Atomic Structure and Function 25 September 2003 Professor Taylor PhysicsChemistry Astronomy Astronomy Geology GeologyBiology Ecology Ecology

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure: consternation Atomic structure: consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding Discovery of the elements Discovery of the elements

Atomic Structure and Function: Take Home Message Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) Role of hypothesis development and testing in the sciences – scientific method Role of hypothesis development and testing in the sciences – scientific method Demystification of physics and chemistry: structure of the atom is a simple concept Demystification of physics and chemistry: structure of the atom is a simple concept Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) Applications – some examples Applications – some examples

Early History in 18 th and 19 th Century Atomic Theory of Dalton Atomic Theory of Dalton Atoms as indivisible “marbles” Atoms as indivisible “marbles” Element (shape and size the same for a given element) Element (shape and size the same for a given element) Molecules/compounds (e.g., water or H 2 O) combine in predictable ways and ratios Molecules/compounds (e.g., water or H 2 O) combine in predictable ways and ratios Atoms as basis for understanding chemistry but without electrons, protons and neutrons (analogy to a marble) Atoms as basis for understanding chemistry but without electrons, protons and neutrons (analogy to a marble) Elements – 60 known (e.g., hydrogen, gold, sodium) Elements – 60 known (e.g., hydrogen, gold, sodium)

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure with consternation Atomic structure with consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding

Thompson’s Discovery of the Electron Idea that there may be some smaller components that comprise atoms Idea that there may be some smaller components that comprise atoms Hypothesis: Atoms consist of subcomponents, one of which is negatively charged and very small in mass Hypothesis: Atoms consist of subcomponents, one of which is negatively charged and very small in mass Experiment to test hypothesis Experiment to test hypothesis

Thompson’s Experiment

Thompson’s Discovery of the Electron Idea that there may be some smaller components that comprise atoms Idea that there may be some smaller components that comprise atoms Hypothesis: Atoms consist of some subcomponents, one of which is negatively charged and very small in mass Hypothesis: Atoms consist of some subcomponents, one of which is negatively charged and very small in mass Hypothesis: accept the hypothesis Hypothesis: accept the hypothesis

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure with consternation Atomic structure with consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding

Rutherford’s Discovery of a Nucleus Idea that there may be some smaller components that comprise atoms, including negatively and positively charges components Idea that there may be some smaller components that comprise atoms, including negatively and positively charges components Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive Experiment to test hypothesis Experiment to test hypothesis Alpha particles (+), gold foil, “bullets”, and “tracks” Alpha particles (+), gold foil, “bullets”, and “tracks”

Rutherford’s Experiment:

Explanation

Observations, Hypothesis and Conclusion Observations: Observations: Almost all alpha’s passed through foil unaffected Almost all alpha’s passed through foil unaffected Very small number of alpha’s small angle Very small number of alpha’s small angle 1/1000 deflected at large angle (struck “head on”) 1/1000 deflected at large angle (struck “head on”) Hypothesis: Atoms consist of some positive subcomponents Hypothesis: Atoms consist of some positive subcomponents Accept the hypothesis Accept the hypothesis

New Model of Atom Small, dense center, positively charged – called the nucleus (later to be called the proton) Small, dense center, positively charged – called the nucleus (later to be called the proton) Small, light (w/o mass), negative charged subcomponent circles the nucleus (electron) Small, light (w/o mass), negative charged subcomponent circles the nucleus (electron) Later additions (1932) Later additions (1932) Neutrons Neutrons Protons Protons Charge (neutrality calls for + and – charges) Charge (neutrality calls for + and – charges)

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure with consternation Atomic structure with consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding

Atomic Structure with Consternation Consternation with Rutherford Model Electrons moving, so must give off energy Electrons moving, so must give off energy 2 nd Law of Thermodynamics 2 nd Law of Thermodynamics Electrons should collapse Electrons should collapse Yet atoms are billions of years old Yet atoms are billions of years old Conclusions: model violated fundamental laws of physics

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure with consternation Atomic structure with consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding

Atomic Structure: Bohr Atom Observations (1913) Observations (1913) Heat hydrogen gas and light is emitted as a discrete wavelength (not continuous spectrum) Heat hydrogen gas and light is emitted as a discrete wavelength (not continuous spectrum) Other gases behave the same in producing discrete wavelength, but each gas unique in wavelengths produced Other gases behave the same in producing discrete wavelength, but each gas unique in wavelengths produced

Atomic Structure: Bohr Atom Hypothesis Hypothesis Electrons circle the nucleus at specific distances from the nucleus in defined orbits Electrons circle the nucleus at specific distances from the nucleus in defined orbits Diagram Diagram

Atomic Structure: Bohr Atom Explanation of light in discrete wavelengths Explanation of light in discrete wavelengths As “excited”/heated electrons drop back from orbit, a packet of energy is lost as a function of distance from the nucleus As “excited”/heated electrons drop back from orbit, a packet of energy is lost as a function of distance from the nucleus Packet of energy is called photon (light) Packet of energy is called photon (light)

Atomic Structure: Bohr Atom Key aspects of the Bohr atom Key aspects of the Bohr atom No space between the orbits for electrons to be so “jump” has to be “all or none” – quantum levels of electrons No space between the orbits for electrons to be so “jump” has to be “all or none” – quantum levels of electrons Excited electrons in outer orbit has 2 options: Excited electrons in outer orbit has 2 options: Drop down to next lower orbit and emit a packet of energy (light) unique for that distance Drop down to next lower orbit and emit a packet of energy (light) unique for that distance Drop back to even lower level (ground state) and emit packet of energy (light) unique for that that distance Drop back to even lower level (ground state) and emit packet of energy (light) unique for that that distance Energy release is unique for each element and is a “window” on atomic structure (lab exercise) Energy release is unique for each element and is a “window” on atomic structure (lab exercise) Ideas verified two decades later using quantum mechanics Ideas verified two decades later using quantum mechanics

Bohr Atom

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure: consternation Atomic structure: consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Atomic Structure: Consternation with the Bohr Atom (theory) Consternation with the Bohr Atom (theory) Theory of Quantum Mechanics (1930’s) Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding Discovery of the elements Discovery of the elements

Theory of Quantum Mechanics Observations Observations Behavior of electrons in heated hydrogen gas were consistent with Bohr Model (orbits, etc.) Behavior of electrons in heated hydrogen gas were consistent with Bohr Model (orbits, etc.) Behavior of other heavier gases could not be explained by Bohr Model Behavior of other heavier gases could not be explained by Bohr Model Investigators (Heisenberg, Shrodinger): wave-particle duality of light Investigators (Heisenberg, Shrodinger): wave-particle duality of light Key: Integrated mechanisms of waves and particles, focusing on “fuzzy electron clouds”/waves 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 Hypothesis: new model of atomic structure and function: quantum mechanics theory of the atom Incorporation of know facts Incorporation of know facts Prediction of new properties yet to be discovered Prediction of new properties yet to be discovered

Quantum Mechanics

Quantum Mechanics Model Key Principles Key Principles Electron behaves as a wave not a particle (Bohr) Electron behaves as a wave not a particle (Bohr) Energy of electron behaves as a “spread-out wave” in three dimensions - not discrete bundle of energy (Bohr) Energy of electron behaves as a “spread-out wave” in three dimensions - not discrete bundle of energy (Bohr) Description is based on quantum numbers Description is based on quantum numbers Distance from nucleus Distance from nucleus Energy level and sublevel Energy level and sublevel Orientation of electrons in space Orientation of electrons in space Direction of electron spin Direction of electron spin

Quantum Mechanics Model: Principle Energy Levels Main energy level of electrons Main energy level of electrons Distance from nucleus Distance from nucleus n = 1 (lowest energy level) n = 1 (lowest energy level) n = 2 n = 2 n = 3 n = 3 n = 4 (highest energy level) n = 4 (highest energy level) etc. etc. Higher the number, the higher the energy Higher the number, the higher the energy

Quantum Mechanics Model: Sub-Energy Levels Electrons in “fuzzy” probability distribution called an “orbital” Electrons in “fuzzy” probability distribution called an “orbital” e.g., n = 1 has 1 sub-energy level/orbital (2 electron orientations) e.g., n = 1 has 1 sub-energy level/orbital (2 electron orientations) Heisenberg Uncertainty Principle Heisenberg Uncertainty Principle Electrons in an orbital can only exist in pairs (2) and the pair spins in opposite directions Electrons in an orbital can only exist in pairs (2) and the pair spins in opposite directions Pauli Exclusion Principle Pauli Exclusion Principle

Pauli’s Exclusion Principle

Quantum Mechanics Model: Electron Configuration Electrons always adopt most energetically stable energy levels/orbitals consistent with Laws of Thermodynamics and previous conditions (above) Electrons always adopt most energetically stable energy levels/orbitals consistent with Laws of Thermodynamics and previous conditions (above) If n = 1, two electrons in 1st orbital (s orbital) If n = 1, two electrons in 1st orbital (s orbital) If n = 2, six electrons in 3 orbitals (p orbitals) If n = 2, six electrons in 3 orbitals (p orbitals)

Electron Configuration in p Orbital

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure with consternation Atomic structure with consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding

Applications of Our Understanding of Atomic Structure Spectroscopy: quantum differences in light emitted or absorbed result in unique signature for each element (“fingerprint”) Spectroscopy: quantum differences in light emitted or absorbed result in unique signature for each element (“fingerprint”) LASER (Light Amplification by Stimulated Emission of Radiation) LASER (Light Amplification by Stimulated Emission of Radiation) Binding of elements to create compounds Binding of elements to create compounds Neurotransmitters in the brain Neurotransmitters in the brain Allergy reactions Allergy reactions Etc. Etc.

Atomic Structure and Function: Take Home Message Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom) Role of hypothesis development and testing in the sciences – scientific method and progress Role of hypothesis development and testing in the sciences – scientific method and progress Consternation, rejection, new models & new hypotheses Consternation, rejection, new models & new hypotheses Demystification of physics and chemistry: structure of the atom is a simple concept Demystification of physics and chemistry: structure of the atom is a simple concept Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) Understanding of atomic structure underpins future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental sciences) Applications – some examples Applications – some examples

Atomic Structure

Atomic Structure and Function: Outline Early history in 18 th and 19 th centuries Early history in 18 th and 19 th centuries Discovery of the electron (1897) Discovery of the electron (1897) Discovery of the nucleus (1911) Discovery of the nucleus (1911) Atomic structure: consternation Atomic structure: consternation Atomic structure: the Bohr atom (1913) Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Mechanics (1930’s) Atomic Structure: Theory of Quantum Mechanics (1930’s) Applications of our understanding Applications of our understanding Discovery of the elements Discovery of the elements