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Unit 8 – Atoms and The Periodic Table A. Early Models a. During the 5th century B.C. Democritus stated that the differences in substances were the direct result of differences in the size of tiny uncuttable particles. b. During the 4th century B.C. Artistole said NO WAY! He thought that only 4 elements actually exist: water, air, fire and earth B. Dalton’s Atomic Theory (1808) Stated that elements are made up of tiny particles called Atoms and each chemical has a unique combination of atoms C. Thomson Model (1897) Discovered that atoms contain negatively charged particles called “corpuscles” later called electrons D. Other discoveries at the time a. Henri Becquerel – (1896) - discovered radioactivity by mistake - he was studying light emission when he discovered that uranium emits energy by itself b. Marie and Pierre Curie – (1898) – separated radioactive elements and coined the term “radioactive decay” E. Rutherford – Bohr Model (1899 – 1920) Rutherford proposed that the atom consists of a tiny positively charged nucleus surrounded by a cloud of negatively charged electrons. The nucleus contains almost all of the mass of the atom and consists of protons and neutrons. The number of electrons surrounding the nucleus, equals the number of protons so as to make the atom neutral. Bohr (1913) Electrons are in orbits in a fixed position around the nucleus. It was proven later that Bohrs theory was wrong F. Atomic Number and Atomic Mass Atomic Number – the number of Protons in the nucleus Protons have a +1 charge Neutral atoms have an equal number of protons and electrons Ions Formed when electrons are either added or subtracted from the neutral atom i. If electrons are added a negative charge is formed ii. If electrons are removed , a positive charge is formed Examples Add a electron to Fluorine ( F ) Protons = 9 Total charge = +9 Electron = 9 + 1 Total charge = -10 Total charge on the atom = -1 shown as F- Example 2 Remove 2 electrons from Barium (Ba) Protons = 56 Total charge = +56 Electron = 56 - 2 Total charge = -54 Total charge on the atom = +2 shown as Ba+2 Atomic Mass If protons and neutrons have a mass of approx. 1 gram, then the total mass of the atom is equal to the total number of protons and neutrons Atomic mass = neutrons + protons Neutrons = atomic mass – atomic number As shown on the periodic table Atomic Number 11 Na Atomic Mass 23 neutrons = 23 – 11 = 14