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 Dalton’s atomic

theory

Describe Avogadro’s Hypothesis

Glimpse into the interior

What we “see” now

with scanning tunneling

microscope could only

be speculated on 2,000

years ago

The scale of things

From a distance sand looks smooth and continuous

Up close it’s made of particles

Each particle looks smooth

Up closer it is made of particles - atoms

400 BC

Beginnings of the atom

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’ idea is not much different from the modern atom with some modifications...

Atoms are not uncuttable

Atoms are all very small

Atoms are themselves made of particles

Stop forward movement:

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

intellectual activity in Europe

Science and maths continued in Persian Empire (golden age)

The major “scientific” activity was alchemy, largely the pursuit of the transformation of matter into gold and the elixir of life

Uncritical acceptance of Greek thinking about matter lingered until the Age of Enlightenmen 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

Models for nature were based on observation and experiment

1780Re-emergence of the atom:

Daniel Bernoulli’s 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 and the elements

Beginning of the periodic table and the concept of elements

Introduced concept of compounds – elements combined

Demonstrated Law of Conservation of Mass

No additional insights into the atom at this stage.

Chemical laws and the case 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 Dalton’s 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

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

22 CO

CO

MassO

MassO

1808

Gay-Lussac law of combining

volumes

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…

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 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

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 hydrogen

Coincidence or significance?

Proposed that atoms of heavier elements are made of hydrogen atoms

Implies that larger atoms comprise smaller units

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

but 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…?