Read Section 2.4 before viewing the slide show.
Unit 7
•John Dalton’s atomic theory (2.4)
•Mass relationships from Dalton’s atomic
theory (2.4)
John Dalton (2.4)
•Dalton was a school teacher at the turn of the 18th
century
•Provided an explanation (theory) for the laws of
conservation of mass, definite proportions, and
multiple proportions (his law)
•The framework for the explanation is known as
Dalton’s atomic theory
Dalton’s Atomic Theory
•Four premises in Dalton’s atomic theory
•All matter is composed of extremely small
indivisible particles called atoms
•All atoms of a given element are alike, and atoms
of different elements are different from each other
•Compounds are formed when atoms of different
elements combine in fixed proportions
•Chemical changes are the result of the
rearrangement of atoms – no atoms are created
or destroyed
Today’s Status of Dalton’s Atomic Theory
•Today we are aware that Dalton’s atomic theory as
originally stated is not entirely correct. However, it
still serves as a solid basis for interpreting chemical
changes. Some of the differences, of which we will
see more soon include:
•Atoms are not indivisible – they are made of
subatomic particles
•Not all elements of the same element are
identical – we will encounter species called
isotopes
How Does Atomic Theory Help?
•Consider the Law of Definite Proportions:
•If each atom of a specific element has the same mass, but
different elements are different from each other, the
requirement for definite proportions becomes clear.
•Consider our old standby water. The ratio of oxygen by
mass to hydrogen by mass is 8:1. If the mass of an oxygen
atom is sixteen times that of hydrogen, then the observed
ratio in water is explained by having two hydrogen atoms
for every oxygen atom. (
oxygen is red, hydrogen gray)
It is not possible to have fractions of atoms, so the mass
ratios in a compound will always depend on the relative
masses and the numbers of atoms of each kind in the
compound.
How Does Atomic Theory Help? (continued)
•Consider the Law of Multiple Proportions:
•If only integral numbers of atoms can combine, the number of
possible compounds formed may be greater than one. For
example, consider hydrogen and oxygen:
Arrangement
Mass oxygen/
Mass hydrogen
16:1
16:2 (8:1)
32:2 (16:1)
16:3
•There is an infinite number of atom combinations for even two
atoms, but not all are chemically viable. For oxygen and
hydrogen only the combinations in the second and third rows
above exist as actual compounds.
Mass Relationships from Dalton’s Atomic
Theory
•An example may be used to illustrate how to use mass
relationships to find useful information
•Octane, a compound often used to simulate gasoline, can be
decomposed to give 5.3 parts by mass of carbon to 1.0 parts by mass
of hydrogen. How many grams of carbon are produced when 150 g of
gasoline is decomposed?
•Solution: Consider that if the compound is composed of 5.3 parts
by mass carbon and 1.0 parts by mass hydrogen, that every 6.3
parts of the compound has 5.3 parts of carbon (the 6.3 is the sum of
the parts = 5.3 +1.0). Thus, the fraction of the compound that is
carbon is 5.3/6.3 = 0.84. If one starts with 150 g, the grams of
carbon in it is 0.84 x 150 g = 126 g carbon.