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Chapter 4 Atom • Smallest particle of an element that retains its chemical identity • Atoms of the same element cannot be separated chemically because they share identical chemical properties Daltons Atomic Theory • Dalton have four postulates (characteristics) of atoms • • • • First postulate Second postulate Third postulate Fourth postulate First Postulate • All elements are composed of tiny indivisible particles called atoms – Atoms cannot be divided into smaller pieces Second Postulate • Atoms of the same element are identical. – Atoms of the same element have identical chemical and physical properties. This is why they cannot be separated from one another by chemical means Third Postulate • Atoms of different elements can physically mix together or can chemically combine in simple whole number ratios to form compounds – Remember when elements mix they do not have to form compounds Fourth Postulate • Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms, however, never change during a chemical reactions Revision of Daltons First Postulate • Daltons first postulate was later revised to read – Atoms cannot be divided by chemical means • No chemical reaction can divide atoms. However, nuclear weapons, and nuclear power have shown how atoms can be divided by nuclear processes Subatomic particles • Three sub atomic particles exist • Protons • Electrons • Neutrons Protons • Discovered by Eugen Goldstein • Positively charged particle found inside the nucleus of an atom • Protons have a mass of approximately 1 amu Neutrons • Neutral particles (carry no charge) • Found in the nucleus of an atom • Neutrons have a mass of approximately 1 amu Electrons • Discovered by J. J. Thompson with the Cathode Ray Tube experiment • Carry a negative charge • Found outside the nucleus of the atom • Have very little mass (approximately 1/1840 the mass of a proton) Cathode Ray Tube Experiment • Passed a “beam of electricity” called a cathode ray through a gas in a cathode ray tube • The beam responded to magnetic and electric forces which indicates it was charged particles • Those particles were later called electrons Cathode Ray Tube Experiment • Because the cathode ray tube experiment could be repeated with many types of anodes and cathodes it was found that electrons resided in all materials. • Hence all elements contained electrons Rutherford Gold Foil Experiment • Discovered atoms had a nucleus • Fired alpha particles into gold foil • Results Results • Most particles went straight through the gold foil – Atoms are mostly space • Some particles bounced back – Atoms have dense tightly packed core