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450
Democritus (460 BC)
Democritus was the first person to develop the idea of the atom. He knew that everything was made of matter, but he wondered what matter was made of. But, because he was not a scientist, he had no access to a lab to experiment. So, he simply asked himlself, "If I break a piece of matter in half, how many breaks until it is indivisible?" He called these indivisible particles atoms. Democritus belived they were tiny and entirely fill up the space they are in. He thought atoms looked like spheres. -
455
Aristotle (350 BC)
Aristotle completely rejected Democritus's idea of the atom. He never believed atoms could be constantly moving if they were in a void. Instead Aristotle said that everthing was made of the four elements- fire, earth, water, air. He believed this because of the things he observed in every day life. At this time in history, Aristotle's ideas were very popular and widely accepted so when he rejected the idea of the atom so did everyone else. -
John Dalton
John Dalton was the first person to develop the Atomic Theory. He discovered all matter is made of atoms which are tiny, indivisible and have mass. Dalton said all atoms of one element are identical but atoms of different elements were not. Also. he knew chemical reactions consist of reorganizing atoms. He knew this because he performed several experiments that lead him to the discovery of the atom. Dalton believed the atom looked like a sphere. -
JJ Thomson
Thomson was the man who discovered the electron. He also developed the plum-pudding model of the atom, which consisted of the negatively charged electrons floating around inside a spherical shaped positively charged substance. Thomson proved Dalton's theory was wrong using a cathode ray experiment in which he noticed the way the cathode rays reacted to positively charged plates similar to the way electrons react to protons. -
Ernest Rutherford
Rutherford discovered electrons were on the outside of the nucleus which disproved the plum-pudding model. Also, he discovered protons were inside the nucleus and the nucleus contained most of the atoms mass. Rutherford proposed the idea of the neutron and found that the electrons were extremely tiny. He is most famous for his gold foil experiment where he shined radium rays onto the atoms on the gold foil. What resulted was that some of the rays bounced off what he thought were electrons. -
Niels Bohr
Bohr came up with two rules- electrons can orbit only at certain distances from the nucleus and atoms radiate energy when an electron moves between orbits. He believed electrons can only exist in these orbits and there were an unlimited number of them. He discovered these rules through theories from observations. After making this discovery he created the Bohr model AKA the planetary model of the atom this is the model most commonly used when teaching the atom and its subatomic particles. -
Werner Heisenberg
Heisenberg discovered matrix mechanics and found the matrice which means electrons are particles not waves. He also developed the Heisnenberg Uncertainty principle. He believed no experiment can measure the postion, speed or momentum of an electron which is basically what the Heisenberg Principle states. So, he used observations.The electron cloud model was created by Heisenberg and is the result of his principle. It looks like a positively charged nucleus surrounded by a cloud of electrons. -
Erwin Schrodinger
Schrodinger actually believed the exact opposite of Heisenberg because he proposed that electrons are waves not particles. He believed the waves could be altered by outside forces and that there were stable and unstable waves. This theory was based off of energy levels in the atom and discovered by observations of the atoms behavior. His model of the atom depicts waves (either stable or unstable) surrounding a positvely charged nucleus and is known as the quantum theory model or wave model. -
Works Cited
"Democritus." The-History-of-the-Atom -. N.p., n.d. Web. 26 Feb. 2014.
"Atomic History Timeline." Timetoast Timelines. Timetoast, n.d. Web. 25 Feb. 2014.
"A Brief History of Modern Atomic Theory." Chemistry Tutorial : History of Atomic Theory. Aus-e-Tute, n.d. Web. 25 Feb. 2014.
Buescher, Lee. "Atomic Structure Timeline." Atomic Structure Timeline. Science Department, Watertown HIghschool, 2004. Web. 25 Feb. 2014. -
More Works Cited
"Chemistry Project : The History Of The Atom." Chemistry Project : The History Of The Atom. Tripod.com, n.d. Web. 24 Feb. 2014. Dewitt, Tyler. "Models of the Atom Timeline." YouTube. YouTube, 06 Dec. 2012. Web. 27 Feb. 2014. Heinricks, Mitchell. "The History of the Atomic Theory." Timetoast Timelines. Timetoast, n.d. Web. 25 Feb. 2014. Walker, Jim. "History of Atoms." History of Atoms. E-Prime, Nov.-Dec. 2004. Web. 23 Feb. 2014.
