Atoms and Elements Atoms and Elements 30 September 200430 September 2004

PhysicsPhysics

ChemistryChemistry AstronomyAstronomy

GeologyGeology

BiologyBiology

TopicsTopics

Discovery of what an atom is Discovery of what an atom is composedcomposed

Models of atomsModels of atoms Bohr atom (1913)Bohr atom (1913) Quantum mechanics (1930’s)Quantum mechanics (1930’s)

Elements and atomsElements and atoms Electrons and shells/orbits Electrons and shells/orbits Valence electronsValence electrons

Synopsis (1Synopsis (1stst lecture) lecture)

Principles of physics, astronomy, chemistry, Principles of physics, astronomy, chemistry, geology, and biology geology, and biology

Dominant theories and laws that underlie how Dominant theories and laws that underlie how the natural world operatesthe natural world operates

Theories and laws are relevant in every day lifeTheories and laws are relevant in every day life Underlying themes across all disciplines (e.g., Underlying themes across all disciplines (e.g.,

emergent properties, models, scientific emergent properties, models, scientific method)method)

Cardinal features of scientific investigations Cardinal features of scientific investigations (e.g., scientific method, data visualization, (e.g., scientific method, data visualization, models, data collection), through a models, data collection), through a combination of computer simulations and combination of computer simulations and hands-on experiments in labhands-on experiments in lab

Models in the SciencesModels in the Sciences

Conceptual models Conceptual models based on based on observationsobservations Example:Example: Dynamics Dynamics

of family interactionsof family interactions

Example:Example: Structure Structure of the atom (protons, of the atom (protons, neutrons and electronsneutrons and electrons

Models are an Models are an abstraction and are abstraction and are meant to be challengedmeant to be challenged Protons and

NeutronElectrons in

orbits

Scientific MethodScientific Method

Observations

Proposal/Hypothesis

Testing Hypothesis

Rejection/Acceptance of Hypothesis

Crystals: Structure and Crystals: Structure and ColorColor

TopicsTopics

Discovery of what an atom is Discovery of what an atom is composedcomposed

Models of atomsModels of atoms Bohr atom (1913)Bohr atom (1913) Quantum mechanics (1930’s)Quantum mechanics (1930’s)

Elements and atomsElements and atoms Electrons and shells/orbits Electrons and shells/orbits Valence electronsValence electrons

Compounds to Subatomic Compounds to Subatomic Particles:Particles:

A Hierarchy A Hierarchy

CompoundCompound

ElementElement

AtomAtom

Subatomic Subatomic ParticleParticle

Thompson’s Discovery of Thompson’s Discovery of the Electronthe Electron

Idea that there may be some smaller Idea that there may be some smaller components that comprise atomscomponents that comprise atoms

HypothesisHypothesis: Atoms consist of : Atoms consist of subcomponents, one of which is subcomponents, one of which is negatively charged and very small in negatively charged and very small in massmass

Experiment to test hypothesisExperiment to test hypothesis

Thompson’s ExperimentThompson’s Experiment

Rutherford’s Discovery of a Rutherford’s Discovery of a NucleusNucleus

Idea that there may be some smaller Idea that there may be some smaller components that comprise atoms, components that comprise atoms, including negatively and positively including negatively and positively charges componentscharges components

HypothesisHypothesis: Atoms consist of multiple : Atoms consist of multiple subcomponents, some negative and subcomponents, some negative and some positivesome positive

Experiment to test hypothesisExperiment to test hypothesis Alpha particles (+), gold foil, “bullets”, Alpha particles (+), gold foil, “bullets”,

and “tracks”and “tracks”

Rutherford’s Experiment:Rutherford’s Experiment:

Observations, Hypothesis Observations, Hypothesis and Conclusionand Conclusion

ObservationsObservations Almost all alpha’s passed through foil Almost all alpha’s passed through foil

unaffectedunaffected Very small number of alpha’s deflected @ Very small number of alpha’s deflected @

small anglesmall angle 1/1000 deflected at large angle (struck 1/1000 deflected at large angle (struck

“head on”)“head on”) HypothesisHypothesis: Atom consists of a positive : Atom consists of a positive

subcomponent that is very small in subcomponent that is very small in volume in comparison to atom itselfvolume in comparison to atom itself

Accept the hypothesisAccept the hypothesis

Atomic Structure: Bohr Atomic Structure: Bohr AtomAtom

Observations (1913)Observations (1913) Heat hydrogen gas and light is emitted Heat hydrogen gas and light is emitted

as a as a discretediscrete wavelength (not wavelength (not continuous spectrum)continuous spectrum)

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

Bohr AtomBohr Atom

A Model of Quantum A Model of Quantum MechanicsMechanics

Theory of Quantum Theory of Quantum MechanicsMechanics

ObservationsObservations Behavior of electrons in heated hydrogen gas Behavior of electrons in heated hydrogen gas

consistent with Bohr Model (orbits, etc.)consistent with Bohr Model (orbits, etc.) Behavior of other heavier gases not explained by Behavior of other heavier gases not explained by

Bohr ModelBohr Model Investigators (Heisenberg, Shrodinger): wave-Investigators (Heisenberg, Shrodinger): wave-

particle duality of lightparticle duality of light Key: integrated mechanisms of waves and Key: integrated mechanisms of waves and

particles, focusing on “fuzzy electron particles, focusing on “fuzzy electron clouds”/clouds”/waveswaves

HypothesisHypothesis: new model of atomic structure and : new model of atomic structure and function: quantum mechanics function: quantum mechanics theorytheory of the atom of the atom Incorporation of know factsIncorporation of know facts Prediction of new properties yet to be discoveredPrediction of new properties yet to be discovered

Models in the SciencesModels in the Sciences

Conceptual models Conceptual models based on based on observationsobservations Example:Example: Dynamics Dynamics

of family interactionsof family interactions

Example:Example: Structure Structure of the atom (protons, of the atom (protons, neutrons and electronsneutrons and electrons

Models are an Models are an abstraction and are abstraction and are meant to be challengedmeant to be challenged Protons and

NeutronElectrons in

orbits

ElementsElements

PatternsPatterns 92 naturally occurring elements (e.g., hydrogen, 92 naturally occurring elements (e.g., hydrogen,

gold, helium)gold, helium) Total of 113Total of 113 25 of 92 are essential to life (e.g., what are they?)25 of 92 are essential to life (e.g., what are they?)

Key pointsKey points any element is the same in its chemical structure any element is the same in its chemical structure

and physical properties (~stable over time)and physical properties (~stable over time) All elements have origin in either the big bang All elements have origin in either the big bang

(hydrogen and helium) or the subsequent (hydrogen and helium) or the subsequent evolution of the universe evolution of the universe

ElementsElements CompoundCompound

Elements combine in very precise ways that Elements combine in very precise ways that are recurrent and predictableare recurrent and predictable

Sodium + Chlorine = Sodium ChlorideSodium + Chlorine = Sodium Chloride NaNa + Cl =+ Cl = NaClNaClmetal + gas = solidmetal + gas = solid

Key points Key points Atoms of sodium (Na) and chlorine (Cl) remain Atoms of sodium (Na) and chlorine (Cl) remain

atoms of eachatoms of each Emergent propertyEmergent property: “emergence” of new : “emergence” of new

properties in a compound not be explained by properties in a compound not be explained by the summation of the two elements (the summation of the two elements (hierarchy hierarchy theorytheory))

ParticlesParticles

Atoms are composed of particles (subatomic Atoms are composed of particles (subatomic particles)particles)

Most stable particlesMost stable particles NeutronsNeutrons ProtonsProtons ElectronsElectrons

Other less stable particles (quarks, neutrinos, etc.)Other less stable particles (quarks, neutrinos, etc.) Relationship among the more stable particlesRelationship among the more stable particles

NeutronNeutron ProtonProton ElectronElectronChargeCharge neutralneutral positivepositive negativenegativeMassMass 2x102x10-24-24 g g 2x102x10-24-24 g g

5x105x10-28-28 g g

Atomic StructureAtomic Structure

Atoms of the same element (e.g., hydrogen, Atoms of the same element (e.g., hydrogen, helium, gold) have the same number of helium, gold) have the same number of subatomic particles and by convention we subatomic particles and by convention we abbreviate as follows:abbreviate as follows:

22 # of protons# of protons

HeliumHelium HeHe Abbreviation of elementAbbreviation of element 44 Atomic mass (g/mole)Atomic mass (g/mole)

11

HydrogenHydrogen HH 11

Mass = Mass = Protons + NeutronsProtons + Neutrons

ElectronsElectrons

Background of quantum mechanicsBackground of quantum mechanics Energy “Energy “baronsbarons” of the atom (motion) ” of the atom (motion)

Energy = ability to do workEnergy = ability to do work Potential energy = energy stored due to Potential energy = energy stored due to

position or locationposition or location Charge is negative (-) and particle is Charge is negative (-) and particle is alwaysalways

in motionin motion Capturing an atom and its orbiting electronCapturing an atom and its orbiting electron

Key to Electron StructureKey to Electron Structure

Count the number of electrons (and Count the number of electrons (and compare with abbreviation of compare with abbreviation of element)element)

Electrons are negative in charge and Electrons are negative in charge and in constant motionin constant motion

Electrons are in orbits around the Electrons are in orbits around the nucleusnucleus

Chemists sometimes refer to orbits as Chemists sometimes refer to orbits as “shells”“shells”

ElectronsElectrons

Example of Sulfur (Example of Sulfur (16163232S)S)

(16 electrons in 3 orbits)(16 electrons in 3 orbits)

Example of Electrons in Example of Electrons in ShellsShells

As electrons move between shells, they As electrons move between shells, they change potential energychange potential energy Hot summer day, bright sun and car topHot summer day, bright sun and car top

Light absorption by pigments and electrons Light absorption by pigments and electrons “jump” to higher shell (potential energy); give “jump” to higher shell (potential energy); give off energy when they drop back (kinetic energy)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 Excited electron “captured” by chlorophyll in

leaf and shuttled to a sugar molecule in its leaf and shuttled to a sugar molecule in its excited state (potential energy) until you release excited state (potential energy) until you release the energy the energy viavia digestion/respiration, allowing digestion/respiration, allowing the electron to “drop back” to a lower level the electron to “drop back” to a lower level (kinetic energy)(kinetic energy)

Some keys to Electron Some keys to Electron StructureStructure

Electrons reside in shells as a function of Electrons reside in shells as a function of quantum mechanics (1-4 orbits per shell)quantum mechanics (1-4 orbits per shell)

Never more than two electrons per orbit Never more than two electrons per orbit (Pauli’s Exclusion Principle)(Pauli’s Exclusion Principle)

Distribution of electrons is key to Distribution of electrons is key to understanding why elements and atoms understanding why elements and atoms behave the way they dobehave the way they do

Outermost electrons are called valence Outermost electrons are called valence electrons and they have a very special electrons and they have a very special significance in chemistrysignificance in chemistry

According to the Rutherford model of the atom, the volume of any atom is largely _______.

A. protons and neutronsB. electronsC. empty spaceD. covertly sequenced nuons

surrounded by proton

QuestionQuestion

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

A. jumping from one atom to anotherB. speeding up or slowing down in its orbitC. jumping from one orbit to anotherD. being removed from the atom

QuestionQuestion

According to the Rutherford model of the atom, the volume of any atom is largely _______.

A. protons and neutronsB. electronsC. empty spaceD. covertly sequenced nuons

surrounded by proton

QuestionQuestion

According to the Rutherford model of the atom, the volume of any atom is largely _______.

A. protons and neutronsB. electronsC. empty spaceD. covertly sequenced nuons

surrounded by proton

QuestionQuestion

Discussion QuestionDiscussion Question

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

Suppose the person next to you says that Suppose the person next to you says that she has an instrument that can identify the she has an instrument that can identify the occurrence of specific atoms (e.g., sodium, occurrence of specific atoms (e.g., sodium, hydrogen, sulfur) based on the energy hydrogen, sulfur) based on the energy patterns/signatures of electrons (PE and KE) patterns/signatures of electrons (PE and KE) in atoms on that planet. Could she be right?in atoms on that planet. Could she be right?

Discussion QuestionDiscussion Question

Models are one of the key investigative tools Models are one of the key investigative tools in the scientific method. Models of our in the scientific method. Models of our understanding of the atom are a classic understanding of the atom are a classic example of the role that models play in the example of the role that models play in the sciences. sciences.

List five features of models – not the specifics List five features of models – not the specifics of any given model, but rather how models of any given model, but rather how models are constructed, how they serve to help are constructed, how they serve to help promote our understanding of natural promote our understanding of natural systems, and their fallibility.systems, and their fallibility.

Discussion QuestionDiscussion Question

You are in the atomic world, having won You are in the atomic world, having won a free all day ride on an electron nested a free all day ride on an electron nested within a carbon atom. Your peers are within a carbon atom. Your peers are jealous (hang with me on this). jealous (hang with me on this).

You are seated on an electron, duly You are seated on an electron, duly belted down with a safety harness and off belted down with a safety harness and off you go. you go.

Twenty-four hours later you return to life Twenty-four hours later you return to life at GMU. List the cardinal features of at GMU. List the cardinal features of your “ride”.your “ride”.