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Atomic History Notes.notebook September 15, 2014 Signs of a Chemical Reaction Physical Change - altering the form of a substance without changing its chemical composition Chemical Change - a change that alters the chemical composition of a substance Sep 57:45 AM Chapter 3 - Elements, Atoms and Ions Elements - a pure chemical substance consisting of one type of atom; examples are Carbon, Hydrogen and Gold Element symbols for the above elements are C, H and Au - 1st letter is always capital, 2nd is lower case - symbols are often derived from Greek and Latin - Pb for lead comes from Latin word plumbum (liq. silver) - Fe for iron comes from Latin word ferrum (sword) Atoms - the smallest particle of elements that still retain the property of the element Sep 410:51 PM 1 Atomic History Notes.notebook September 15, 2014 Democritus - 460 - 370 BC Often credited with being the father of atomic theory. Proposed that matter was made up of tiny indivisible particles called atoms. Democritus' theory was largely ignored in his time and wasn't revived until the early 1800's by John Dalton. Sep 411:07 PM Dalton devised an atomic theory (early 1800's) based on the following points: 1) Elements are composed of extremely small and indivisible particles called atoms. 2) Atoms of the same element are identical. 3) Atoms combine chemically in simple whole number ratios, H2O is a 2:1 ratio of hydrogen and oxygen. - A chemical compound is a distinct substance made up of atoms or two or more elements (like water above) 4) Chemical reactions occur when atoms are separated, joined or rearranged. 5) Atoms of one element are never changed into atoms or other elements. (This is known as transmutation.) Sep 411:15 PM 2 Atomic History Notes.notebook September 15, 2014 Atomic Structure Electron - discovered by Thomson in 1897 - negatively charged ( -1 ) subatomic particle - almost 2000x smaller than H atom Proton - discovered by Goldstein in 1887 (positive particles) - Rutherford 1911 discovered nucleus and proton in 1917 - positively charge (+1) subatomic particle - 1840X more massive than electron Neutrons - discovered by Chadwick in 1932 - neutral charge - approximately same mass as proton Sep 411:28 PM Rutherford's Gold Foil Experiment Rutherford discovered the nucleus of the atom. Most alpha particles (+) passed straight through, but a small percentage were deflected. This indicated that the positive charge in an atom most be concentrated in a small area, which he referred to as the nucleus. If the Metrodome was an atom, the nucleus would be about the size of a pea. Sep 411:44 PM 3 Atomic History Notes.notebook September 15, 2014 History of Science Video Sep 411:49 PM 4