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Unit 3: History of the Periodic Table 400 BC – Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void. o “Atomos” Greek word for indivisible o Could not be proven scientifically 350 BC - Greek philosopher Aristotle said all matter is made of Earth, Wind, Fire and Water. o He was wrong, but his theory persisted for 2000 years 1808 John Dalton developed the “Atomic Theory” based on Experiments 1. All matter is made of small particles called atoms. 2. Atoms of an element are identical in size, mass and other properties. Atoms of different elements differ in size, mass, and other properties. 3. Atoms cannot be subdivided, created or destroyed. 4. Atoms of different elements combine in simple whole number ratios to form compounds 5. In chemical reactions, atoms combine, separate o Dalton’s theory explains the law of conservation of mass and the law of constant composition. o Later some parts of his theory were proven wrong. (by the development of new technology) o Dalton’s atom was compared to the Billiard ball, solid and indivisible. o 1871 Dmitri Mendeleev, a Russian Chemist arranged the elements in order based on increasing atomic mass o He noticed a repeating pattern of chemical and physical properties Hence the name “Periodic Table” o Mendeleev left 3 blank spaced and predicted the discovery of those elements. 1897 J.J. Thompson o He used a Cathode Ray Tube to discover the electron and its charge. o He could not find the positive charge but new it had to exist. o Plum Pudding model. Electrons in a sea of positive material. o 1911 Ernest Rutherford o Gold foil experiment using alpha particles to discover the dense positive core of an atom, and that an atom is made of mostly empty space. 1911 Henry Mosley rearranged the periodic table based on increasing atomic number o He established Periodic Law with chemical and physical properties as a function of their atomic number. 1913 Niels Bohr o Electrons in fixed paths, and why they did not attract to positive nucleus o Electron location cannot be predicted at a given moment o Electrons travel so fast the form an electron cloud. 1932 Sir James Chadwick proved/discovered Neutrons Periodic Families and Properties Group 1: Alkali Metals o In the pure state, they have a silvery appearance and are soft enough to cut with a knife. o They react violently with water and must be stored in oil. o A very reactive family on the P. table Group 2: Alkali Earth Metals o Less reactive than Alkali Metals, but are still too reactive to be found in the pure form naturally. Group 17: Halogens o Most reactive nonmetals o They react with metals to form salts Group 18: Noble Gases o Generally unreactive because they have a full valence shell of electrons. Periodic Trends Atomic Radius: ½ of the distance between the nuclei of identical atoms that are bonded together o Increases going down because in an increase in the size of the electron cloud Ionization Energy: o An Ion is an atom or group of bonded atoms that has a positive or negative charge Cation is Positive charge Anion is Negative charge o Any process that results in the formation of an ion is called Ionization. o Ionization Energy is the energy required to remove 1 electron from a neutral atom of an element. Electron Affinity is the energy change that occurs when an electron is acquired by a neutral atom. Electronegativity is a measure of an atom’s ability to attract electrons from another element Protons, Neutrons, and Electrons An Atom has 3 Parts o Proton – Positive o Neutron – No Charge o Electron = Negative The Proton and Neutron are found in the nucleus Electrons Orbit the nucleus The Bohr Model Lewis Dot Structure Atomic Mass = Protons + Neutrons Atomic Mass – Protons = Neutrons Protons = Electrons (in a neutral Atom) Protons = Atomic Number Isotopes – Atoms of the same element with a different mass o Isotopes have the same number of protons, but a different number of Neutrons o Hyphen Notation Uranium-235 o Nuclear Symbol Calculating Average Atomic Mass (Mass x Abundance) + (Mass x Abundance)……= Avg Atomic Mass Example 1 Mass 15.995 16.995 17.999 Oxygen 16 Oxygen 17 Oxygen 18 % Abundance 99.759% .037% .204% (15.995 x .99759) + (16.995 x .00037) + (17.999 x .00204) = 15.956 + .00629 + .0367 = 15.999 Avg Atomic Mass