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400 BCE
Democritus
Figured that if a stone was cut in half a certain number of times, there would be a small, invisible piece of that matter:
- He called these invisible pieces "atomos" (greek for invisible)
- Believed that atoms belonged to the material they were composed into
- Believed atoms were of different size and shape, which were constantly in motion -
John Dalton
Proposed modern theory of atoms in 1803:
- All matter is made up of atoms (indestructible and invisible)
- Atoms of the same matter are identical
- Atoms of different matter have different weight and chemical compound
- Different atoms can combine to form compounds
- Atoms cannot be created or destroyed -
Dmitri Mendeleev
Known for creation of the first periodic table in 1869:
- Worked with periodic law
- Periodic Law states that when elements are arranged according to their atomic number, then elements with similar properties will appear -
Eugene Goldstein
Discovered that canal rays, or positive rays, are positively charged ions that accelerate toward and through a perforated cathode in a tube.
- Eugene was the first to observe the proton in 1886 using this method -
J.J. Thomson
Performed experiments to study shape of atoms and discovered electrons in 1897:
- Figured an atom had a positive charge with negative electrons
- Studied electric discharge in a high vacuum cathode-ray tube
- Won a Nobel Prize in 1906
- Found that electrons were 1,000 units smaller than atoms- Used the Plum Pudding Model of an atom where the negative electrons of an atom sat in a positively charged space
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Max Planck
In 1900, he proposed his quantum theory of energy:
- Work contributed to the work of atomic and subatomic processes
- Won a Nobel Prize in 1918
- Planck's Law describes that energy given off by atoms is not emitted or absorbed continuously, rather it is emitted or absorbed in the form of wave packets (pioneering theory of modern physics and quantum theory) -
Robert Millikan
Conducted the "oil drop experiment" in 1909:
- He determined the number of electrons on an atom by seeing how much electric charge it took to keep a drop of oil from falling
-Using the electric field in the tub with the gravity and mass, Robert determined the charge on oil droplets -
Ernest Rutherford
Ernest Rutherford discovered the nucleus of an atom in 1911
- He split the atom in an experiment between nitrogen and protons
- It was reported by Ernest that the nitrogen ejected a hydrogen atom
- Used the gold foil experiment to find nucleus of atom
- Saw through the experiment, that when alpha particles were shot towards gold foil, that some hit the back section, and some bounced back
- Figured the atom consisted of mainly empty space -
Niels Bohr
Niels found that atoms (firstly in hydrogen atoms), travel around in a specified orbit around the nucleus of an atom.
- Figured out that the number of electrons in the outer orbit determines the property of an element
- With Rutherford's description of the nucleus and Planck's theory, Bohr showed what happens inside an atom and developed a picture of atomic structure -
Erwin Schrodinger
In 1924, Schrodinger proposed a theory of wave mechanics for the electrons of an atom. These could either be viewed as an orbit or a wave pattern.
- Found wave pattern because of dissatisfaction with Bohr's findings -
Werner Heisenberg
Werner published his theory of quantum mechanics in 1925.
- The theory gave the atom's electrons fixed and quantized (fixed value) orbits around the nucleus, and was found through radiation emitted from the atom by what he observed
- "We cannot, always assign to an electron a position in space at a given time, nor follow it in its orbit." - Werner Heisenberg -
James Chadwick
James was the first to discover the neutron in an atom in 1932.
- This experiment consisted of Beryllium that, when bombarded with alpha particles, emitted an energetic stream of radiation. When a substance rich with protons was hit with a radiation beam, protons were knocked loose which the geiger counter picked up.
- Figured that most of the mass in an atom was in the nucleus
