Mass and moles, structure of matter [Summary].
- Atomic mass, isotopes (atomic structure)
.
- Molecular mass, molecular structures,
periodical table [link to WEB-ELEMENTS, Sheffield]
,
bonding (ionic and covalent bonds),
compounds (table of selected compounds).
- Concentrations (mass and molar concentrations, mass and molar fractions)
- Mass balance (mass balances of compounds and elements, example: burner).
- Stoichiometry of chemical reactions.
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It was not and is not easy to identify the correct structures,
given only the summary chemical formulas. For example the structure of benzene,
C6H6, remained a puzzle for a long time, though four possible
arrangements with open-end molecules were suggested. None of them could explain
some peculiarities of benzene (for example the non-existence of
C6H5Cl isomers), until the chemist August Kekul‚ (1858)
found the explanation while staring into his fireplace:
"...Again the atoms were gamboling before my eyes.
My mental eye, rendered more acute by repeated visions of the kind,
could now distinguish larger structures of manifold conformations;
long rows, sometimes more closely fitted together, all twining and twisting
in snake-like motion. But look! One of the snakes had seized hold of its own
tail, and the form whirled mockingly before my eyes. As if by a flash of
lightning I awoke; and this time also I spent the rest of the night in
working out the consequences of the hypothesis."
But Kekul‚ also added:
"Let us learn to dream, gentlemen, then perhaps we shall find the truth.
But let us beware of publishing our dreams till they have been tested by the
waking understanding"
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