/
Atomic Theory: Why do we believe in atoms? Atomic Theory: Why do we believe in atoms?

Atomic Theory: Why do we believe in atoms? - PowerPoint Presentation

ani
ani . @ani
Follow
342 views
Uploaded On 2022-06-08

Atomic Theory: Why do we believe in atoms? - PPT Presentation

Dr John Mitchell ID1004 Approaches to Reality For Years People Wondered What is the World Made of What is Matter I s Water Made of the Same Stuff as Rock Is Matter Continuous Is Matter Continuous ID: 915143

atomic atoms matter molecules atoms atomic molecules matter theory number elements weights water oxygen gas chemical law structure continuous

Share:

Link:

Embed:

Download Presentation from below link

Download Presentation The PPT/PDF document "Atomic Theory: Why do we believe in atom..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.


Presentation Transcript

Slide1

Atomic Theory:Why do we believe in atoms?

Dr John Mitchell

ID1004: Approaches to Reality

Slide2

For Years People Wondered …

Slide3

What is the World Made of?

Slide4

What is Matter?

Slide5

Is Water Made of the Same Stuff as Rock?

Slide6

Is Matter Continuous?

Slide7

Is Matter Continuous?

Slide8

Is Matter Continuous?

Slide9

Indivisible, no microscopic structure.

Is Matter Continuous?

Slide10

Is Matter Discrete?

Slide11

Is Matter Discrete?

Slide12

Is Matter Discrete?

Composed of microscopic particles,

p

robably with void between them.

Slide13

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?

Slide14

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.

Slide15

Leucippus and his pupil Democritus

Leucippus

Democritus

Time: approximately mid- to-late 5

th

century BC.

Place: Greece.

Slide16

Leucippus &Democritus’s Beliefs

Matter is composed of indivisible, indestructible atoms, in continuous motion through an empty void.

There are many

d

ifferent kinds of atom, varying in shape and size. Atoms can be connected together. Shape, size and connections relate to the observed properties of materials.

Slide17

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).

Slide18

Reasonable Reasons for their Beliefs

Movement requires a void.

Materials have an indivisible ‘essence’.

A vase is still clay, even if it is broken.

Slide19

Reasonable Reasons for their Beliefs

Movement requires a void.

Materials have an indivisible ‘essence’.

A vase is still clay, even if it is broken.

Movement was surmised, not observed.

Logically possible for various materials all to be continuous, but made of ‘different stuff’.

Not proofs!

Slide20

Plato’s Perfect Solids as Atoms

The cube is the only Platonic solid to tessellate without gaps – appropriate for solidity of earth.

Plato

Slide21

The Rise of Experimental Science

Slide22

The Rise of Experimental Science

By the late 18

th

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 18

th

century, at least 50 more in 19

th

century).

This allowed compounds to be analysed to obtain compositions in terms of component elements.

Slide23

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.

Slide24

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.

Slide25

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.

Slide26

Lavoisier had worked as a tax collector and as an aristocrat found himself on the wrong side in the French revolution.

Slide27

Lavoisier had worked as a tax collector and as an aristocrat found himself on the wrong side in the French revolution.

Slide28

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 mass.

Example: All samples of mercury oxide are 92.6% mercury by mass and 7.4% oxygen.

Jons

Jacob Berzelius (1779-1848) later confirmed and popularised this finding.

Slide29

3. Law of Multiple Proportions

Manchester chemist John Dalton (1766-1844) 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.

Slide30

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.

Slide31

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.

Slide32

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.

Slide33

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 numbers.

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.

Slide34

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 (O

2

, H

2

, N

2

).

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.

Slide35

The Composition of Water

2

litres of hydrogen gas + 1 litre of

oxygen

gas

 2 litres of

water vapour.

2H

2

+ O

2

 2

H

2

O

How can we explain this ratio using Avogadro’s Law?

The formula of water is H

2

O, not HO as previously assumed by Dalton. Thus, the atomic weight of oxygen is 16 times that of hydrogen (not 8 times).

Slide36

Cannizzaro and the K

arlsruhe 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.

Slide37

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.

Slide38

Couper and Molecular Structure

Scottish chemist Archibald Scott

Couper

(1831-1892) 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 …

Slide39

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.

Slide40

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.”

Slide41

Structural Formulae

Slide42

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.

Slide43

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.

Slide44

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).

Slide45

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.

Slide46

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

Slide47

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)

Slide48

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).

Slide49

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)

Slide50

How Big is the Atom?

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

-19

C

N

A

= F/e = 6.02

10

23

mol

-1

Michael Faraday

(1791-1867)

Robert Millikan

(1868-1953)

Slide51

X-ray Crystallography

Max von Laue (1879-1960, Nobel Laureate 1914)

r

ealised in 1912 that X-rays would have the correct wavelength to be diffracted by layers of atoms in a crystal.

Slide52

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.

father

son

Slide53

X-ray Crystallography

The diffraction pattern of spots on a photographic plate allowed the positions of atoms to be inferred,

via

some intimidating mathematics.

Slide54

X-ray Crystallography

The technique doesn’t image individual atoms or molecules, but relies on the repeating nature of the structure.

Slide55

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!

Slide56

3-D Structure

X-ray crystallography showed chemists the three dimensional structures of molecules, as well as confirming 2D structures.

Dorothy Hodgkin

1910-1994

Pioneering British

crystallographer

Slide57

Including that of DNA

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.

Rosalind Franklin

1920-1958

Maurice Wilkins

1916-2004

Slide58

Atomic Theory in the 20th Century

By the early 20

th

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.

Slide59

Seeing Atoms

Recent advances in Atomic Force Microscopy (AFM) allow images to be generated down to the atomic scale.

Slide60

Epilogue

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).

Slide61

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.