Mileposts on the road to the atom All matter is made from elements What makes elements unique? A component unique to each element A common component but assembled in a different way

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

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 lingered until the Age of Enlightenment The modern scientific era began in the 17th century, pioneered by a few inspired individuals

1780 Daniel Bernoulli’s development of the kinetic theory of gases based on the concept of atoms Atoms were assumed to be hard round spheres which behaved much like billiard balls. The kinetic theory of gases, which was developed using this hypothesis, was remarkably successful in describing the experimental behaviour of gases. It is still used, with scarcely any modification, today

Introduced concept of compounds 1790 Antoine Lavoisier compiled a list of elements: substances that could not be broken down into simpler substances. The beginning of assembly of the periodic table and the notion that there are some materials (elements), from which all the other substances are derived. Introduced concept of compounds Demonstrated Law of Conservation of Mass 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 - musing on atoms: John Dalton’s atomic theory 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

Significance of the Law of Multiple Proportions 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

1808 Gay-Lussac law of combining volumes for gases. 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 Dalton’s unproven belief that the formula of water was HO

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

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 they are diatomic molecules? Matter conserved, and each volume contains same number of particles

1811 Amadeo Avogadro’s hypothesis Solved riddle of gas volumes by positing that the molecules in the gas contained two atoms. Most elemental gases are diatomic Second part of hypothesis was that the same volumes of all gases contain the same number of particles. Results conflicted with Dalton’s views and were not recognized for nearly 50 years 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…

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 the atoms cannot be modified what would the attractive forces be? What about those other experimental observations accumulating…?