Concept of the Atom Leucippus and Democritus, circa 400 B.C.E., Greece Chem 101 Concept of the Atom Democritus Leucippus and Democritus, circa 400 B.C.E., Greece Quotes attributed to Democritus: I would rather discover one scientific fact than become King of Persia. Quoted in D MacHale, Comic Sections (Dublin 1993) Everything existing in the Universe is the fruit of chance and necessity. Nothing is so easy as to deceive one's self; for what we wish, that we readily believe. Nothing exists except atoms and empty space; everything else is opinion. Diogenes Laertius The Development of Atomic Theory

Experimental Evidence for the Existence of Atoms Antoine Lavoisier (1743-1794), French Law of Conservation of Matter: Matter (mass) is neither gained nor lost in a chemical reaction. Like most scientists, he came from a wealthy family—science was not a real profession yet. Wrote the first chemistry textbook Is often called the “Father of Chemistry” Was executed by guillotine during the French Revolution Lavoisier Law of conservation of Matter: Mass of reactants = Mass of products (no mass is lost from a reaction)

Experimental Evidence for the Existence of Atoms Joseph Louis Proust (1754-1826), French Law of Constant Composition In a pure compound, the elements combine in definite proportions to each other. Also known as the Law of Definite Proportions all samples of a given chemical compound have the same elemental composition. Example for both: Pure water is always 8/9 oxygen and 1/9 hydrogen Proust

Experimental Evidence for the Existence of Atoms Berzelius Jöns Jacob Berzelius (1779 – 1848), Swedish Also arrived at the Law of Constant Composition Parents died before he reached 12 years old He supported himself in school with tutoring Became a physician and later a professor of chemistry Gave elements abbreviations (called chemical symbols) that we still use today! He also wrote chemical formulas. Berzelius discovered the elements cerium, selenium, and thorium. He isolated silicon, zirconium, and titanium Like Lavoisier and Dalton, also known as the Father of Chemistry!

Experimental Evidence for the Existence of Atoms Dalton’s table of elements and their combinations John Dalton (1766-1844 ), British Put the pieces together to form the first atomic theory! If elements combined in whole-number ratios, there must be atoms! (matter is made of discrete particles!) Was the first to create a table of atomic weights He was a quaker school teacher at age 12 He was color-blind; never married More than 40 thousand people marched in his funeral procession. Dalton

Dalton’s Atomic Theory 1. Elements are composed of atoms solid, tiny, hard, unbreakable, spheres   2. All atoms of a given element are identical all carbon atoms have the same chemical and physical properties   3. Atoms of a given element are different from those of any other element carbon atoms have different chemical and physical properties than sulfur atoms  

Dalton’s Atomic Theory   4. Atoms of one element combine with atoms of other elements to form compounds (in simple ratios). Law of Constant Composition all samples of a compound contain the same proportions (by mass) of the elements  5. Atoms are indivisible in a chemical process. Reactions only cause a rearrangement of atoms. Law of Conservation of Mass: all atoms present at beginning are present at the end atoms are not created or destroyed, just rearranged atoms of one element cannot change into atoms of another element cannot turn Lead into Gold by a chemical reaction

Discovery of the electron Joseph John Thomson (1856-1940), British Used the cathode ray tube (modified by Crookes, 1850s) to do experiments that led to the idea of electrons. Electrons are negatively charged particles that exist in an atom Thomson won the Nobel Prize in Physics, 1906 for evidence of electrons as particles. Ironically, his son won the Nobel in 1937 for evidence of electrons as waves (it has been accepted since then that electrons have both wave and particle properties). “Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" -- J.J. Thomson.

Plum Pudding Model of the Atom Atoms (matter) is electrically neutral. If electrons are negatively charged, there must be positive charges to balance the electrons (later, protons were discovered by Goldstein). Lord Kelvin (William Thomson, no relation to JJ) JJ Thomson

Discovery of the nucleus Ernest Rutherford (1871-1937), British His famous gold foil experiment proved the existence of the nucleus in atoms and that The nucleus was composed of positive charge (protons). Atoms are mostly empty space Born and raised on a farm in New Zealand Won scholarship to study at Cambridge Became a student of JJ Thomson Performed extensive research in radioactivity Nobel Prize in 1908 in Chemistry Was a great mentor—several of his students also won Nobel Prizes “All science is either physics or stamp collecting” -Ernest Rutherford

Gold Foil Experiment Rutherford and JJ Thomson (horsing around?)

Rutherford’s Atom Atoms are not hard spheres! Due to the gold foil experiment: 1. Atoms are mostly empty space (the alpha particles went straight through the gold foil most of the time) Atoms contain a dense nucleus composed of positive charge (later, neutrons were discovered and believed to stabilize the protons in the nucleus)