1. 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

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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

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

11. 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…?

12. Enter Avogadro: What if they are diatomic molecules?
Matter conserved, and each volume contains same number of particles

13. 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.

14. 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…

15. 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…?