Lec 10 Elements of Chemistry

Question 1

Little background

Answer 1

• Controversy over formulas for common compounds.
• > e.g. vinegar had 19 formulas
• Nobody can agree on atomic weights of elements.
• people confused atoms and molecules

Cannizzaro showed how you can do tons of math
->Avogadro’s hypothesis to determine gas weights
->Gay-Lussac’s law of combining volumes
->Berzelius’ atomic weights
Can cut through anything, show H gas = H2 not H4,6

Question 2

Mendeleev’s Predictions

Answer 2

Many had tried to sort the elements
->Newlands realised 1=8, 2=9, the “law of octaves”

Played Patience (solitaire) with all 63 elements.

• > realised no value in ordering by atomic weight
• > arranged into 7 groups - similar properties appear every 8th element when sorted by atomic weight

Amazing insight:

• > Gold was in Pt’s spot - rightfully stated the current measurements for Au’s weights wrong
• > when elements didn’t fit in scheme, acknowledged he didn’t have all the cards

1869, predicts some new elements, stating their atomic weights and chemical properties. Predicted they would have properties similar to some known ones. Called:

• > eka-aluminum (gallium)
• > eka-silicon (germanium)
• > eka-boron (later prediction, 1871)

Big deal because he had no experiments.

• > 1875, Gallium is found, almost exactly eka-Al
• > yields 2 new lines on spectroscope

Question 3

Spectrum Analysis

Answer 3

Newton’s continuous spectrum is actually many rays and spectra. By admitting light with narrow slit, you get a “pure spectrum”

First examined by Wollaston, who observed spectrum not continuous

• > examined prism with naked eye
• > dark lines at intervals

JOSEPH VON FRAUNHOFER

• > enhanced image by looking with telescope
• > incident, refracted rays form ~equal angle with faces
• > minimal deflection, giving clear/sharp images
• > saw 600+ dark lines, each a unique wavelength of light

Question 4

Spectroscope

Answer 4

Developed by Kierchhoff and Bunsen

• > light to slit to prism
• > slit controls source, therefore display of wavelengths
• > allowed Brewster to see some gas absorbe certain rays of the sun
• > used Bunsen burner to heat chemicals to incandescence (heat where they give off light) and saw each chemical has pattern of lines
  • > unique, is the THUMBPRINT

Allows you to investigate matter, in terms of the light emitted by it

• > elements clearly characterised by their unique spectrum
• > identified numerous new elements
  • > including the Gallium or eka-Al

Question 5

The Zero Group

Answer 5

Mendeleev’s ordering couldn’t be explained
->required the atom’s nucleus & structure
Ramsay noticed that natural nitrogen heavier than chemical nitrogen
->some inert gas hiding - was argon
->a year later, finds Helium
->then Neon, Krypton, Xenon

Becomes known as the Zero group - 7 atoms which will not combine with anything (the Nobles)