Atoms and Elements 29 September 2005 Physics Chemistry Chemistry Astronomy Astronomy Geology Geology Biology Biology

Topics Discovery of atom structure and function Discovery of atom structure and function Models of atoms Models of atoms Bohr atom (1913) Bohr atom (1913) Quantum mechanics (1930’s) Quantum mechanics (1930’s) Elements and atoms Elements and atoms Electrons and orbits/shells Electrons and orbits/shells Valence electrons Valence electrons

Scientific Method Observations Hypothesis Testing Hypothesis Rejection/Acceptance

Models in the Sciences Example: Dynamics of family interactions Example: Structure of the atom (protons, neutrons and electrons Models are an abstraction and meant to be challenged Protons and Neutron Electrons in orbits

Crystals: Structure and Color

Compounds to Subatomic Particles: A Hierarchy Compound Element ElementAtom Subatomic Particle Subatomic Particle Emergent Properties

Thompson’s Discovery of the Electron Idea that there are smaller components that comprise atoms Idea that there are smaller components that comprise atoms Hypothesis: Atoms consist of subcomponents, one of which is negatively charged with a very small mass Hypothesis: Atoms consist of subcomponents, one of which is negatively charged with a very small mass Testing the hypothesis Testing the hypothesis

Thompson’s Experiment

Rutherford’s Discovery of a Nucleus Idea that there are other smaller components of atoms, including some that are positively charged Idea that there are other smaller components of atoms, including some that are positively charged Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive Testing the hypothesis Testing the hypothesis Alpha particles (+), gold foil, “bullets”, and “tracks” Alpha particles (+), gold foil, “bullets”, and “tracks”

Rutherford’s Experiment:

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 (reflected back) 1/1000 deflected at large angle (reflected back) Hypothesis: Atom consists of a positive subcomponent that is very small in volume in comparison to atom itself Hypothesis: Atom consists of a positive subcomponent that is very small in volume in comparison to atom itself Accept/reject? Accept/reject?

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 is unique in wavelengths emitted Other gases behave the same in producing discrete wavelength, but each gas is unique in wavelengths emitted

Hypothesis and Model

Model: Quantum Mechanics

Theory of Quantum Mechanics Observations Observations Behavior of electrons in heated hydrogen gas consistent with Bohr Model (orbits, etc.) Behavior of electrons in heated hydrogen gas consistent with Bohr Model (orbits, etc.) Behavior of other heavier gases not explained by Bohr Model Behavior of other heavier gases not 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 known facts Incorporation of known facts Prediction of new properties yet to be discovered Prediction of new properties yet to be discovered

Questions Is a model in the sciences a form of an hypothesis? Is a model in the sciences a form of an hypothesis? All of these models (when proven correct) resulted in scientific revolutions in physics. How are revolutions in the sciences analogous to revolutions in society? All of these models (when proven correct) resulted in scientific revolutions in physics. How are revolutions in the sciences analogous to revolutions in society?

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. covertly sequenced nuons surrounded by pompous protons Question

The existence of a tiny, massive, and positively charged atomic nucleus was deduced from the observation that ___. A. fast, massive, and positively charged alpha particles all 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 Question

According to the Bohr model of the atom, an electron gains or looses energy only by ______. 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 Question

Topics Discovery of atom structure and function Discovery of atom structure and function Models of atoms Models of atoms Bohr atom (1913) Bohr atom (1913) Quantum mechanics (1930’s) Quantum mechanics (1930’s) Elements and atoms Elements and atoms Electrons and orbits/shells Electrons and orbits/shells Valence electrons Valence electrons

Elements Patterns Patterns 92 naturally occurring elements (e.g., hydrogen, gold, helium) 92 naturally occurring elements (e.g., hydrogen, gold, helium) Total of 113 Total of of 92 are essential to life (e.g., what are they?) 25 of 92 are essential to life (e.g., what are they?) Key points Key points any element is the same in its chemical structure and physical properties (~stable over time) any element is the same in its chemical structure and physical properties (~stable over time) All elements have origin in either the big bang (hydrogen and helium) or the subsequent evolution of the universe All elements have origin in either the big bang (hydrogen and helium) or the subsequent evolution of the universe

Elements Compound Compound Elements combine in very precise ways that are recurrent and predictable Elements combine in very precise ways that are recurrent and predictable Sodium + Chlorine = Sodium Chloride Sodium + Chlorine = Sodium Chloride Na+ Cl = NaCl Na+ Cl = NaCl metal + gas = solid Key points Key points Atoms of sodium (Na) and chlorine (Cl) remain atoms of each Atoms of sodium (Na) and chlorine (Cl) remain atoms of each Emergent property: “emergence” of new properties in a compound not be explained by the summation of the two elements (hierarchy theory) Emergent property: “emergence” of new properties in a compound not be explained by the summation of the two elements (hierarchy theory)

Compounds to Subatomic Particles: A Hierarchy Compound Element ElementAtom Subatomic Particle Subatomic Particle Emergent Properties

Particles Atoms are composed of particles (subatomic particles) Atoms are composed of particles (subatomic particles) Most stable particles Most stable particles Neutrons Neutrons Protons Protons Electrons Electrons Other less stable particles (quarks, neutrinos, etc.) Other less stable particles (quarks, neutrinos, etc.) Relationship among the more stable particles Relationship among the more stable particles Neutron ProtonElectron Chargeneutralpositivenegative Mass 2x g 2x g 5x g

Atomic Structure Atoms of the same element have the same number of subatomic particles, we abbreviate this as follows: Atoms of the same element have the same number of subatomic particles, we abbreviate this as follows: 2# of protons 2# of protons HeliumHeAbbreviation of element 4Atomic mass (g/mole) 4Atomic mass (g/mole) 1 HydrogenH 1 Mass = Protons + Neutrons Mass = Protons + Neutrons

Electrons Background of quantum mechanics Background of quantum mechanics Energy “barons” of the atom (motion) Energy “barons” of the atom (motion) Energy = ability to do work Energy = ability to do work Potential energy = energy stored due to position or location Potential energy = energy stored due to position or location Charge is negative (-) and particle is always in motion Charge is negative (-) and particle is always in motion Capturing an atom and its orbiting electron may be difficult Capturing an atom and its orbiting electron may be difficult

Key to Electron Structure Count the number of electrons (and compare with abbreviation of element) Count the number of electrons (and compare with abbreviation of element) Electrons are negative in charge and in constant motion Electrons are negative in charge and in constant motion Electrons are in orbits around the nucleus Electrons are in orbits around the nucleus

Electrons Example of Sulfur ( S) Example of Sulfur ( S) (16 electrons in 3 shells) What are shells and what are orbits?

Example of Electrons in Shells As electrons move among shells, they change potential energy Hot summer day, bright sun and car top Hot summer day, bright sun and car top Light absorption by pigments and electrons “jump” to higher shell (potential energy); give off energy when they drop back (kinetic energy) Light absorption by pigments and electrons “jump” to higher shell (potential energy); give off energy when they drop back (kinetic energy) Banana, orange juice or bagel this AM? Banana, orange juice or bagel this AM? 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/respiration, allowing the electron to “drop back” to a lower orbit (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/respiration, allowing the electron to “drop back” to a lower orbit (kinetic energy)

Some keys to Electron Structure Electrons reside in shells as a function of quantum mechanics (1-4 orbits per shell) 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) 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 Distribution of electrons is key to understanding why elements and atoms behave the way they do Outermost electrons are called valence electrons and they have a very special significance in chemistry Outermost electrons are called valence electrons and they have a very special significance in chemistry

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 Concept: most stable state for an atom is one in which the outermost shell is filled with the maximum number of electrons 1 st Shell (1 orbit; 2 electrons) 1 st Shell (1 orbit; 2 electrons) Hydrogen ( 1 1 H; 1 electron; stable ?) Hydrogen ( 1 1 H; 1 electron; stable ?) Helium ( 2 4 He; 2 electrons; stable ?) Helium ( 2 4 He; 2 electrons; stable ?) Periodic Table’s 1 st Row Periodic Table’s 1 st Row Hydrogen and Helium Hydrogen and Helium

Periodic Table of Elements 2 nd shell has 4 orbits with 2 electrons (maximum) per orbit (total of 8 electrons/shell) 2 nd shell has 4 orbits with 2 electrons (maximum) per orbit (total of 8 electrons/shell) Most stable configuration is: Most stable configuration is: 1 st shell filled with 2 electrons 1 st shell filled with 2 electrons 2 nd shell filled with 8 electrons 2 nd shell filled with 8 electrons Total of 10 electrons ( Ne) Total of 10 electrons ( Ne) 2 nd row of Periodic Table 2 nd row of Periodic Table 8 elements 8 elements

Periodic Table of Elements 3 nd shell has 4 orbits with 2 electrons maximum per orbit (total of 8 electrons/shell) 3 nd shell has 4 orbits with 2 electrons maximum per orbit (total of 8 electrons/shell) Most stable configuration is the following: Most stable configuration is the following: 1 st shell filled with 2 electrons 1 st shell filled with 2 electrons 2 nd shell filled with 8 electrons 2 nd shell filled with 8 electrons 3 rd shell filled with 8 electrons 3 rd shell filled with 8 electrons Total of ___ electrons ( Ar) Total of ___ electrons ( Ar) 3 nd row of Periodic Table 3 nd row of Periodic Table 8 elements (list and relate to the above) 8 elements (list and relate to the above)

Periodic Table of Elements Number of elements in a row is not chance but reflects the maximum number of electrons in the outermost shell Number of elements in a row is not chance but reflects the maximum number of electrons in the outermost shell Row 1 = 2 Row 1 = 2 Row 2 = 8 Row 2 = 8 Row 3 = 8 Row 3 = 8 Row 4 = 18 Row 4 = 18 etc etc

Discussion Question As you scan the night sky, you see multiple objects, and you question whether these objects are similar to or different from Earth. 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?

Discussion Question Models are one of the key investigative tools in the scientific method. Models of our understanding of the atom are a classic example of the role that models play in the sciences. List five features of models – not the specifics of any given model, but rather how models are constructed, how they serve to help promote our understanding of natural systems, and their fallibility.

Discussion Question 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? How does this model compare with that of Rutherford’s model of the atom or the quantum mechanics model of the atom?