Timeline of Atomic Structure. Marissa Kopelman. Democritus (460 B.C.-370 B.C.). Democritus, a Greek philosopher, believed that all matter was made up of indivisible and indestructible particles called atoms.
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All elements are made of indivisible particles called atoms.
All atoms of the same element have identical properties and mass.
Compounds are formed by the combination of atoms of different elements.
Chemical reactions are the separation, combination, or rearrangement of atoms.
Dalton’s atomic symbols1803John Dalton (continued)
At the time of this discovery, Thomson knew that the same charges repelled each other and the opposite charges attracted.
Thomson’s results allowed him to develop the first model of the atom with Lord Kelvin in 1904. It was called the “plum pudding” model because the negative electrons represented the plums embedded in the uniform positive charge, representing the pudding.1897 J.J. Thomson (continued)
Millikan used an oil drop apparatus to measure the electron’s charge. Oil droplets were sprayed into a chamber by a perfume atomizer. Some droplets fell through a pinhole into a middle chamber, ionized by x-rays, with a positive plate on the top and a negative plate on the bottom. Particles that captured electrons floated upward or fell slowly because they were attracted to the positive plate.
Millikan watched how fast the rise and fall of the drops were and calculated the charge of the drop. He used Thomson’s charge-to-mass ratio of an electron and his own results to calculate the mass of an electron.1910 Robert A. Millikan
Millikan’s diagram from his Physical Review paper
the structure of the atom.
model which shows that there is a higher
probability of finding an electron in the
cloud’s denser regions.
began the bombardment of neutrons
with many different materials.
Prentice Hall Chemistry Text Book