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Democritus
He hypothesized that atoms are indivisible and indestrcutible, solid but invisible, homogeneous, differ in size, shape, mass, position, and arrangement. He stated that all matter consists of atoms. -
John Dalton
By using experimental methods, he made an atomic theory that stated: all elements are made of identical atoms, compounds have a fixed ratio of atoms, and atoms are rearranged in a chemical reactions. -
Sir J.J. Thomson
By experimenting with cathode rays, he discovered that atoms could be broken down to even smaller particles. He called these small particles "corpuscles", now called electrons. He then set up another experiment to test the ratio of an electron's charge to its mass. He discovered that the ratio was constant and that it was not affected by the gas in the cathode tube. He concluded that electrons made up atoms of all elements. His atomic theory produced the "plum pudding model" of the atom. -
Robert Andrews Millikan
He used the oil-drop experiment to discover the quantity of the charge of an electron as well as its mass. He discovered the mass of an electron to be 1/1840 and to have one unit of negative charge. -
Ernest Rutherford
By performing the gold foil experiment, which aimed alpha particles at a piece of gold foil, he discovered that the atom was mostly open space with a very small postively charged core at the center, which he deemed the nucleus. He created the Rutherford Atomic model, which is still used today, which is also known as the nuclear atom. In the nuclear atom, the protons and neutrons are in the nucleus with electrons evenly distributed around the nucleus and occupy most of the atom's volume. -
Niels Bohr
He developed the quantized orbit model of the atom. He stated that electromagnetic radiation only occurred in an atom when an electron jumped to a lower-energy orbit. He stated that electrons could only reside in certain energy levels. Even though it was radical and unacceptable to most physicists, the Bohr atomic model backed up the large number of experimental data. He also won the Nobel Prize in 1922. -
Henry Moseley
He discovered, using X-rays and meauring their frequencies, that an element's atomic number is identical to how many protons it has. He stated that an element is determined by the number of protons it has; he stated that each element was unique due to every element having a different number of protons and every atom of the same element had the same number of protons. -
Erwin Schrödinger
He developed the quantum mechanical model of the atom by creating a mathmatical model for the distribution of electrons in an atom. His model expanded the idea that electrons behaved as particles as well as waves. His model is made up of wave functions, with three variables, because unlike Bohr's one-dimensional model, it was three-dimensional. The variables are principal (n), angular (l), and magnetic (m) quantum numbers to describe the size, shape, and orientation in space in the atom. -
James Chadwick
He discovered the neutron in 1932, which led to him winning the Nobel Prize in 1935, by bombarding Beryllium atoms with alpha particles.
