Atomic theory is the idea that matter is made up of small particles made of atoms. The atomic theory first had 5 points which were stated by John Dalton who came up with it. The first one was that all elements were made up of small units called atoms. The second one was that all atoms of a given element are identical to each other. Third, all the atoms were different from all the other atoms. Fourth, atoms of different elements combined together in equal numbers to form compounds. Fifth and final, atoms cannot be broken down, created or divided.
John Dalton was a chemist in the early 19th century and his work was focused on the behavior and properties of gases. He used his work on gases to come up with his law of proportions which was true to the smallest particle or atom. He then found out that he could use that to find the atomic weight of each element. When John Dalton created his atomic theory, it was based on simple deductive reasoning and was later; testing by his successors proved that the majority of his theory was correct.
From the atomic theory created by John Dalton, Neil Bohr a physicist was able to create his own theory, the Bohr Atomic theory that was about an atoms structure and more on its atomic weight. John Dalton’s atomic theory was not perfect, it had its flaws, but it went above and beyond during its time. Some of the problems with Dalton’s theory are that his second law that all atoms of the same element are the same is false because today we have found that there are other kinds of atoms called isotopes. An isotope is an atom that has the same amount of electrons and protons, but it has more neutrons that increase its atomic mass.
Some examples of isotopes are Carbon 12, carbon 13, carbon 14, Uranium (234, 235, and 238) and lithium (6, 7). Carbon 14 is especially useful because we know its rate of decay and can use to find how old an object is due to the fact that everything absorbs carbon 14 until they die and then they start to decay. Using carbon 14, you find how old something is. Another problem with Dalton’s theory was discovered in rule 4, but in the second part that states that all atoms join together in equal numbers. This is obviously contradicted because we have a compound such as H2O which is ot in equal numbers. If his rule was completely true, it would have to be HO which is a hydrogen ion. The last part of his theory, rule number 5 which talks about that atoms cannot be created, destroyed, or divided is partly false because of nuclear fission, we know that it is possible to split and atom and as a result, you release a lot of energy. Nuclear fission is the process of shooting a neutral particle at an unstable atom like uranium and having it break apart sending more particles to other atoms of uranium exponentially increasing in power.
Even thought Dalton’s theory had its problems, his ideas where made from basic reasoning and since it was discovered in the 1800’s, this discovered was ahead of its time. With the development of the microscope and future experiments made, we soon learned that atoms are made of 3 sub-particles called electrons, protons, and neutrons. We then went on to scientifically create a field around the atom where we can predict the path of electrons and how they can interact with other atoms to form certain bonds; bonds such as ionic bonds and covalent bonds which create most of the compounds that we interact with in our daily life. TimeLine:
Year Scientist(s) Discovery Greek eraDemocritus”by convention bitter, by convention sweet, but in reality atoms and void” 1704Isaac NewtonProposed a mechanical universe with small solid masses in motion. 1803John DaltonProposed an “atomic theory” with spherical solid atoms based upon measurable properties of mass. 1832Michael 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. 1859 J. Plucker Built one of the first gas discharge tubes (“cathode ray tube”). 869Dmitri MendeleevArranged elements into 7 groups with similar properties. He discovered that the properties of elements”were periodic functions of their atomic weights”. This became known as the Periodic Law. 1873James Clerk MaxwellProposed electric and magnetic fields filled the void. 1879 Sir William CrookesDiscovered 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. 1886E.
Goldstein Used a CRT to study “canal rays” which had electrical and magnetic properties opposite of an electron. 1894 G. J. StoneyProposed that electricity was made of discrete negative particles he called electrons “. (Link to info on electrons) 1895 Wilhelm RoentgenUsing 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”. 1896 Henri BecquerelWhile studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very penetrating rays. 897 J. J. ThomsonUsed a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1. 759 x 10 8 coulombs/gram. 1897 J. J. ThomsonStudied “canal rays” and found they were associated with the proton H +. 1898RutherfordStudied radiations emitted from uranium and thorium and named them alpha and beta. 1898Marie Sklodowska CurieStudied uranium and thorium and called their spontaneous decay process “radioactivity”. She and her husband Pierre also discovered the radioactive elements polonium and radium. 900SoddyObserved 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. 1900Max PlanckUsed the idea of quanta (discrete units of energy) to explain hot glowing matter. 1903Nagaoka Postulated a “Saturnian” model of the atom with flat rings of electrons revolving around a positively charged particle. 1904AbeggDiscovered that inert gases had stable electron configurations which lead to their chemical inactivity. 905Albert EinsteinPublished the famous equation E=mc 2 1906Hans GeigerDeveloped an electrical device to “click” when hit with alpha particles. 1909 R. A. MillikanOil 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. 1911 Ernest RutherfordUsing 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. 1914 H.
G. J. MoseleyUsing 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. 1919 AstonDiscovered the existence of isotopes through the use of a mass spectrograph. 1922Niels BohrDeveloped an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. 923 de BroglieDiscovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein. 1927HeisenbergDescribed atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminacy – you cannot know both the position and velocity of a particle. 1929Cockcroft / WaltonBuilt an early linear accelerator and bombarded lithium with protons to produce alpha particles 1930 Schrodinger Viewed electrons as continuous clouds and introduced “wave mechanics” as a mathematical model of the atom. 930Paul DiracProposed anti-particles. Anderson discovered the anti-electron (positron) in 1932 and Segre/Chamberlain detected the anti-proton in 1955… 1932 James ChadwickUsing alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. 1938Lise Meitner, Hahn , StrassmanConducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction. 941 – 51Glenn SeaborgSynthesized 6 transuranium elements and suggested a change in the layout of the periodic table. 1942Enrico FermiConducted the first controlled chain reaction releasing energy from the atoms nucleus. 1950’s -New findings/particlesFollow this link to current theories about atomic structure. Work Cited Atomic Structure Timeline. Web. 23 Oct. 2011. . “John Dalton’s Atomic Theory. ” Universe Today — Space and Astronomy News. Web. 23 Oct. 2011. . “What Is Atomic Theory? ” WiseGEEK: Clear Answers for Common Questions. Web. 23 Oct. 2011. .