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Democritus • Ancient Greek philosopher and mathematician • Born 460 BC • Died 370 BC • Considered the father of atomic theory for his idea that the world was made up of small, indivisible parts called “atomos”. • “Atomos” is the Greek word for “uncuttable.” This is where we get the current word “atom.” • His ideas were not popular, as most thinkers of the time held that the world was made of four elements (earth, water, air, fire) and that combinations of these elements made up everything. • It was only much later in science that his ideas were confirmed. Democritus’ Atom: a featureless sphere with properties determined by size. John Dalton • British chemist, physicist, and meteorologist • Born 1766 AD • Died 1844 AD • Formulated the first modern version of atomic theory: 1. All matter is made of indivisible units called atoms. 2. All atoms of the same element are identical. 3. Compounds (molecules) are formed when two or more atoms bond together. 4. Chemical reactions are the rearrangement of atomic bonds. • Color-blindness was often called “Daltonism” due to his own color blindness and the research he did on it. Dalton’s Atom: like Democritus he envisioned atoms as small, featureless spheres. J. J. Thomson • • • • British physicist Born 1856 AD Died 1940 AD Credited with the discovery of the electron, invalidating Dalton’s first atomic law. • Using cathode ray tubes, he was able to determine that when a strong electric current was run through it, the metals would give off a small, highly charged particle. • He is also known for his discovery of isotopes, invalidating Dalton’s second atomic law. Thomson’s Atom: also known as the “plum pudding” model, Thomson believed that a mass of positive charge was embedded with small, negative electrons. Ernest Rutherford • • • • New Zealand physicist Born 1871 AD Died 1937 AD Conducted a key experiment to test the structure of the atom by firing alpha particles at a gold foil. 1. If Thomson’s model was correct, then the alpha particles should pass straight through. 2. Instead, Rutherford had some alpha particles bounce right back. 3. This led him to hypothesize that there was a small positive nucleus. Rutherford’s atom: a small, positive nucleus with electrons orbiting around it like planets around a sun. Niels Bohr • • • • Danish physicist Born 1885 AD Died 1962 AD One of the fathers of modern quantum mechanics • Considered both a scientist and philosopher • Was trying to solve two problems: 1. Why don’t negative electrons get pulled into a positive nucleus? 2. Why does metal emit the same wavelengths of light every time it is heated? • In order to solve these, he proposed his new version of the atom. Bohr’s Atom: electrons don’t exist in any orbit, but in specific orbitals around the atom. They cannot exist between these. Werner Heisenberg • • • • German physicist Born 1901 AD Died 1976 AD One of the fathers of modern quantum mechanics • Commonly known for his “uncertainty principle.” • “Heisenberg’s Uncertainty Principle” states that it is impossible to measure the location and velocity of a particle at the same time. • Because of this uncertainty, electrons were better viewed as “clouds of charge” or “probability” rather than solid balls. Heisenberg’s atom: electrons still exist in set levels like Bohr, but rather than having a specific location they are spread out.