To Understand The Atom Is To To Understand The Atom Is To Understand Chemistry: Mileposts Understand Chemistry: Mileposts

on the road to the atomon the road to the atomAll matter is made from elementsAll matter is made from elementsWhat makes elements unique?What makes elements unique?

A component unique to each elementA component unique to each elementA common component but assembled in a different wayA common component but assembled in a different way

Learning ObjectivesLearning Objectives

Describe what is meant by “atom”Describe what is meant by “atom” Describe Law of Conservation of Mass and Describe Law of Conservation of Mass and

Definite ProportionsDefinite Proportions Identify main features of Dalton’s atomic Identify main features of Dalton’s atomic

theorytheory Describe Avogadro’s HypothesisDescribe Avogadro’s Hypothesis Identify main features of the periodic tableIdentify main features of the periodic table

Glimpse into the interiorGlimpse into the interior

What we “see” now What we “see” now with scanning tunneling with scanning tunneling microscope could only microscope could only be speculated on 2,000 be speculated on 2,000 years agoyears ago

400 BC400 BCProposal that matter is particulate Proposal that matter is particulate

Democritus posed the question: could matter be Democritus posed the question: could matter be subdivided forever? He answered no: there is a limit subdivided forever? He answered no: there is a limit to the extent to which matter can be subdivided, and to the extent to which matter can be subdivided, and he coined the term atom from the Greek for he coined the term atom from the Greek for uncuttable uncuttable a-tomos.a-tomos.

Democritus’ legacyDemocritus’ legacy

No indication about the size of these No indication about the size of these atomsatoms. In . In fact Democritus atoms could be extremely fact Democritus atoms could be extremely largelarge

Democritus’ idea has endured with some Democritus’ idea has endured with some modifications:modifications: Atoms are not uncuttableAtoms are not uncuttable Atoms are all very smallAtoms are all very small Atoms are themselves made of particlesAtoms are themselves made of particles

Science in the dark agesScience in the dark ages The decline of Greek civilization saw a concomitant The decline of Greek civilization saw a concomitant

decline of intellectual activity in Europedecline of intellectual activity in Europe The major “scientific” activity was alchemy, largely The major “scientific” activity was alchemy, largely

the pursuit of the transformation of matter into gold the pursuit of the transformation of matter into gold Uncritical acceptance of Greek thinking about matter Uncritical acceptance of Greek thinking about matter

lingered until the Age of Enlightenmentlingered until the Age of Enlightenment Matter is made of the four elements (earth, wind and Matter is made of the four elements (earth, wind and

fire...and water)fire...and water) The modern scientific era began in the 17The modern scientific era began in the 17 thth century, century,

pioneered by a few inspired individuals who broke pioneered by a few inspired individuals who broke free from long-established conventions. Modern free from long-established conventions. Modern science is based on observation and reasonscience is based on observation and reason

17801780Re-emergence of the atom:Re-emergence of the atom:

Daniel Bernoulli’s kinetic theory of gasesDaniel Bernoulli’s kinetic theory of gases

Atoms were assumed to be hard round spheres Atoms were assumed to be hard round spheres which behaved much like billiard balls. which behaved much like billiard balls.

Success of kinetic theory in describing gases Success of kinetic theory in describing gases lends support to atomic description of matter. lends support to atomic description of matter.

It is still used, with scarcely any modificationIt is still used, with scarcely any modification

17901790Order out of chaosOrder out of chaosAntoine Lavoisier defines elements: substances that Antoine Lavoisier defines elements: substances that could not be broken down into simpler substances.could not be broken down into simpler substances. Beginnings of periodic table Beginnings of periodic table

and definition of and definition of elementselements Introduced concept of Introduced concept of

compoundscompounds (combination of (combination of elements)elements)

Demonstrated Law of Demonstrated Law of Conservation of MassConservation of Mass

No additional insights into the No additional insights into the atom at this stage.atom at this stage.

Chemical composition laws and Chemical composition laws and arguments for an atomic worldarguments for an atomic world

Law of Conservation of Mass:Law of Conservation of Mass:Matter is neither created nor destroyed in the course of a Matter is neither created nor destroyed in the course of a

chemical reaction. chemical reaction. Does not apply to nuclear changesDoes not apply to nuclear changes

Law of Definite Proportions:Law of Definite Proportions:In forming compounds, elements combine together in In forming compounds, elements combine together in

definite mass ratiosdefinite mass ratios No knowledge of actual atom ratios at this stage, but how No knowledge of actual atom ratios at this stage, but how

else to explain fixed quantities unless the elements were else to explain fixed quantities unless the elements were present as discrete bodiespresent as discrete bodies

Strong indication for combination of the atoms in simple Strong indication for combination of the atoms in simple ratiosratios

1803 - musing on atoms:1803 - musing on atoms:John Dalton’s atomic theory John Dalton’s atomic theory

Sample of any element contains Sample of any element contains tiny particles called atomstiny particles called atoms

Atoms cannot be subdivided, Atoms cannot be subdivided, created or destroyedcreated or destroyed LawLaw ofof ConservationConservation ofof MatterMatter

All atoms of the same element are All atoms of the same element are the samethe same

All atoms of different elements are All atoms of different elements are differentdifferent

Atoms combine together in simple Atoms combine together in simple whole number ratioswhole number ratios Law of Multiple Proportions: The ratio of Law of Multiple Proportions: The ratio of

the masses of one element combined with the masses of one element combined with the same mass of another element is a the same mass of another element is a simple whole numbersimple whole number

Significance of the Law of Multiple Significance of the Law of Multiple ProportionsProportions

With benefit of knowledge of chemical With benefit of knowledge of chemical composition, consider example of carbon composition, consider example of carbon dioxide (COdioxide (CO22) and carbon monoxide (CO)) and carbon monoxide (CO)

Combination of finite components (atoms) of Combination of finite components (atoms) of C and O in simple number ratios is the most C and O in simple number ratios is the most sensible explanation of the Lawsensible explanation of the Law

22 CO

CO

MassO

MassO

18081808Gay-Lussac law of combining Gay-Lussac law of combining volumes: atomic implications volumes: atomic implications

Gases react with other gases to give products, in Gases react with other gases to give products, in volumes which have simple whole number ratios. volumes which have simple whole number ratios.

““Mystery” of the reaction of hydrogen and oxygen to Mystery” of the reaction of hydrogen and oxygen to provide water.provide water.

2 vols of hydrogen + 1 vol of oxygen gives 2 vols of 2 vols of hydrogen + 1 vol of oxygen gives 2 vols of water (Why not 1 vol?)water (Why not 1 vol?)

The ratio of 2:1 was inconsistent with Dalton’s belief The ratio of 2:1 was inconsistent with Dalton’s belief that formula of water was HO (principle of that formula of water was HO (principle of simplicity)simplicity)

Mystery of the gas volumesMystery of the gas volumes

Combining H and O in 1:1 ratio (Dalton’s Combining H and O in 1:1 ratio (Dalton’s proposed ratio) does not satisfy Conservation proposed ratio) does not satisfy Conservation of Matter…of Matter…

H:O = 2:1H:O = 2:1

However, the 2:1 ratio (as suggested by However, the 2:1 ratio (as suggested by volume ratio of reactants) predicts only 1 vol volume ratio of reactants) predicts only 1 vol of Hof H22O, not 2 vols as observed…?O, not 2 vols as observed…?

Enter Avogadro:Enter Avogadro:What if they are What if they are diatomicdiatomic molecules? molecules?

Matter conserved, Matter conserved, andand each volume contains each volume contains same number of particlessame number of particles

18111811Amadeo Avogadro’s hypothesis Amadeo Avogadro’s hypothesis

Solved riddle of gas volumes by positing Solved riddle of gas volumes by positing that the gas molecules contained that the gas molecules contained twotwo atoms. atoms. Most elemental gases are diatomicMost elemental gases are diatomic

Second part of hypothesis was that the Second part of hypothesis was that the same volumes of all gases contain the same volumes of all gases contain the same number of particles.same number of particles.Results conflicted with Dalton’s views and were Results conflicted with Dalton’s views and were

not recognized for nearly 50 yearsnot recognized for nearly 50 years

Key result: Able to calculate the relative Key result: Able to calculate the relative atomic masses of the elements: atomic masses of the elements: development of a scale of atomic weight.development of a scale of atomic weight.

18151815William Prout weights of atoms appear to be William Prout weights of atoms appear to be

simple multiples of the hydrogen atom.simple multiples of the hydrogen atom.

Coincidence or significance?Coincidence or significance?

This led to the proposal that atoms of heavier This led to the proposal that atoms of heavier elements were made from hydrogen atomselements were made from hydrogen atoms

Implication that larger atoms comprise smaller unitsImplication that larger atoms comprise smaller units Partial truth: there are common factors between atoms Partial truth: there are common factors between atoms

of different elements…of different elements… but they are not H atoms…but they are not H atoms…

Development of atomic weight scale Development of atomic weight scale – elimination of the missing link– elimination of the missing link

Experimentally mass could be measuredExperimentally mass could be measured Without knowledge of atomic mass, impossible to Without knowledge of atomic mass, impossible to

know how many atoms of one element combine with know how many atoms of one element combine with anotheranother

Essential to know number of atoms to understand Essential to know number of atoms to understand chemistry – write chemical formulaechemistry – write chemical formulae

Atomic weight scale, largely enabled by Avogardro, Atomic weight scale, largely enabled by Avogardro, provides link between experimental observables and provides link between experimental observables and numbers of atomsnumbers of atoms

Unanswered questionsUnanswered questions

What features of the atoms are responsible for What features of the atoms are responsible for the differences in element properties?the differences in element properties?

How can atoms actually bind together in How can atoms actually bind together in compounds to give substances of different compounds to give substances of different properties if atoms cannot be modified? What properties if atoms cannot be modified? What are the attractive forces?are the attractive forces?

What about those other experimental What about those other experimental observations accumulating…? observations accumulating…?