Atomic Theory By PresenterMedia.com

Democritus First to propose that matter is comprised of fundamental indivisible particles called atomos

Billiard Ball Model Dalton 1. Elements are composed of extremely small particles called atoms that cannot be subdivided, created, or destroyed. 2. All atoms of a given element are identical in mass and properties and the atoms of one element differ from the atoms of all other elements. 3. Compounds are composed of atoms of more than one element. Atoms combine in fixed whole number ratios to form compounds. 4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

Thomson To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions: The cathode ray is composed of negatively-charged particles The particles must exist as part of the atom, since the mass of each is MUCH smaller than the mass of a hydrogen atom These subatomic particles can be found within atoms of all elements.

Thomson’s Model Plum Pudding Model Proposed that atoms could be described as negative particles floating within a soup of diffuse positive charge Thomson knew that atoms had an overall neutral charge. Therefore, he reasoned that there must be a source of positive charge within the atom to counterbalance the negative charge on the electrons. electron (e−) negatively charged subatomic particle

Rutherford Gold Foil Experiment

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Based on his experimental results, Rutherford made the following conclusions about the structure of the atom: The positive charge must be localized over a very tiny volume of the atom, which also contains most of the atoms mass. This explained how a very small fraction of the alpha particles were deflected drastically, presumably due to the rare collision with a gold nucleus Since most of the alpha particles passed straight through the gold foil, the atom must be made of mostly empty space!

Rutherford’s Model Nuclear Model Proposed that an atom consists of a very small, positively charged nucleus surrounded by the negatively charged electrons Nucleus small dense central core of the atom; contains all the mass Proton (p+) positively charged subatomic particle

Further questions? What were the electrons doing in the atom? How did the electrons keep themselves from collapsing into the nucleus since opposite charges attract? How does internal structure of the atom relate to the discrete emission lines produced by excited elements?

Line Emission Spectrum of Hydrogen Atoms Bohr Energize the sample Atomic line spectra are another example of quantization. When an element or ion is heated by a flame or excited by electric current, the excited atoms emit light of a characteristic color. The emitted light can be refracted by a prism, producing spectra with a distinctive striped appearance due to the emission of certain wavelengths of light. Line Emission Spectrum of Hydrogen Atoms

e- can only have specific (quantized) energy values light is emitted as e- moves from one energy level to a lower energy level

Planetary Model Proposed that an atom consists of a electrons revolving around a positively charged nucleus like the rings around Saturn Ground state lowest energy state of the electron Excited state higher energy state than the ground state of the electron

Further questions? If electrons are orbiting the nucleus, why don’t they fall into the nucleus as predicted by classical physics?

Schrödinger Quantum Mechanical Model Proposed that an atom where electrons behave like waves and particles and occupy particular regions of space around the nucleus