Atomic Timeline

A History of the Atom

Democritus460 BC

• Matter is composed of indivisible units• These particles he described by using the Greek word

"atomos”• Democritus observed the massive sea coast of the

Ageian Sea. He observed that the massive beaches were a result of countless numbers of grains of sand. Each grain was tiny and insignificant by itself, but taken together, they formed the massive beaches. Therefore, Democritus extrapolated that matter was composed of these tiny particles that formed the mass of the substances.

Indivisible Solid Sphere Model

Aristotle384 BC – 322 BC

• He claimed that there was no smallest part of matter and that different substances were made up of proportions of fire, air, earth, and water

• Aristotle along with other Greek Philosophers questioned whether matter was composed of divisble or indivisible particles.

• To Aristotle, all material was made of atoms with the exception of the Prime Mover(God)

Antoine Lavoisier1778

• Now known as the Father of Modern Chemistry. • He proved the Law of Conservation of Mass,

showing that the mass of the reactants had to equal the mass of the products.– Burned tin in a sealed container– Mass before equaled mass after– Matter cannot be created or destroyed, only changed

from one form to another

Charles Coulomb1785

• Explained the laws of attraction and repulsion between electric charges and magnetic poles

• Perhaps most famous for the law of physics bearing his name. Coulomb's law describes the relationship between force, charge and distance.

• He thought that the attraction and repulsion were due to different kinds of fluids

Coulomb’s Law

John Dalton1803

• John Dalton developed the first useful atomic theory of matter

• All matter is made of atoms. Atoms are indivisible and indestructible.

• All atoms of a given element are identical in mass and properties

• Compounds are formed by a combination of two or more different kinds of atoms

William Crookes1879

• In his investigations of the conduction of electricity in low pressure gases, he discovered that as the pressure was lowered, the negative electrode (cathode) appeared to emit rays

• The so-called cathode rays, now known to be a stream of free electrons

• Was a pioneer in the construction and use of vacuum tubes for the study of physical phenomena

Crookes Tubes

Wilhem Rontgen1895

• Röntgen's name is chiefly associated with his discovery of the rays that he called X-rays.

• He was studying the phenomena accompanying the passage of an electric current through a gas of extremely low pressure.

• Previous work in this field had already been carried out by J. Plucker J. W. Hittorf C. F. Varley E. Goldstein, Sir William Crookes H. Hertz

• Röntgen's work on cathode rays led him, however, to the discovery of a new and different kind of rays.

Henri Becquerel1896

• Discovered that uranium salts emitted rays that resembled X-rays in their penetrating power.

• He demonstrated that this radiation, unlike phosphorescence, did not depend on an external source of energy but seemed to arise spontaneously from uranium itself.

• Becquerel had in fact discovered radioactivity.

J.J. Thomson1897

• He achieved the most brilliant work of his life - an original study of cathode rays culminating in the discovery of the electron

• Thomson's discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in him being awarded a Nobel Prize in Physics in 1906

Cathode Ray Tube• The long glass finger (in the photo) is where the entire

tube was evacuated down to as good as a vacuum as could be produced, then sealed.

• The two plates about midway in the CRT were connected to a powerful electric battery thereby creating a strong electrical field through which the cathode rays passed.

• Thomson also could use magnets, which were placed on either side of the straight portion of the tube just to the right of the electrical plates.

• This allowed him to use either electrical or magnetic or a combination of both to cause the cathode ray to bend.

Plum Pudding Model

• From these ideas Thomson developed the idea that atoms are made of negative electrons embedded in a gel of positive charge (a "plum pudding" model).

Marie Curie1897

• Marie decided to look into uranium rays as a possible field of research for a thesis.

• Her first result was the finding that the activity of the uranium compounds depended only on the amount of uranium present.

• She had shown that the radiation was not the outcome of some interaction between molecules but must come from the atom itself.

• In scientific terms, this was the most important single piece of work carried out by her

Marie & Pierre Curie1898

• Pierre and Marie together published a paper announcing the existence of an element which they named "polonium," in honor of her native Poland,

• On December 26, 1898, the Curies announced the existence of a second element, which they named "radium" for its intense radioactivity — a word that they coined.

Marie & Pierre Curie

• In an unusual decision, Marie Curie intentionally refrained from patenting the radium-isolation process so that the scientific community could do research unhindered.

• Since they were unaware of the effects of radiation exposure on their chronic unprotected work with radioactive substances, Marie and Pierre had no idea what price they were paying for their research

Max Planck1900

• Quantum theory evolved as a new branch of theoretical physics during the first few decades of the 20th century in an endeavor to understand the fundamental properties of matter

• He showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts if it was to correctly describe observed phenomena

Planck’s Constant 6.626068 × 10-34 m2 kg / s

• The idea that energy could be emitted or absorbed only in discrete energy quanta seemed odd, since it could not be fitted into the traditional framework of physics

• Planck's idea only gained credence when Einstein used it in 1905 to explain the photoelectric effect. - After all, if matter is not infinitely divisible, why should energy be?

Robert Millikan1910

• An atomizer sprayed a fine mist of oil droplets into the upper chamber.

• Some of these tiny droplets fell through a hole in the upper floor.

• Millikan first let them fall until they reached terminal velocity due to air resistance.

• Using the microscope, he measured their terminal velocity, and by use of a formula calculated the mass of each oil drop.

Oil Drop Experiment

Oil Drop Experiment

• Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays.

• This caused the air to become ionized, which basically means that the air particles lost electrons.

• A part of the oil droplets captured one or more of those extra electrons and became negatively charged.

Ernest Rutherford1910

• Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons.

• This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.

• Rutherford came to this conclusion following the results of his famous gold foil experiment.

Gold Foil Experiment• This experiment involved the firing of radioactive

particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide

• Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.

Rutherford Atom

Niels Bohr1913

• Danish physicist Niels Bohr managed to explain the spectrum of atomic hydrogen by an extension of Rutherford's description of the atom.

• In that model, the negatively charged electrons revolve about the positively charged atomic nucleus because of the attractive electrostatic force according to Coulomb's law

Bohr Model aka Planetary Model

• In 1913, 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.

Bohr Atom

Lise Meitner1917

• Lise Meitner was part of the team that discovered nuclear fission an achievement for which her colleague Otto Hahn was awarded the Nobel Prize

• Meitner is often mentioned as one of the most glaring examples of scientific achievement overlooked by the Nobel committee

• Discovered the cause, known as the Auger effect, of the emission from surfaces of electrons with 'signature' energies

Erwin Schrodinger1926

• Schrödinger combined the equations for the behavior of waves to generate a mathematical model for the distribution of electrons in an atom.

• The advantage of this model is that it consists of mathematical equations known as wave functions that satisfy the requirements placed on the behavior of electrons.

• The disadvantage is that it is difficult to imagine a physical model of electrons as waves.

Wave mechanics model

• The Schrödinger model assumes that the electron is a wave and tries to describe the regions in space, or orbitals, where electrons are most likely to be found.

• Instead of trying to tell us where the electron is at any time, the Schrödinger model describes the probability that an electron can be found in a given region of space at a given time.

• This model no longer tells us where the electron is; it only tells us where it might be.

Problems with Bohr

• The Bohr model was a one-dimensional model that used one quantum number to describe the distribution of electrons in the atom.

• The only information that was important was the size of the orbit, which was described by the n quantum number.

• Schrödinger's model allowed the electron to occupy three-dimensional space. It therefore required three coordinates, or three quantum numbers, to describe the orbitals in which electrons can be found.

Wave Mechanic or Cloud Charge Model

James Chadwick1932

• Chadwick repeated the Curie’s experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge.

• His experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's.

• He published his findings with characteristic modesty in a first paper entitled "Possible Existence of Neutron." In 1935 he received the Nobel Prize for his discovery

Glenn T. Seaborg1942-1946

• Seaborg was the principal or co-discoverer of ten elements: plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendeleviuum, nobeliuum and element 106, which was named seaborgiuum in his honor while he was still living

• He also developed more than 100 atomic isotopes, and is credited with important contributions to the separation of the isotope of uranium used in the atomic bomb at Hiroshimma

bibliography• http://members.aol.com/profchm/democ.htm• limprobable.org/era/physics/atom.html• http://www.geocities.com/CapeCanaveral/7997/aristot.htm• http://atomictimeline.net/index.php• mattson.creighton.edu/History_Gas_Chemistry/Lavoisier.html• http://www.iun.edu/~cpanhd/C101webnotes/composition/dalton.html• http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Disc-of-Electron-Images.html• http://en.wikipedia.org/wiki/J._J._Thomson• www.rsc.org/chemsoc/timeline//pages/1911.html • http://abyss.uoregon.edu/~js/images/thomson.gif• http://images.google.com/imgres?imgurl=http://abyss.uoregon.edu• http://www.green-planet-solar-energy.com/images/bohr_atom.gif• http://hi.fi.tripod.com/timeline/images/indivisible_solid_sphere_model2.jpg

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