Proof for Modern Atoms

1
Proof for Modern Atoms
2
No Atoms?

• Aristotle said that matter was made of earth,
  air, fire, and water.

3
Atoms?

• Democritus said that matter was made of tiny,
  indivisible particles called atomos.

4
Smackdown!

• Aristotles theory was easier to understand and
  had visual proof so was accepted by the
  scientific community for about 2000 years.

5
What Changed?

• Joseph Proust (1754-1826) noted that when
  mixing chemicals together to form new
  substances, they ALWAYS reacted in a particular
  mass ratio
• It didnt matter how much you used, you always go
  the same result.

6
Prousts Experiment
oxygen
hydrogen
water



8 g
0 g
1 g
0 g
9 g
10 g
1 g
9 g
2 g
0 g
8 g
2 g
9 g
0 g
1 g
16 g
2 g
18 g
0 g
0 g
16 g
3 g
18 g
0 g
1 g
Conclusion oxygen and hydrogen combined in only
an 81 ratio Anything else will be unreacted and
left over.
7
Was is always 81?

• For oxygen and hydrogen, yes.
• For nitrogen and hydrogen, NO.
• It was found that nitrogen to hydrogen ALWAYS
  combined in a 51 ratio of masses.
• He did MANY experiment like these and found that
  for every reaction, there was some ratio of
  masses of the ingredients.

8
So What?

• How did these elements of oxygen and hydrogen
  know to only combine in an 81 ratio?
• Proust put forth the Law of Definite Proportions
  
• Elements combine in definite mass ratios to form
  compounds.
• What about these elements made it so?

9
Then Came Dalton...

• John Dalton, a self-educated English
  schoolteacher, began to do experiments that
  cumulated in 1803 with a series of postulates.

10
Daltons Hypotheses

• Elements were made of atoms
• These atoms cant be created or destroyed
• All atoms of the same element are identical
• Atoms combine in whole-number ratios to form
  compounds
• Atoms of different elements had different masses

11
Blah, blah, blah
Water

• Dalton said that because hydrogen and oxygen
  always combine in a 81 ratio, water must be a
  combination of 1 hydrogen atom and 1 oxygen atom
  that weighs 8 times as much!

O
H
12
Finally...

• Dalton was able to make sense of what Proust saw
  in his experiments.
• Lets review...

oxygen
hydrogen
water



8 g
0 g
1 g
0 g
9 g
nitrogen
hydrogen



ammonia
5 g
0 g
1 g
0 g
6 g
13
Relative Masses

• Oxygen must be 8 times as heavy as hydrogen
• Nitrogen must be 5 times as heavy as hydrgoen
• Thus you could set up a table of RELATIVE atomic
  masses

H 1 amu
O 8 amu
N 5 amu
14
So Thats It?!!!

• No way
• Whats wrong with Daltons idea of water?

15
Hello?

• Dont we know water now to be H2O?
• Dalton would have you believe that water is just
  HO (snicker, snicker)
• Also, look at the Periodic table
• What is the mass of Hydrogen? Oxygen?
• Those arent an 81 ratio, huh.
• How did this change?

16
Again with the French...

• In 1808, Joseph Gay-Lussac
• (again, snicker, snicker) reported that when
  gases react, their volumes react in ratios of
  small whole numbers

17
By the way...

• Were Dalton and Gay-Lussac separated at Birth?

(Actually they were born 12 years apart, Dalton
in 1766, Gay-Lussac in 1778)
18
Anyhoo...

• Didnt that sound familiar?
• Gay-Lussac When gases react, their volumes are
  in the ratio of small whole numbers
• Proust Elements comine in definite mass ratios
  to form compounds
• Hmm..again with the ratios but with different
  properites!

19
So what did he find?

• Gay-Lussac found that when hydrogen and oxygen
  combined, their VOLUMES reacted in a 212 ratio
• 2 L Hydrogen 1 L Oxygen --gt 2 L Water
• Conclusion water should be H2O, not HO as
  Dalton says

20
But how could this be?

• Dalton was not buying this theory because of his
  own atomic theory
• Atoms would have to split to accommodate this new
  data...

21
The Visual Aid...
Hydrogen
Water
Oxygen
2 H plus 1 O should only make 1 water, not two
22
For that to happen...
Water
Hydrogen
Oxygen
Notice how there are now twice as many oxygen
atoms on the right?
23
But Data is Data...

• Although there was a fierce debate between these
  two heavyweights, no conclusion could be reached
• Daltons atomic ideas had lots of support
• Gay-Lussacs results couldnt be denied
• So howd they resolve it?
• Thunderdome!
• No.

24
They didnt.

• Somebody else did.
• In 1811 (thats three YEARS later for those of
  you playing along at home), Amadeo Avogadro
  (1776-1856) rectified Daltons atomic theory with
  the experimental proof of Gay-Lussac

25
His Proposal...

• Atoms of hydrogen and oxygen were actually
  Diatomic
• This meant that instead of being just one atom,
  they were TWO atoms bonded together

H
H
H
Daltons Hydrogen
Avogadros Hydrogen
26
Did this break any of the rules?

• Daltons rules
• Elements were made of atoms
• These atoms cant be created or destroyed
• All atoms of the same element are identical
• Atoms combine in whole-number ratios to form
  compounds
• Atoms of different elements had different masses

• Avogadros idea

27
Its so simple...
Water
Hydrogen
Oxygen
Count em up on BOTH sides!
28
Other effects...

• This also messed with Daltons relative mass
  table
• If you remember
• H 1 O 8
• From Prousts experiment
• Dalton said that Oxygen was 8 times as heavy as
  hydrogen

29
But examine the atoms...
Water
Hydrogen
Oxygen
There are twice as many H as O
30
Add up the masses
Hydrogen
Water
Oxygen
10 x 8 80
Hey, thats not an 81 ratio!
20 x 1 20
80/20 41 ratio
31
So how do explain it?

• The 81 ratio cannot be denied
• But now the atoms are Diatomic
• Thus, oxygen must weigh twice as much or 16 times
  as much as hydrogen

32
Add up the masses
Hydrogen
Water
Oxygen
10 x 16 160
Hey, that fixes it!
20 x 1 20
160/20 81 ratio
33
Check it out.

• Avogadros ideas fit not only Daltons theories
  but Gay-Lussacs data
• Check out the Periodic Table
• The mass of Oxygen.16
• The mass of Hydrogen1
• So it all worked out for everybody, right?

34
Any they lived happily ever after...

• Actuallyno.
• Dalton refused to embrace Avogadros solution and
  tried for the rest of his life to disprove
  Gay-Lussacs results.
• The debate raged on for years until everybody
  died (Dalton in 1844, Gay-Lussac in 1850, and
  Avogadro in 1856) and the ideas of Avogadro were
  reintroduced by Cannizzaro in 1860.

35
The Moral of the Story...

• A new scientific truth does not triumph by
  convincing its opponents and making them see the
  light, but rather because its opponents
  eventually die and a new generation grows up.
• -Max Plank (Quantum Physicist of the early 20th
  century)

36
So where does that leave us?

• In the time period of about 1790 through 1860 the
  following ideas were embraced
• Atoms exist
• They cannot be destroyed so mass is conserved in
  a chemical reaction
• They combine in small whole-number ratios
• Their relative masses can be calculated

37
Whats next?

• Think like a scientist
• Youve come to accept that there are small pieces
  of matter called atoms, what would your next
  question be?

O
N
H