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Jan 4, 1200
Aristotle
Aristotle believed that there was no limit to the amount of times matter could be divided. His theory was actually produced in the B.C times. Aristotles theory was accepted for many centuries. Until the 1800's when scientists had enough data from experiements to support an atomic model of matter. -
Jan 1, 1417
Philosepher Democritus
Philosepher Democritus believed that all matter consists of extremely small particles that couldn't be divided.He called the partices atoms. He thought that liquid atoms were round and smooth, and solid atoms were rough and prickly. His work was done in the B.C. times. Democritus performed no experiments to come up with his theory. -
John Dalton
Some time in 1803 John Dalton made his theory public. He believed that atoms were tiny solid spheres. He discovered that no matter how big or small the sample, the ratio of the masses of elements in the compound is always the same. He concluded that a gas consists of individual particals. He gathered evidence of atoms exhistance by measuring the masses of elements that combine when compounds form. -
J.J Thomson
J.J Thompson believed that objects with opposite charges attract. A flow of charged particles is called an electric current. His experiment consisted of a sealed tube of gas with either a positive or negative charge on the ends. When he added a positive or negative charge on top the beam had bent toward the positively charged plate and glowed. He had concluded that the gas in the tube had a negatie chargebecause it bent toward the positive plate. -
Hantaro Nagaoka
Nagaoka believed that an atom had a central nucleus. The electrons move in orbits like the rings surrounding Saturn. His model was in direct analogy to a propasal by James Clerk Maxwell in his thoughts of Saturn's rings. He deducted from the frequencies a qualitive picture of the regularity in spectral lines showing that successive lines would be more closely packed together at the end of the spectrum. He then acknowledged that his model did not match the experimental data. -
Robert Milikan
Millikan measured the charge on an electron with his oil-drop apparatus. An "atomizer" from a perfume bottle sprayed
oil or water droplets into the sample chamber. Some of the drops fell through a pin hole into to plates(one positive,one negative), This middle chamber was ionized by x-rays. Particles that did not capture any electrons fell to the bottom plate due to gravity. Particles that did capture one
or more electrons were attracted to the
positive upper plate and floated up or down slow. -
Ernest Rutherford
In Rutherford's experiment he hypothesised that the mass and charge at any location in gold foil would be to small to change the path of an alpha partical. He had Marsden aim a narrow beam of particles at a piece of gold. After Rutherfords gold foil experiment he concluded that the positive charge isn't distributed evenly throughout the atom. It's concentrated in a very small, central area that Rutherford called the nucleus. -
Niels Bohr
Bohr had assigned something called energy levels to electrons. These are the possible energies that electrons in an atom could have. An electron cannot exhist in between energy levels. The electron changes power levels when the atom loses or gains energy. He came off from the idea of going up/down stairs. When going down stairs you can measure your position changes. Whether going up or down you can only move in whole step incriments. You can't stand in between stairs. -
Erwin Schrodinger
Schrodinger developed mathematical equations to describe the motion of electrons in the atoms. His work created the electron cloud model. -
Ernest Marsden
Marsden worked with Rutherford in his gold foil experiment as a student. Rutherford and Marsden had thought up a group of atoms called alpha atoms. In the experiment Marsden aimed a beam of the alpha particles at a sheet of gold foil. They predicted the particles would all go straight through the gold. Very few would deflect. However, more deflected than they expected. They concluded that the positive charge wasn't spread evenly throughout the atom. Most was in a central area called the nucleus -
James Chadwick
James Chadwick discovered the neutron. He figured out that neutrons have no charge. While doing this he discovered that atomic nuclei contain neutrons and positively charged protons. -
Louis De Broglie
Broglie proposed that moving particles like electrons have some properties of waves. With in a few years evidence was collected confirming his idea. He helped scientists understand that the atom didn't behave like a solar system. -
Werner Heisenberg
Heisenberg stated that the act of observation interfered with the location and velocity of small particles such as electrons. This is the case because observation requires light and light has momentum. When light bounces off an electron momentum exchange can occur between light and the electron which means the electrons location and velocity have been altered by the act of measurement. This idea has important implications to what we can measure at the atomic level.
