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The Structure of an Atom Chapter 3 Early Theories • Greek Philosophers – 4 Elements • • • • Air Fire Wind Water – Democritus • Atoms make up matter – Aristotle • Refuted Democritus and atom’s existence forgotten Deomcritus Atoms Differences in atoms Dalton’s Atomic Theory • All matter is composed of extremely small particles called atoms. • All atoms of a given element are identical. • Atoms cannot be created, divided into smaller particles, or distroyed. • Different atoms combine in simple whole number ratios to form compounds. • In a chemical reaction, atoms are separated, combined or rearranged. Dalton’s Atomic Theory • All matter is composed of extremely small particles called atoms. • All atoms of a given element are identical. • Atoms cannot be created, divided into smaller particles, or destroyed. (This part proven wrong) • Different atoms combine in simple whole number ratios to form compounds. • In a chemical reaction, atoms are separated, combined or rearranged. Deomcritus Atoms Differences in atoms Dalton •Atoms •Sameness •Created/destroyed •Combination •Rearragement Subatomic Particles and the Atom • Cathode ray tube – Stream of charged particles (electrons). – See pg 60 for picture • J. J. Thompson – Used cathode ray tube to prove existence of electron. – Proposed “Plum Pudding Model” Subatomic Particles and the Atom • Plum Pudding • J. J. Thompson • Plum Pudding Model Deomcritus Thompson Atoms •Atoms composed of electrons Differences in atoms Dalton •Atoms •Sameness •Created/destroyed •Combination •Rearragement Ernest Rutherford • Gold Foil experiment – Used to prove the existence of a positively charged core (Nucleus) • The results were “like firing a large artillery shell at a sheet of paper and having the shell come back and hit you!” Ernest Rutherford • What should have happened • What DID happened Deomcritus Thompson Atoms •Atoms composed of electrons Differences in atoms Rutherford Dalton •Atoms •Sameness •Created/destroyed •Combination •Rearragement •Positively Charged Nucleus Subatomic Particles and the Atom • Chadwick – Worked with Rutherford. – Noted there was energy in the nucleus, but wasn’t the protons. – Concluded that neutral particles must aslo exist in nucleus. • Mosley – Assigned atomic number to atoms • Bohr – Designed “Bohr Model” of the atom. Deomcritus Thompson Atoms •Atoms composed of electrons Differences in atoms Rutherford Dalton •Atoms •Positively Charged Nucleus •Sameness Chadwick •Created/destroyed •Neutrons exist in Nucleus •Combination •Rearragement Subatomic Particles and the Atom • Three main particles: (see chart on pg 97) – Proton • Positive • In nucleus – Neutrons • Neutral • In nucleus – Electrons • Negative • Orbiting the nucleus (not inside) Atomic Differences • Atomic Number – Distinguishes one atom from another. – = #p+ (& #e- for a neutral atom) • Isotopes – Differ in mass number • Mass # = #p+ + #no – Change is in # no • Atomic Mass Unit – Defined as 1/12 mass of a Carbon-12 atom • p+ = 1 amu • no = 1 amu • e- = 0 amu Atomic Differences • Mass on the periodic table is based on the percent abundance of each isotope of that element. • Therefore it is a decimal. • Ex: Chlorine exist naturally as Cl-37 and Cl-35. Cl-37 is about 75% and Cl-35 is about 25%. So, (37 X .75) + (35 X .25) = 36.5 amu. • See Ex problem on pg 103 for more ideas. Questions? • Ask now, or forever come in the morning for them!