Presentation on theme: "Atomic Theory Lesson Objectives"— Presentation transcript:
1 Atomic Theory Lesson Objectives Understand the experimental design and conclusions used in the development of modern atomic theory includingDalton’s PostulatesThomson’s discovery of electron propertiesRutherford’s nuclear atomBohr’s nuclear atom
2 A brief look at the history of the atom. Atomic StructureA brief look at the history of the atom.
3 Democritus of AbderaBegan the discussion of the possible existence of ATOMS!ATOM: from the Greek meaning “cannot be cut”.But, he had no empirical evidence, so for centuries people believed Aristotle.
4 Democritus' IdeasMatter is composed of empty space through which atoms move.Atoms are solid, homogeneous, indestructible, and indivisible.Diff. kinds of atoms have different sizes and shapesThe differing properties of matter are due to the size, shape, and movement of atoms.Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves.
5 The Aristotelian Approach to Knowledge The scientific method is still another 2000 yrs. away.They believed one should separate the mind from the body (emotions) in order to understand anything. Attaining knowledge by pure reason, or Rationalism.What’s the purpose of knowledge??To lead you to the truth.
6 The Empirical Approach Come to knowledge by relying on our senses the beginning of experimentation.So, compare Rationalism to Empiricism.What’s better??Rational – problems with our reason. Plato’s Academy.Empirical – problems with trusting our senses.
7 Development of Atomic Theory First credible atomic theory introduced by John Dalton in 1808Dalton studied the ratio in which elements combine when reacted with each otherBased theory on experimental results gathered by other scientists (Lavosier’s conservation of mass and Proust’s law of definite proportions)
8 Dalton’s Experimental Theory Dalton’s PostulatesElements are made of small and indivisible particles called atomsAtoms of a given element are identical to one another but differ from atoms of other elementsChemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratiosAtoms are neither created nor destroyed in chemical reactionsShortcomings of Dalton’s PostulatesAtoms of an element can have masses that differ slightlyAtoms can be divided into subatomic particles
9 Atoms and ElectricityScientists noticed that some substances decomposed when exposed to electrical currentElements are held together by electrical forcesScientists discovered the cathode rayCathode Ray Tube
10 Discovery of the Electron Rays in a cathode tube were attracted to a metal plate that had a positive chargeCathode ray – stream of negatively charged particlesThomson calculated the electric charge to mass ratio of the particles in cathode raysMass of these particles was much less than the atom with the least massElectron (e-) – negatively charged subatomic particleCathode Ray Tube
11 Thomson’s Atomic Model Thomson formulated the “Plum Pudding” modelAtoms are made of a positively charged mass and electrons are scattered throughout the mass, like plums mixed in a puddingPlum Pudding ModelFailed to correctly establish the structure of the atom
12 Rutherford’s Gold Foil Experiment Bombarded a thin foil of gold with alpha particlesAlpha particles have a +2 chargeDiscovery of nucleusNucleus – positively charged, dense center of an atom
13 By en:User:Cburnett [GFDL] Rutherford’s Nuclear AtomRutherford estimated that diameter of the nucleus was only 1/10000 of the diameter of the atomNucleus accounts for almost all of an atom’s massNucleus contains protons and neutronsProton – positively charged subatomic particle (p+)Neutron – neutrally charged subatomic particle (n0) - J. ChadwickRutherford’s nuclear atom showed a positive nucleus orbited by electronsBy en:User:Cburnett [GFDL]
14 Bohr’s Nuclear AtomFollowing discovery of the nucleus, Bohr proposed his own atomic theoryTried to explain why the emission spectrum of the hydrogen atom included only some light frequenciesProposed a diagram that explained electron movement and energy relationsHydrogen gas discharge tubeSiltPrismDetecting Screen
15 Bohr’s Nuclear Atom Proposed that electrons Are arranged in circular orbits around the nucleusDo not lose energy as they orbitIn each orbit have fixed amount of energyIn an orbit closest to the nucleus are in their lowest energy stateGain and lose energy in certain, fixed amounts called quantaMay receive energy and move to a higher energy orbitReturn back to lower energy orbit when they release energyPhoton – particle of electromagnetic radiation that possesses a quantum of energyEnergy of each photon corresponds to a certain frequencye-Photon
16 Short Comings of Bohr’s Nuclear Atom Did not explain emission spectrums of other elementsDid not explain chemical properties of atomsFurther experimentation indicated that electrons don’t revolve in circular orbitsMovement of electrons is much more complex