Atoms: discovering the stuff that stuff is made of · Atoms: discovering the stuff that stuff is...

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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?

Transcript of Atoms: discovering the stuff that stuff is made of · Atoms: discovering the stuff that stuff is...

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

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

large

Democritus’ idea has endured with some

modifications:

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 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 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 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: 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 Dalton’s belief that formula of water was HO (principle of simplicity)

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 hydrogen and oxygen are

diatomic molecules? Matter conserved, and each volume contains

same number of particles

Seems perfect right? Dalton couldn’t accept

either formula of water or atoms of same

element combining

1811

Amadeo Avogadro’s hypothesis

Hypothesis that same volumes of all gases

contain same number of particles.

Results conflicted with Dalton’s 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

another

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…?