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Democritus
Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” -
Issac Newton
Proposed a mechanical universe with small solid masses in motion. -
Antoine Lavoisier
it contained a list of elements, or substances that could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc, and sulfur. His list, however, also included light, and caloric, which he believed to be material substances. -
Law of Conservation of Mass
states that the mass of an isolated system (closed to all transfers of matter and energy) will remain constant over time. -
Dalton's Atomic Theory
that elements combine at the atomic level in fixed ratios. This ratio would naturally differ in compounds due to the unique atomic weights of the elements being combined. -
John Dalton
"Experimental Essays" on the constitution of mixed gases; on the pressure of steam and other vapours at different temperatures, both in a vacuum and in air; on evaporation; and on the thermal expansion of gases. These four essays were published in the Memoirs -
Michael Faraday
Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism. -
J. Plucker
Built one of the first gas discharge tubes ("cathode ray tube"). -
Dmitri Mendeleev
Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law. -
James Clerk Maxwell
Proposed electric and magnetic fields filled the void. -
Sir William Crookes
Discovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass. -
E. Goldstein
Used a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron. -
G.J. Stoney
Proposed that electricity was made of discrete negative particles he called electrons ". -
Wilhelm Roentgen
Using a CRT he observed that nearby chemicals glowed. Further experiments found very penetrating rays coming from the CRT that were not deflected by a magnetic field. He named them "X-rays" -
Henri Becquerel
While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays. -
Cathode Ray Tube
A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. -
J.J. Thomas
Thomson discovered this through his explorations on the properties of cathode rays. Thomson made his suggestion on 30 April 1897 following his discovery that Lenard rays could travel much further through air than expected for an atom-sized particle. -
Rutherford
Studied radiations emitted from uranium and thorium and named them alpha and beta. -
Marie Curie
Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium. -
Soddy
Observed spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay. -
Max Planck
used the idea of quanta (discrete units of energy) to explain hot glowing matter. -
Nagaoka
Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle. -
Plum Pudding Model
Imagine a plum pudding wherein the pudding itself is positively charged and the plums, dotting the dough, are the negatively charged electrons. -
Abegg
Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity. -
Albert Einstein
Published the famous equation E=mc 2 -
Hans Gieger
Developed an electrical device to "click" when hit with alpha particles. -
Gold Foil Experimant
This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). -
R.A. Millikan
Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron. -
Rutherford Model
A source which undergoes alpha decay is placed in a lead box with a small hole in it. Any of the alpha particles which hit the inside of the box are simply stopped by the box. Only those which pass through the opening are allowed to escape, and they follow a straight line to the gold foil. -
Earnest Rutherford
Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus. -
H.G.J. Mosely
Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass. -
Bohr Planetary Model
In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun -
Aston
Discovered the existence of isotopes through the use of a mass spectrograph. -
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, as a quantum mechanically-influenced model, we shouldn’t know exactly where they are, and hence describe their probable location around the nucleus only as an arbitrary -
Niels Bohr
Discovered the existence of isotopes through the use of a mass spectrograph. -
Quantum Mechanical Model
Electrons are defined as standing waves. The electron probability distribution gives the areas in which the probability of electron presence is high. The exact position of the electron is never known as stated by the Heisenberg uncertainty principle: -
De Broglie
Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein. -
Heisenberg
Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle. -
Cockcroft/Walton
Built an early linear accelerator and bombarded lithium with protons to produce alpha particles -
Erwin Schrodinger
Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom. -
James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. -
Aristotle
He was known for being a reliable philosopher. As a result, his ideas had a much greater impact on people. His ideas when it came to matter, however, were slightly different than his predecessor, Democritus. Aristotle believed all things were made of: matter and essence. Aristotle thought organisms of the same species were made of the same but different matter. Aristotle believed matter could be broken down into the four elements. His beliefs were accepted over democritus's.
