Atomic Theory - University of St Andrews

ID1004: Approaches to Reality
Atomic Theory:
Why do we believe in atoms?
Dr John Mitchell
For Years People Wondered …
What is the World Made of?
What is Matter?
Is Water Made of the Same Stuff as Rock?
Is Matter Continuous?
Is Matter Continuous?
Is Matter Continuous?
Is Matter Continuous?
Indivisible, no
microscopic structure.
Is Matter Discrete?
Is Matter Discrete?
Is Matter Discrete?
Composed of microscopic particles,
probably with void between them.
Matter: Continuous or Discrete?
Related questions:
What makes rock rocky?
What makes water watery?
Do materials have some essence, independent of objects formed from them?
Ancient Times: Philosophy, not Experiment
In ancient times, the question
of the continuity or discreteness
of matter was not amenable to
experimental investigation …
… it was a matter for philosophical
speculation, though the Greek
expertise in geometry and
simple observations of the
world could also be relevant.
Leucippus and his pupil Democritus
Time: approximately mid- to-late 5th century BC.
Place: Greece.
Leucippus &Democritus’s Beliefs
Matter is composed of indivisible,
indestructible atoms, in continuous motion
through an empty void.
There are many different kinds of atom,
varying in shape and size. Atoms can be
connected together. Shape, size and
connections relate to the observed properties
of materials.
Leucippus &Democritus’s Beliefs
Salt atoms: pointed, sharp (presumably relates
to taste).
Water atoms: smooth (flow freely).
Air atoms: light and rapidly moving.
Iron atoms: tightly bound together by
connections (hence a strong solid material).
Reasonable Reasons for their Beliefs
Movement requires a void.
Materials have an indivisible ‘essence’.
A vase is still clay, even if it is broken.
Reasonable Reasons for their Beliefs
Not proofs!
Movement requires a void.
Movement was surmised, not observed.
Materials have an indivisible ‘essence’.
A vase is still clay, even if it is broken.
Logically possible for various materials all to be
continuous, but made of ‘different stuff’.
Plato’s Perfect Solids as Atoms
The cube is the only Platonic solid
to tessellate without gaps –
appropriate for solidity of earth.
The Rise of Experimental Science
The Rise of Experimental Science
By the late 18th century, alchemy was giving way to chemistry.
The existence of chemical elements was recognised.
Numerous elements were being discovered (about 20 new
ones in the 18th century, at least 50 more in 19th century).
This allowed compounds to be analysed to obtain
compositions in terms of component elements.
The Rise of Experimental Science
Some key principles emerged that gave an experimental
basis for discussions of the nature of matter.
Atomic theory was to gain ground gradually, rather than
through one dramatic discovery, because the ideas of atoms
and molecules would allow chemists to make sense of the
chemical world.
Elements Combine in Fixed Proportions
The key idea underlying the acceptance of atomic theory
is that elements combine in fixed and simple proportions
to form compounds.
This is easily explained if we believe in atoms, much harder to
understand otherwise.
However, because different atoms have
different masses, correctly identifying these
ratios was not trivial.
1. Law of Conservation of Mass
Antoine Lavoisier (1743-1794) found that the
total mass of reactants is equal to the total
mass of products in a chemical reaction.
Lavoisier had worked as a tax collector and as
an aristocrat found himself on the wrong side in
the French revolution.
Lavoisier had worked as a tax collector and as
an aristocrat found himself on the wrong side in
the French revolution.
2. Law of Definite Proportions
Joseph Proust (1754-1826) stated in 1806
that every sample of a given chemical
compound has an identical elemental
composition, in terms of percentages by
Jons Jacob Berzelius (1779-1848) later
confirmed and popularised this finding.
Example: All samples of mercury oxide are 92.6% mercury by mass and 7.4% oxygen.
3. Law of Multiple Proportions
Manchester chemist John Dalton (17661844) stated in 1803 that whenever
elements combine to form more than one
compound, the relative amounts of two
given elements in each compound will be in
ratios of small whole numbers.
Example: combine tin and oxygen.
One compound has 88.1% tin and 11.9% oxygen by mass (ratio 7.4 : 1).
The other compound has 78.8% tin and 21.2% oxygen (ratio 3.7 : 1).
Ratio of ratios = 7.4 : 3.7 = 2 : 1, a simple whole number ratio.
Put another way, 100g of tin combines either with 13.5g or 27g of oxygen – a factor of two.
Dalton’s Atomic Theory
John Dalton saw that these observed laws
made sense if the elements were formed of
atoms which could combine in simple ratios
to form compounds.
Dalton’s Atomic Theory
Dalton believed that …
Elements consist of tiny indivisible atoms.
All atoms of the same element are identical.
Atoms of different elements have different atomic weights.
Atoms of different elements combine to form
compounds of fixed stoichiometry.
Dalton’s Atomic Weights
Dalton also believed that stoichiometry
followed a Rule of Greatest Simplicity …
… which meant that he assumed water
to be HO and ammonia to be HN.
Since the correct stoichiometry of compounds was
unknown, early attempts to compile atomic weights were
error-prone. Incorrect assumptions about atomic weights no
doubt delayed the acceptance of atomic theory by impairing
its ability to explain the observations.
4. Law of Combining Volumes
Joseph Gay Lussac (1778-1850) found in
1808 that the ratios of the volumes of each
reactant and product gas in a chemical
reaction are ratios of simple small whole
3 litres of hydrogen gas + 1 litre of nitrogen gas  2 litres of ammonia gas.
2 litres of hydrogen gas + 1 litre of oxygen gas  2 litres of water vapour.
5. Avogadro’s Law
Amedeo Avogadro (1776-1856) made two
significant advances in atomic theory.
Firstly, he clarified the distinction between
atoms and molecules. Oxygen, hydrogen
and nitrogen, though elements, form
diatomic molecules (O2, H2, N2).
Secondly, around 1811, he interpreted the Law of Combining
Volumes in terms of atomic, or molecular, theory.
Equal volumes of any gas, at the same temperature and
pressure, contain equal numbers of molecules.
The Composition of Water
2 litres of hydrogen gas + 1 litre of oxygen gas  2 litres of water vapour.
How can we explain this ratio using Avogadro’s Law?
2H2 + O2  2H2O
The formula of water is H2O, not HO as previously assumed by
Dalton. Thus, the atomic weight of oxygen is 16 times that of
hydrogen (not 8 times).
Cannizzaro and the Karlsruhe Congress
Stanislao Cannizzaro (1826-1910) gave a paper at
the 1860 Karlsruhe congress, in which he
advocated Avogadro’s approach.
Cannizzaro is credited with persuading the
somewhat sceptical chemical community to
adopt a recognisably modern view of atoms,
molecules and atomic weights. This included
the reformed atomic weights H=1, C=12, O=16.
Structure in Chemistry
Over the second half of the 19th century it became
apparent that atomic and molecular theory could
both make sense of the disparate body of knowledge
about substances and reactions that chemistry had
become and form the basis for reliable predictions.
The key idea was that molecules each had a defined
structure based on connections between atoms.
The atoms themselves each had a valency, a set
number of connections they could make.
Couper and Molecular Structure
Scottish chemist Archibald Scott Couper (18311892) was a pioneer of the concept of valency
and of the use of simple structural formulae to
represent molecules.
Because of the (incorrect) atomic weights in use at the time, these structures have
too many oxygen atoms.
Sadly, Couper’s career and health were broken by a spectacular
row with his Professor when he was beaten into print by …
August Kekulé
August Kekulé (1829-1896), who is
most famous for proposing the ring
structure of benzene in 1865.
Equally important was his 1857 proposal of
tetravalent carbon.
Crum Brown & Molecular Structural Formulae
Edinburgh chemist Alexander Crum Brown
(1838-1922) developed the diagrammatic
representation of molecules from 1864, his
diagrams clearly showing single and double
bonded connections between atoms.
He also discovered the
double bond in ethene.
His diagrams were topological and deliberately two dimensional: “I do not mean to indicate
the physical, but merely the chemical position of the atoms.”
Structural Formulae
The Periodic Table
Dmitri Mendeleev (1834-1907), possibly the greatest of all chemists,
had created his periodic table on the basis of periodic chemical
properties and atomic weights that increased down a group and,
broadly speaking, increased along a period.
The Periodic Table
Dmitri Mendeleev (1834-1907), possibly the greatest of all chemists,
had created his periodic table on the basis of periodic chemical
properties and atomic weights that increased down a group and,
broadly speaking, increased along a period.
Arguments over Atomic Theory
Although most chemists by then accepted the reality of atoms, there
was still significant opposition as late as 1900. Amongst notable
opponents was physical chemist Wilhelm Ostwald (1853-1932,
Nobel Chemistry Laureate 1909).
A Useful Model?
Ludwig Boltzmann (1844-1906) developed the kinetic theory of
gases which seemed only to make sense if atoms or molecules really
existed. He was a strong advocate of atomic theory, but prepared to
accept a compromise whereby atomic theory was agreed to be a
useful model, but no absolute belief in its reality was required.
Frustration at the lack of acceptance of his theories, as well as probable
psychiatric illness, may well have contributed to his suicide.
Atomic Structure: A Surprise
Ernest Rutherford’s 1911 paper interpreting the results of Geiger
and Marsden (firing alpha particles at gold foil) brought a surprise …
… almost all the mass of the atom was concentrated in a
tiny region in the centre, the nucleus.
Ernest Rutherford, NZ-British physicist, 1871-1937
Atomic Number as an Observable
Manchester physicist Henry Moseley
(1887-1915) found that the frequencies of
the X-rays emitted by elements depended
directly on their atomic numbers.
This meant that atomic number was an experimentally
observable quantity. Moseley saw this as confirmation of
van den Broek’s hypothesis that the atomic number was
the positive charge of the nucleus.
Antonius van den Broek, Dutch lawyer & physicist (1870-1926)
Atomic Number as an Observable
Moseley’s observations also justified the need to leave
gaps in the periodic table, as Mendeleev had famously
done, which would be necessary if no known element had
a given value of atomic number (as then was true for 43,
61, 72, and 75; Tc, Pm, Hf, Re).
Moseley also showed that the
ordering of elements by atomic
number was sometimes different
from that by their atomic weights
(examples are Co, Ni and Ar, K).
Moseley’s death at Gallipoli in 1915 is widely credited with
changing British policy towards sending outstanding scientists
to the front line.
“In view of what he [Moseley] might still have accomplished ... his death might well have been
the most costly single death of the War to mankind generally.” (Isaac Asimov)
How Big is the Atom?
Michael Faraday
Robert Millikan
Avogadro’s number is the number of atoms in 12g of carbon, or
molecules in 18g of water, or formula units in 58.5g of NaCl etc.
The electric charge of a mole (Avogadro’s number) of electrons
was known to be equal to a constant found by Michael Faraday.
When Robert Millikan measured the charge on the electron in
1910, Avogadro’s number was now known:
F = 96,500 C mol-1
e = 1.610-19C
NA= F/e = 6.02 1023 mol-1
X-ray Crystallography
Max von Laue (1879-1960, Nobel Laureate 1914)
realised in 1912 that X-rays would have the
correct wavelength to be diffracted by layers of
atoms in a crystal.
X-ray Crystallography
British physicist Lawrence Bragg
(1890-1971) developed this idea
into Bragg’s Law.
He became the youngest ever Nobel Laureate in 1915,
sharing the Physics prize with his father William Bragg.
X-ray Crystallography
The diffraction pattern of spots on a
photographic plate allowed the positions
of atoms to be inferred, via some
intimidating mathematics.
X-ray Crystallography
The technique doesn’t image individual
atoms or molecules, but relies on the
repeating nature of the structure.
How Big is the Atom? (2)
Since X-ray crystallography tells us the size
and shape of the repeating unit in the
crystal, as well as the positions and number
of atoms, it allows us to count the atoms in
a given volume.
Combined with the (macroscopic) density,
this also means that we can count , for
instance, how many silicon atoms there are
in 28 grams of silicon. This is another
method of finding Avogadro’s number.
Note that atoms in crystals don’t have obvious boundaries!
3-D Structure
X-ray crystallography showed
chemists the three dimensional
structures of molecules, as well
as confirming 2D structures.
Dorothy Hodgkin
Pioneering British
Including that of DNA
Rosalind Franklin
Maurice Wilkins
In the early 1950s, the X-ray work of Maurice Wilkins and
Rosalind Franklin would show that DNA was a helix, as
interpreted by James Watson and Francis Crick.
Atomic Theory in the 20th Century
By the early 20th century it became apparent that classical physics
was inadequate for describing atomic properties and quantum
mechanical models were required. The partially successful Bohr
model of 1913 was replaced by the modern Schrödinger model
around 1926.
Seeing Atoms
Recent advances in Atomic Force Microscopy (AFM) allow
images to be generated down to the atomic scale.
Many materials, including water, are molecular,
their smallest particles consisting of from two
upwards of what we now call atoms. Molecules
have different shapes, sizes and properties.
Molecules, or the occasional monatomic
atoms, are indeed in continuous motion;
especially obvious in gases and liquids, but
they also vibrate in solids. Matter does indeed
contain empty space.
A typical rock, or indeed other substances like
glass, sand, salt or diamond, does not have
identifiable molecules. These materials are still
structured arrays of connected atoms (or ions).
What we call the “atom” did not turn out to
be the ultimate indivisible particle. In our
current physics, atoms are made of
electrons, protons and neutrons. While
electrons are indivisible, protons and
neutrons are composed of quarks. Not all
atoms of the same element are identical,
since isotopes have different atomic weights.
Typically, the atoms in our world are effectively
indestructible and remain an atom of the same
element indefinitely. However, we know that
some nuclei undergo radioactive decay and
nuclear fusion is an essential process in stars.

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