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  • Atoms: discovering the

    stuff that stuff is made of

    We know all matter is made from elements

    What makes each element unique?A component unique to each element?

    A common component but assembled in a different way?

  • Learning Objectives

    Describe what is meant by atom

    Describe Law of Conservation of Mass and

    Definite Proportions

    Identify main features of Daltons atomic


    Describe Avogadros Hypothesis

  • Glimpse into the interior

    What we see now

    with scanning tunneling

    microscope could only

    be speculated on 2,000

    years ago

  • 400 BC

    Proposal that matter is particulate

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

  • Democritus legacy

    No indication about the size of these atoms. In

    fact Democritus atoms could be extremely


    Democritus idea has endured with some


    Atoms are not uncuttable

    Atoms are all very small

    Atoms are themselves made of particles

  • Science in the dark ages

    The decline of Greek civilization saw a concomitant decline of intellectual activity in Europe

    The major scientific activity was alchemy, largely the pursuit of the transformation of matter into gold

    Uncritical acceptance of Greek thinking about matter lingered until the Age of Enlightenment Matter is made of the four elements (earth, wind and

    fire...and water)

    The modern scientific era began in the 17th century, pioneered by a few inspired individuals who broke free from long-established conventions. Modern science is based on observation and reason

  • 1780Re-emergence of the atom:

    Daniel Bernoullis kinetic theory of gases

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

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

    It is still used, with scarcely any modification

  • 1790

    Order out of chaos

    Antoine Lavoisier defines elements: substances that

    could not be broken down into simpler substances. Beginnings of periodic table

    and definition of elements

    Introduced concept of compounds (combination of elements)

    Demonstrated Law of Conservation of Mass

    Discredited phlogiston theory

    No additional insights into the atom at this stage.

  • Chemical composition laws and

    arguments for an atomic world Law of Conservation of Mass:

    Matter is neither created nor destroyed in the course of a chemical reaction. Does not apply to nuclear changes

    Law of Definite Proportions:

    In forming compounds, elements combine together in definite mass ratios No knowledge of actual atom ratios at this stage, but how

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

    Strong indication for combination of the atoms in simple ratios

  • 1803

    John Daltons atomic musings Sample of any element contains tiny

    particles called atoms

    Atoms cannot be subdivided, created or destroyed Law of Conservation of Matter

    All atoms of the same element are the same

    All atoms of different elements are different

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

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

  • Significance of the Law of Multiple


    With benefit of knowledge of chemical composition, consider example of carbon dioxide (CO2) and carbon monoxide (CO)

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

    22 CO




  • 1808

    Gay-Lussac law of combining

    volumes: atomic implications Gases react with other gases

    to give products, in volumes which have simple whole number ratios.

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

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

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

  • Mystery of the gas volumes

    Combining H and O in 1:1 ratio (Daltons

    proposed ratio) does not satisfy Conservation

    of Matter

    Needs another box of O atoms

  • H:O = 2:1

    However, the 2:1 ratio (as suggested by

    volume ratio of reactants) predicts only 1 vol

    of H2O, not 2 vols as observed?

  • Enter Avogadro:

    What if hydrogen and oxygen are

    diatomic molecules? Matter conserved, and each volume contains

    same number of particles

    Seems perfect right? Dalton couldnt accept

    either formula of water or atoms of same

    element combining

  • 1811

    Amadeo Avogadros hypothesis

    Hypothesis that same volumes of all gases

    contain same number of particles.

    Results conflicted with Daltons views and were

    not recognized for nearly 50 years

    Solved riddle of gas volumes by positing

    that gas molecules contained two atoms.

    Most elemental gases are diatomic

    Key result: Able to calculate the relative

    atomic masses of the elements:

    development of a scale of atomic weight.

  • 1815

    William Prout weights of atoms appear to be

    simple multiples of the hydrogen atom.

    Coincidence or significance?

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

    Implication that larger atoms comprise smaller units

    Partial truth: there are common factors between atoms of different elements

    but they are not H atoms

  • Development of atomic weight scale

    elimination of the missing link

    Experimentally mass could be measured

    Without knowledge of atomic mass, impossible to

    know how many atoms of one element combine with


    Essential to know number of atoms to understand

    chemistry write chemical formulae

    Atomic weight scale, largely enabled by Avogadro,

    provides link between experimental observables and

    numbers of atoms

  • Unanswered questions

    What features of the atoms are responsible for

    the differences in element properties?

    How can atoms actually bind together in

    compounds to give substances of different

    properties if atoms cannot be modified? What

    are the attractive forces?

    What about those other experimental

    observations accumulating?


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