Atoms and Elements Atoms and Elements 29 September 200529 September 2005

PhysicsPhysics

ChemistryChemistry AstronomyAstronomy

GeologyGeology

BiologyBiology

TopicsTopics

Discovery of atom structure and Discovery of atom structure and functionfunction

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 orbits/shells Electrons and orbits/shells Valence electronsValence electrons

Scientific MethodScientific Method

Observations

Hypothesis

Testing Hypothesis

Rejection/Acceptance

Models in the SciencesModels in the Sciences

Example: Dynamics of Example: Dynamics of family interactionsfamily interactions

Example: Structure of Example: Structure of the atom (protons, the atom (protons, neutrons and neutrons and electronselectrons

Models are an Models are an abstraction and abstraction and meant to be meant to be challengedchallenged Protons and

NeutronElectrons in

orbits

Crystals: Structure and Crystals: Structure and ColorColor

Compounds to Subatomic Compounds to Subatomic Particles:Particles:

A Hierarchy A Hierarchy

CompoundCompound

ElementElement

AtomAtom

Subatomic Subatomic ParticleParticle

Emergent Properties

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

Idea that there are smaller Idea that there are 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 with a very negatively charged with a very small masssmall mass

Testing the hypothesisTesting the hypothesis

Thompson’s ExperimentThompson’s Experiment

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

Idea that there are other smaller Idea that there are other smaller components of atoms, including components of atoms, including some that are positively chargedsome that are positively charged

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

Testing the hypothesisTesting the 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 (reflected 1/1000 deflected at large angle (reflected

back)back) 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/reject?Accept/reject?

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

Hypothesis and ModelHypothesis and Model

Model: Quantum MechanicsModel: Quantum Mechanics

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 known factsIncorporation of known facts Prediction of new properties yet to be discoveredPrediction of new properties yet to be discovered

QuestionsQuestions

Is a model in the sciences a form of Is a model in the sciences a form of an hypothesis?an hypothesis?

All of these models (when proven All of these models (when proven correct) resulted in scientific correct) resulted in scientific revolutions in physics. How are revolutions in physics. How are revolutions in the sciences analogous revolutions in the sciences analogous to revolutions in society?to revolutions in society?

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 pompous protons

QuestionQuestion

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

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

TopicsTopics

Discovery of atom structure and Discovery of atom structure and functionfunction

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 orbits/shells Electrons and orbits/shells Valence electronsValence electrons

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 EmergentEmergent propertyproperty: “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))

Compounds to Subatomic Compounds to Subatomic Particles:Particles:

A Hierarchy A Hierarchy

CompoundCompound

ElementElement

AtomAtom

Subatomic Subatomic ParticleParticle

Emergent Properties

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 have the same Atoms of the same element have the same number of subatomic particles, we number of subatomic particles, we abbreviate this as follows:abbreviate this 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 position Potential energy = energy stored due to position

or locationor location Charge is negative (-) and particle is Charge is negative (-) and particle is alwaysalways

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

may be difficultmay be difficult

Key to Electron StructureKey to Electron Structure

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

ElectronsElectrons

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

(16 electrons in 3 shells)(16 electrons in 3 shells)

What are shells and what are orbits?

Example of Electrons in Example of Electrons in ShellsShells

As electrons move among shells, they As electrons move among 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 orbit the electron to “drop back” to a lower orbit (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 Outermost electrons are called valence valence electronselectrons and they have a very special and they have a very special significance in chemistrysignificance in chemistry

Periodic Table of ElementsPeriodic Table of Elements

Concept: most stable state for an atom Concept: most stable state for an atom is one in which the outermost shell is is one in which the outermost shell is filled with the maximum number of filled with the maximum number of electronselectrons

11stst Shell (1 orbit; 2 electrons) Shell (1 orbit; 2 electrons) Hydrogen (Hydrogen (11

11H; 1 electron; stable ?)H; 1 electron; stable ?)

Helium (Helium (2244He; 2 electrons; stable ?)He; 2 electrons; stable ?)

Periodic Table’s 1Periodic Table’s 1stst Row Row Hydrogen and HeliumHydrogen and Helium

Periodic Table of ElementsPeriodic Table of Elements

22ndnd shell has 4 orbits with 2 electrons shell has 4 orbits with 2 electrons (maximum) per orbit (total of 8 (maximum) per orbit (total of 8 electrons/shell)electrons/shell)

Most stable configuration is:Most stable configuration is: 11stst shell filled with 2 electrons shell filled with 2 electrons 22ndnd shell filled with 8 electrons shell filled with 8 electrons Total of 10 electrons (Total of 10 electrons (1010

2020Ne)Ne)

22ndnd row of Periodic Table row of Periodic Table 8 elements8 elements

Periodic Table of ElementsPeriodic Table of Elements

33ndnd shell has 4 orbits with 2 electrons shell has 4 orbits with 2 electrons maximum per orbit (total of 8 maximum per orbit (total of 8 electrons/shell)electrons/shell)

Most stable configuration is the following:Most stable configuration is the following: 11stst shell filled with 2 electrons shell filled with 2 electrons 22ndnd shell filled with 8 electrons shell filled with 8 electrons 33rdrd shell filled with 8 electrons shell filled with 8 electrons Total of ___ electrons (Total of ___ electrons (1818

4040Ar)Ar) 33ndnd row of Periodic Table row of Periodic Table

8 elements (list and relate to the above)8 elements (list and relate to the above)

Periodic Table of ElementsPeriodic Table of Elements

Number of elements in a row is not Number of elements in a row is not chance but reflects the maximum chance but reflects the maximum number of electrons in the outermost number of electrons in the outermost shellshell Row 1 = 2Row 1 = 2 Row 2 = 8Row 2 = 8 Row 3 = 8Row 3 = 8 Row 4 = 18Row 4 = 18 etcetc

Discussion QuestionDiscussion Question

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

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

Discussion QuestionDiscussion Question

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

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

Discussion QuestionDiscussion Question

In many ways, the Periodic Table is a In many ways, the Periodic Table is a “model” to help scientists understand “model” to help scientists understand patterns in the natural world. List ways patterns in the natural world. List ways in which this model helps scientists to in which this model helps scientists to understand the natural world?understand the natural world?

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