrary ‘cloud’.
