Atoms, Molecules and Ions
The Atomic Theory of Matter
Chemists make their observations in the macroscopic world and
seek to understand the fundamental properties of matter at the level of
the microscopic world (i.e. molecules and atoms). The reason why
certain chemicals react the way they do is a direct consequence of their
atomic structure.
The Atomic Theory of Matter
The word "atom" is derived from the Greek word "atomos", meaning indivisible.
The philosopher Democritus (460-370 B.C.) believed that matter was composed
of fundamentally indivisible particles, called "atomos".
Dalton's atomic theory of 1803:
-
Each element is composed of extremely small particles called atoms
-
All atoms of a given element are identical; the atoms of different elements
are different and have different properties (including different masses)
-
Atoms of an element are not changed into different types of atoms by chemical
reactions; atoms are neither created nor destroyed in chemical reactions
-
Compounds are formed when atoms of more than one element combine; a given
compound always has the same relative number and kind of atoms.
Atoms are the basic building blocks of matter; they are the smallest
units of an element:
-
An element is composed of only one kind of atom
-
In compounds the atoms of two or more elements combine in definite
arrangements
-
Mixtures do not involve the specific interactions between elements
found in compounds, and the elements which comprise the mixture can be
of varying ratios
Atoms are the smallest particle of
an element which retains the chemical properties of that element
Simple "laws" (i.e. theories) of chemical combination which were known
at the time of Dalton:
-
The law of constant composition (in a given compound the
relative number and kind of atoms are constant)
-
The law of conservation of mass (the total mass of materials
present after a chemical reaction is the same as the total mass before
the reaction)
Dalton used these "laws" to derive another "law" - the law of multiple
proportions (if two elements, A and B, can combine to form more
than one compound, then the ratios of the relative masses of each element
which can combine can be represented by characteristically small whole
numbers).
Where's the water?
Hydrogen and oxygen gas can react to form water (H2O).
This is a violent reaction which liberates considerable amounts of heat.
A large child's balloon will hold about four liters of gas. This
would represent about 5 grams of oxygen and 0.3 grams of hydrogen if we
mixed them in the appropriate (molar) ratios. Thus, we would expect,
at most, to form about 5.3 grams of water (about 1 tablespoon full).
Since some gasses escape before reacting, and since we may not add the
gasses in exactly the correct ratio, the amount of water formed may be
far less. The water that is formed is going to be in the vapor phase.
This demonstrates one of the attractive aspects
of hydrogen as a fuel: the only pollution (i.e. product) is water!
1996 Michael Blaber