Atoms: Dalton to J'J' Thomson

1
Atoms Dalton to J.J. Thomson

• Fall 2007

2
Start of the Modern Era of Chemistry

• Daltons Atomic Hypothesis (1808)
• 1. All matter is made of atoms. Atoms are
  indivisible and indestructible.
• 2. All atoms of a given element are identical in
  mass and properties different elements
  different properties
• 3. Compounds are formed by a combination of two
  or more different kinds of atoms.
• 4. A chemical reaction occurs when atoms are
  rearranged or combined to form molecules.

3
Daltons Ideas Atoms to Compounds

• Based on Atomic Hypothesis and assumption that
  atoms of same element repel
• Elements and compounds have definite arrangements
  of atoms.
• Atoms combine in ratio of small whole numbers to
  form compound molecules.
• Rule of greatest simplicity used to determine
  the atomic structure of compounds

4
Rule of Greatest Simplicity

• Dalton suggested a rule of greatest simplicity
• - if two elements form only one compound, assume
  the compound atom has only one atom of each
  element (XO)
• - if two elements form two compounds, assume
• XO and X20 or XO2
• - if two elements form three compounds, assume
• XO and X20 and XO2

5
Rule of greatest simplicity and relative atomic
weights
1 atom O 1 atom N ? 127100 127 units by
weight of oxygen to 100 units of nitrogen implies
atoms of oxygen outweigh nitrogen atoms by ratio
of 1.271
6
Rule of greatest simplicity and relative atomic
weights
1 atom O 1 atom N ? 58? 5850 58 units by
weight of oxygen to 100 units of nitrogen implies
atoms of oxygen outweigh nitrogen atoms by a
ratio of 1.161
7
Rule of greatest simplicity and relative atomic
weights
1 atom O 1 atom N ? 100 ? 119.5100 239
units by weight of oxygen to 100 units of
nitrogen implies atoms of oxygen outweigh
nitrogen atoms by a ratio of 1.1951
8
Rule of greatest simplicity and relative atomic
weights

• weight of oxygen relative to nitrogen 1.151
• Daltons calculations (1.21) reflect imprecision
  in measurement of weight at that time

9
Significance of Daltons Atomic Ideas

• Continued to break down earlier views of
  elements
• Found way to calculate the first relative atomic
  weights
• - by assuming many compounds were binary (11),
  and
• - by knowing reacting weight proportions of
  elements.
• (bridge gap between laboratory data and
  hypothetical atom)

10
Significance of Daltons Atomic Ideas

• Explained observation all samples of a compound
  are made of same weight proportions of its
  elements.
• 1799 Prousts Law of Constant
  Proportions
• Proportions by weight of the two elements would
  reveal the relative weights of the atoms of the 2
  elements
• 18th - early 19th centuries ratios determined by
  synthesis of compounds from pure elements
• Reinforced Law of Mass Conservation
• - Start mass of reaction End mass of
  reaction
• - B/c only reorganizing unchangeable atoms

11
Gay-Lussacs supporting observations using
volumes of gases

• Studied combining volumes of reacting gases
• Ratios of volumes were always small whole numbers
  
• (at constant T and P)

Gay-Lussac (1778 1850)
12
Gay-Lussac - combining nitrogen oxygen
Expect 2 volumes N 1 volume O ? 1 volume N2O
But got 2 volumes N2O
13
Amedeo Avogadro (1776 - 1856)

• Distinction atoms molecules
• Explained Gay-Lussac with Daltons theory
• Equal volumes (same T P) same number of
  molecule
• Compound gases produced have same number of
  molecules per volume as gases from which they are
  made. (i.e., same volumes)

14
Amedeo Avogadro (1776 - 1856)

• Explains
• 2 vol nitrogen 1 vol oxygen gives 2 (not 1!)
  vol N2O
• - if N is actually N2 and O is actually O2
• - then 2 parts N2 1 part O2 gives 2 parts N2O
• (4 atoms N and 2 atoms O at start and end)

15
Avogadros Solution 2 Vol N2 1 Vol O2 ? 2 Vol
N2O
 
 
16
On Dalton some inconsistencies found

• The basic state of an element one atom?
• Perhaps basic natural state of an element
• may be a molecule made of 2 or more
  atoms.
• Dalton Thou knowsno man can split the atom.
  
• No, split in 1932 radioactivity,
  atomic particles.
• Atoms of given element have same mass and
  properties?
• Nearly so different-mass isotopes
  exist.
• Todays elements consist of unique atoms, but are
  
• substances that cannot be broken down
  into simpler substances by a chemical reaction.

17
Meanwhile . . . New elements were being
identified 36 between 1735-1843!

• How identified?
• Physical properties
• Chemical properties types of reactions?
• Relative atomic weights
• more reliable values
• Flame test for solids/solutions

18
(No Transcript)
19
How to make sense of all these elements?
Scientists like a place for everything, and
everything in its place.
And no more places and things than necessary.
20

• Dmitri
• Mendeleev
• (1834-1907)
• Creator of the
• Periodic Table
• (but there were earlier attempts by Dobereiner
  and Newlands, and Meyer probably formulated the
• periodic idea at same time as Mendeleev)

21
Mendeleev

• Properties of elements are periodic functions of
  atomic weight (rows)
• Resulted in properties repeated in series of
  periodic intervals (columns)
• Left gaps for undiscovered elements
• Used to predict existence of new elements (of ten
  found 7 other 3 do not exist)

22
Prediction of the properties of an unknown Group
4 element below Silicon

eka one beyond
23
News flash atoms arent fundamental

• J. J. Thomson (1897)
• experimented with cathode rays
• and then... made a bold speculative leap.
  Cathode rays are not only material particles, he
  suggested, but in fact the building blocks of the
  atom they are the long-sought basic unit of all
  matter in the universe.
• (http//www.aip.org/history/electron/
  jjrays.htm)

Schematic of actual 1897 apparatus (vacuum
inside)
24
Cathode-Ray Tubes ever seen one?
25
Thomsons conclusions

• We have, in the cathode rays, matter in a new
  state...a state in which all matter...is of one
  and the same kind this matter being the
  substance from which all the chemical elements
  are built up."

• I can see no escape from the conclusion that
  cathode rays are charges of electricity carried
  by particles of matter.
• but...
• What are these particles? Are they atoms, or
  molecules, or matter in a still finer state of
  subdivision? - J. J. Thomson

26

• Thomsons plum pudding atom model

• Cathode rays (electrons) are...
• tiny corpuscles of negative charge
• surrounded by a sort of cloud of positive charge

27
If electrons exist, how big are they?

• Thomson calculated the mass-to-charge ratio for
  cathode ray particles it was over 1000 times
  smaller than for a charged hydrogen atom
• This fact suggested - either cathode rays
  carried a huge charge, - or they had very small
  mass

28
Answer very, very small

• Robert Millikan measured the charge of a cathode
  ray particle in 1910.
• From that Thompsons mass-to-charge ratio, he
  could calculate the mass 1800 times lighter
  than a hydrogen atom