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Unformatted text preview: 7.46/22.54) = 3.437 g Cl
In compound B, we would expect
1 gram (85.12/14.88) = 5.720g Cl Dalton would predict that the ratio of
masses of Cl is a “simple ratio of
integers”. 5.720 g
3.437 g = 1.664 ≈ N.B.!PCl3!and!PCl5!are!
3 Atomic Theory
According to Dalton, compounds are made
of units which he called atoms. These
atoms behave according to a fixed set of
rules. John Dalton
Father of the Atomic Theory Basic postulates of Dalton’s atomic theory:
" Matter is composed of indivisible particles called atoms
" Atoms of a given element and are identical in all respects
" Atoms from different elements differ in mass and chemical
" Compounds are composed of two or more atoms of different
elements. Their combination results in molecules.
" In a reaction, atoms can be rearranged, (re)combined but
neither created nor destroyed. " The law of conservation of mass follows directly from Dalton’s ideas.
" Similarly, the law of constant composition follows from Dalton’s idea
that atoms are indivisible.
" Dalton’s atomic theory enables us to set-up a scale of relative masses
since we know the relative ratios of elements in compounds.
We know that in LiCl, Li is 16.4% and Cl is 83.6%. Since we know that
the mass of Li relative to Cl must be the same as the mass % in LiCl
then: mass of Cl atom
mass of Li atom
16.4% € N.B.!Mass!of!Li:!6.9!
Mass!of!Cl:!35.5! Following this approach, we can establish a table of
relative atomic masses for all elements. Definition:
We define atomic mass as the ratio of the mass of
a given atom to the mass of some reference atom*.
The current standard is Carbon.
N.B. Atomic masses are dimensionless.
Molecular mass is simply the sum of the masses of the
constituent elements in a specified molecule.
H2SO4 = 98
*Not all atoms of a given element have the same mass (isotopes) (e.g.
1H, 2H, 3H).! John!Dalton’s!Atoms:! nitrogen! Numerous!mistakes;!both!in!masses,!and!in!which!are!atoms!vs.!
compounds!(lime,!soda,!potash!are!all!compounds).! Dalton’s symbols for
chemical elements. Some of these
“elements” are no...
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This document was uploaded on 11/06/2013.
- Fall '12