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Atoms and Periodic Properties Topics Honors 227 Physics Discovery of atomic structure and function Models of atoms Chemistry Astronomy Geology Biology Scientific Method Elements and atoms Electrons and orbits/shells Valence electrons Models in the Sciences Example: Dynamics of family interactions Observations Reject/Accept Bohr atom (1913) Quantum mechanics (1930’s) Hypothesis Test of Hypothesis Example for atoms: 1. Atom as a marble 2. Atom as the solar system 3. Atom with protons, neutrons and electrons Models: abstraction and meant to be challenged Protons and Neutron Crystal Morphology and Color: Atomic Structure Electrons in orbits Leaf Color and Atomic Structure 1 Compounds to Subatomic Particles: A Hierarchy Compound Emergent Properties? Thompson’s Investigation of the Electron Element Atom Idea that smaller components comprise atoms Hypothesis: Atoms consist of subcomponents, one of which is negatively charged with a very small mass Testing the hypothesis Subatomic Particles Rutherford’s Investigation of the Nucleus Thompson’s Experiment Methodology Observations: 1. Deflection of negatively- charged beam 2. Very small mass relative to charge 3. Mass same for all elements Idea that other smaller components are part of atoms, including ones that are positively charged Hypothesis: Atoms consist of multiple subcomponents, some of which are positive Testing the hypothesis tested Alpha particles (+), gold foil, “bullets”, and “tracks” Hypothesis: Accept Particle name: Electron Rutherford’s Experiment: Methodology Observations, Hypothesis and Conclusion Observations Almost all alpha’s (+) passed through foil unaffected Very small number of alpha’s (+) deflected at a small angle 1/1000 bounced back Hypothesis: Atom consists of a positive subcomponent that is small in volume in comparison to atom itself Accept/reject? Conclusions: radius of nucleus is 10-4 x radius of atom Name of particle is the proton 2 Hypothesis and Model Atomic Structure: Bohr Atom Observations (1913) Heat hydrogen gas and light emitted as a discrete wavelengths (not continuous spectrum) Other gases behave the same in producing discrete wavelength, but each gas is unique in wavelengths emitted (e.g., violet versus yellow versus red) Quantum Concepts: 1. Only allowed orbits for electrons 2. Discrete frequency of photon (emitted light) Theory of Quantum Mechanics Model: Quantum Mechanics Observations Investigators (Heisenberg, Shrodinger): wave-particle duality of light (remember Dr. Geller’s lecture) Key: integrated mechanisms of waves and particles, focusing on “fuzzy electron clouds”/waves Hypothesis: new model of atomic structure and function: quantum mechanics theory of the atom Questions Is a model in the sciences a form of an hypothesis? A B Yes No All of these models (when accepted or rejected) resulted in scientific revolutions in physics. Are revolutions in the sciences somewhat analogous to revolutions in society? Behavior of electrons in heated hydrogen gas consistent with Bohr Model (orbits, etc.) Behavior of other heavier gases not explained by Bohr Model Incorporation of known facts (Bohr model as particles) Wave properties of electrons in orbits Prediction of new properties yet to be discovered Question According to the Rutherford model of the atom, the volume of any atom is largely _______. A. protons and neutrons B. electrons C. empty space D. noxiously sequenced nuons surrounded by pompous protons E. none of the above 3 Question Question According to the Bohr model of the atom, an electron gains or looses energy only by ______. The existence of a tiny, massive, and positively charged atomic nucleus was deduced from the observation that ___. A. jumping from one atom to another B. speeding up or slowing down in its orbit C. jumping from one orbit to another D. being removed from the atom A. fast, massive, and positively charged alpha particles move in straight lines through gold foil B. alpha particles were deflected by a magnetic field C. some alpha particles were deflected by metal foil D. all of the above Elements Topics Discovery of atom structure and function Models of atoms Bohr atom (1913) Quantum mechanics (1930’s) Symbol Name 1 2 H He Hydrogen Helium 6 7 8 C N O Carbon Nitrogen Oxygen 17 18 Cl Ar Chlorine Argon 26 27 Fe Co Iron Cobalt 53 I Iodine 73 74 Ta W Tantalum Tungsten 79 80 Au Hg Gold Mercury 94 Pu Plutonium 104 Rf Rutherfordium 92 naturally occurring elements (e.g., hydrogen, gold, helium) Total of 113 25 of 92 are essential to life (e.g., what are they?) Key points Elements and atoms Electrons and orbits nested within shells Valence electrons At No Patterns any element is the same in its chemical structure and physical properties (stable over time, with one exception - radioactive elements) All elements have origin in either the big bang (hydrogen and helium) or the subsequent evolution of the universe (Dr. Geller will speak to this a bit later) Elements and Compounds Compound Elements combine in very precise ways that are recurrent and predictable Sodium + Chlorine = Sodium Chloride Na + Cl = NaCl metal + gas = solid Key points Atoms of sodium (Na) and chlorine (Cl) remain atoms of each Product (NaCl) is recurrent and predictable Emergent property: “emergence” of new properties in a compound not be explained by the summation of the two elements (hierarchy theory) Other example: Hydrogen + oxygen = __________ 4 Compounds to Subatomic Particles: A Hierarchy Subatomic Particles Compound Emergent Properties Atoms are composed of subatomic particles Most stable particles Element Neutrons Protons Electrons Other less stable particles (quarks, neutrinos, etc.) Relationship among the more stable particles Atom Subatomic Particle Old Model of Atom and its Particles Charge Mass Atoms of the same element have the same number of subatomic particles, abbreviated as follows: 2 Helium Electrons Abbreviation of element Atomic mass (g/mole) 1 Hydrogen # of protons He 4 H 1 Mass = Electron negative 5x10-28 g Modern Model of Atom and Particles Atomic Structure Proton positive 2x10-24 g Electrons in orbits Protons and Neutron Neutron neutral 2x10-24 g Protons + Neutrons Background of quantum mechanics Energy “barons” of the atom (motion) Energy = ability to do work Potential energy = energy stored due to position or location (analogy to water in a tower) Charge is negative (-) and particle is always in motion Capturing an atom and its orbiting electron may be difficult (why?) 5 Electrons Key to Electron Structure Count the number of protons as the number of electrons = number of protons Electrons are negative in charge and in constant motion (as a wave) Electrons are in orbits around the nucleus and orbits are nested within shells Example of Sulfur (1632S) (16 electrons in 3 shells) What are shells and what are orbits? Example of Electrons in Shells Quantum Mechanics and Orbits As electrons move among shells, they change potential energy Hot summer day, bright sun and car top Banana, orange juice or bagel this AM? Some Keys to Electron Structure Electrons reside in shells as a function of quantum mechanics (1-4 orbits per shell) Never more than two electrons per orbit (Pauli’s Exclusion Principle) Distribution of electrons is key to understanding why elements and atoms behave the way they do Outermost electrons are called valence electrons; they have special significance in chemistry Light absorption by pigments and electrons “jump” to higher shell (potential energy); give off energy when they drop back (kinetic energy) Excited electron “captured” by chlorophyll in leaf and shuttled to a sugar molecule in its excited state (potential energy) until you release the energy via digestion, allowing the electron to “drop back” to a lower orbit (kinetic energy) Periodic Table of Elements Concept: most stable state for an atom is one in which the outermost shell is filled with the maximum number of electrons 1st Shell (1 orbit; 2 electrons) Hydrogen (11H; 1 electron; stable ?) Helium (24He; 2 electrons; stable ?) Periodic Table’s 1st Row Hydrogen and Helium 6 Periodic Table of Elements 2nd shell has 4 orbits with 2 electrons (maximum) per orbit (total of 8 electrons/shell) Most stable configuration is: 1st shell filled with 2 electrons 2nd shell filled with 8 electrons Total of 10 electrons (1020Ne) 2nd row of Periodic Table 8 elements Periodic Table of Elements 3nd shell has 4 orbits with 2 electrons maximum per orbit (total of 8 electrons/shell) Most stable configuration is the following: 1st shell filled with 2 electrons 2nd shell filled with 8 electrons 3rd shell filled with 8 electrons Total of ___ electrons (1840Ar) 3nd row of Periodic Table 8 elements (list and relate to the above) Periodic Table of Elements Predictive Value of Periodic Table Number of elements in a row is not chance but reflects the maximum number of electrons in the outermost shell Row 1 = 2 Row 2 = 8 Row 3 = 8 Row 4 = 18 etc 7 Discussion Question As you scan the night sky, you see multiple objects and question whether these objects are similar to or different from the Earth in chemical composition (i.e., elements). Suppose the person next to you says that she has an instrument that can identify the occurrence of specific atoms (e.g., sodium, hydrogen, sulfur) based on the energy patterns/signatures of electrons in atoms on that planet. Could she be right? A Yes B No Question In the list below, an atom of _______ has the greatest ability to attract electrons. A. silicon B. sulfur C. krypton D. chlorine E. sodium Question In the list below, an atom of _______ has the least ability to attract electrons. A. silicon B. sulfur C. krypton D. chlorine E. sodium Discussion Question Discussion Question Models are one of the key investigative tools in the scientific arsenal. Models of our understanding of the atom are a classic example of the role that models play in the sciences. In many ways, the Periodic Table is a “model” to help scientists understand patterns in the natural world. List ways in which this model helps scientists to understand the natural world? List four features of models – not the specifics of any given model, but rather how models are constructed, how they promote our understanding of natural systems, and how their fallibility is an asset. How does this model compare with that of Rutherford’s model or the quantum mechanics model of the atom? 8