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History of the Atomic Theory Democritus and Aristotle • Ancient Greek philosophers that pronounced that the atom was the smallest unit of an element and it was indivisible Dalton John Dalton developed his atomic theory from observations gathered from many experiments. http://www.britannica.com/eb/art/print?id=15469&articleTypeId=0 Dalton’s Atomic Theory •All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Dalton’s Atomic Theory • Atoms of the same element are exactly alike, and atoms of different elements are different Dalton’s Atomic Theory • Atoms join with other atoms to make new substances. Dalton’s Atomic Image J.J. Thomson o In 1897, J.J. Thomson made a discovery that identified an error in Dalton’s theory. o There are small particles inside the atom. Thomson’s Results o Thomson discovered atoms can be divided into even smaller parts. o What he discovered was the “electron.” Thomson called it a “corpuscle” back then. J.J. Thomson His atomic theory said that the atom was a central sphere of positive charge and was off set by negative charges attached to the outside of the atom. Basically a “plum pudding” structure Revision of the Atomic Theory Because Thomson knew atoms have no overall charge, he realized that positive charges must be present to balance the negative charges of the electron. Rutherford (1909) Ernest Rutherford, a former student of Thomson, designed an experiment to investigate the structure of the atom. http://www.pbs.org/wgbh/aso/databank/entries/bpruth.html Rutherford’s Model Rutherford Theorized that the atom was made of a dense nucleus with a positive charge and negative charged particles circling the nucleus in a “planetary type orbit.” Rutherford’s Atomic Image Bohr (1913) Neils Bohr was a Danish scientist who worked with Rutherford. He suggested electrons travel around the nucleus in definite paths. http://www.pbs.org/wgbh/aso/databank/entries/bpbohr.html These paths are located in energy levels at certain distances away from the nucleus. Bohr’s Atomic Model electron neutron proton Chadwick (1932) James Chadwick, an English scientist, discovered the “NEUTRON.” He won the Nobel Prize for Physics in 1935, and his research prepared the way for the development of the atomic bomb. http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html Chadwick’s Atomic Image Electrons do not travel in definite paths around the nucleus like Bohr thought. This discovery was made by scientist Erwin Schrodinger; it is called an Electron Cloud Erwin Schrodinger Still more changes… Many 20th century scientists have contributed to our current understanding of the atom. http://particleadventure.org/frameless/modern_atom.html In the modern atomic theory… The exact path of a moving electron cannot be predicted, but there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds. https://www.youtube.com/watch?v=07yDiELe83Y Atom Smasher in Switzerland • A 17 mile diameter atom smasher was built in Switzerland and many scientist were worried that when it went into operation there might be particles found that possibly might destroy mankind. Picture of atom Smasher ATOM An atom is the smallest particle into which an element can be divided and still be the same substance. Atoms • Atoms are basic building blocks of matter that make up everyday objects. Atoms • Atoms have three parts. • They are Protons, Neutrons, and Electrons. Electrons • Electrons orbit around an atom. In this picture, they are the small yellow bits. • They have a negative charge. • They are lighter than protons or neutrons. • The number of protons=number of electrons in a neutral atom Protons • They are positively charged. • Are located in the nucleus. • Atomic Number=Number of protons • In this picture, the protons are the blue pieces in the center of the atom. Neutrons • Neutrons are neither positive nor negative. • Neutrons are in the nucleus of an atom. • The atomic mass – number of protons = number of neutrons • In this picture, neutrons are the purple pieces in the center of the atom. Atomic Number •The atomic number is the number of protons or electrons in a neutral atom 8 is the atomic number of Oxygen =the number of protons 11 is the atomic number of Sodium= the number of protons Atomic Mass •Atomic mass is the number of protons and neutrons in a neutral atom. Number of protons and neutrons is the atomic mass number Number of protons and neutrons is the atomic mass number Chemical Symbol •The chemical symbol is either a capitalized single letter or a capitalized letter followed by a lower case letter. Capitalized Symbol if only one letter Capitalized first letter and lower case second letter if two letters Element Name •The name of the element is usually written below the chemical symbol.