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Democritus

Democritus, a Greek philosopher, co-originated the thought (with his teacher, Leucippus) that all matter is composed of indivisible elements.
• Isaac Newton

1642-1727
Newton's Laws of Motion:
1. An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
2. Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).
3. For every action there is an equal and opposite re-action.
• Rudjer Josip Bosocovich

1711-1787
Boscovich’s atomic theory has three parts, he based them off of Newton and Gottfried Wilhelm Leibniz.
a. Material permanence without spatial extension: Quasi-material point-centers of action are substituted for the rigid finite units of matter of earlier atomists.
b. Spatial relations without absolute space: Internal spatial coordinates (the distances between the two members of pairs of puncta) are used instead of external coordinates.
c. Kinematic action without Newtonian forces: In m
• Joseph Priestly

1733-1804
Best known for his discovery of oxygen, he was able to isolate it and identify it. He produced it on August 1, 1774, by heating red mercuric oxide, and then it was clear that air was not an element.
• John Dalton

1766-1844
His Atomic Theory:
1. All matter is made of atoms. Atoms are indivisible and indestructible.
2. All atoms of a given element are identical in mass and properties
3. Compounds are formed by a combination of two or more different kinds of atoms.
4. A chemical reaction is a rearrangement of atoms.
• Berzelius

1779-1848
In dealing with so many elements in so many compounds he created a simple and logical system of symbols; H, O, C, Ca, Cl, and so forth which is basically the same system we use today. He also introduced the present system of chemical formulation.
• Dimitri Mendeleev

1834-1907
His Periodic Table was formed on the basis of arranging the elements in ascending order of atomic weight, grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error.
• Wilhelm C Roentgen

1845-1923
He theorized that when the cathode rays struck the glass wall of the tube, some unknown radiation was formed that traveled across the room, struck the chemical, and caused the fluorescence. He investigated it further and revealed that paper, wood, and aluminum are transparent to this new form of radiation, called an x-ray.
• Henry Becquerel

1852-1908
Becquerel determined that the phosphorescent substance emits radiation which penetrates paper. This meant that the uranium emitted radiation without an external source of energy such as the sun. He was awarded the 1903 Nobel Prize for his discoveries.
• Marie Curie

1867-1934
Curie studied uranium radiations. She found that the radiations from the ore were more intense than those from uranium she realized that unknown elements were even more radioactive than uranium were present. Marie Curie was the first to use the term 'radioactive' to describe elements that give off radiations as their nuclei break down. Marie Curie was the first female recipient of the Nobel Prize.
• Ernest Rutherford

1871-1937
He discovered alpha and beta rays, which set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. Most importantly he discovered the nuclear structure of the atom by observing experiments done in Rutherford's laboratory which showed that when alpha particles are fired into gas atoms, a few are violently deflected. He also discovered half-life of radioactive elements.
• Robert Millikan

1868-1953
In 1909 Millikan began a series of experiments to determine the electric charge carried by a single electron. He began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is an electric charge, but the experiment was not accurate enough to be convincing. He obtained more precise results in 1910 with his famous oil-drop experiment in which he replaced water with oil.
• James Chedwick

1891-1974
He made a fundamental discovery in the domain of nuclear science where he proved the existence of neutrons. Chadwick's discovery was advanced experimental work for all scientists, and in his experiments since neutrons have no electrical charge, any neutrons fired from a source has the ability to go through deep layers of materials.
• Niels Bohr

1865-1962
Bohr published a theory about the structure of the atom based on an earlier theory of Rutherford's. Bohr expanded upon this theory by proposing that electrons travel only in certain larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom's chemical properties. For his research he received the Nobel Prize in physics in 1922.
• Werner Heisenberg

1901-1976
He found out that you could measure the position of an electron or you could measure its momentum, but the more precisely you measure one property, the more you throw the other off. He used this information to make an equation using Planck's constant, and called it the uncertainty principle.
• Lise Meitner

1878-1968
Was the first to articulate a theory of how the nucleus of an atom could be split by the ejection of neutrons with high amounts of energy and established why no elements beyond uranium existed naturally, as the expulsion by protons would be too strong.