5.1 Early Thoughts
Democritus a Greek philosopher along with
his teacher Leucippus asked the question of
what would happen if material was divided
into two parts and then divided again with
the processes continued for many steps.
Could the process be carried out indefinitely?
Democritus and Leucippus reasoned that
this process would come to a stop at the
point which the most fundamental particle of
that material had been reached.They called
this piece atom , individual or singular
particle. This idea did not find wide
acceptance in later years and the theory of
Aristotle regarding earth, fire, water, and
air prevailed for a long time.
John Dalton developed a theory of the atom
that in it's general form is the basis of today's
atomic theory. Dalton's atomic theory contains
the following points:
1. Atoms as small individual particles of matter
that can no longer be subdivided.
2. An element is composed of only one type of
atom.
3. A compound is composed of two or more
elements combined in definite proportions.
4. A chemical reaction is the formation of
a new combination of atoms to form new
substances from the starting substances.
5. Atoms can combine in simple ratios to
form compounds.
6. Two elements can combine in different
ratios to form different compounds.
7. The atoms are neither created nor
destroyed in a chemical reaction.
A particular compound always has the same
elements combined in exactly the same way. It
does not matter where or how it was prepared,
it is the same structure.
*Law of definite Composition- A compound
is made up of two or more elements combined
in a definite proportion by mass.
The same atoms if combined in different ways
and differing ratios can form different
substances. The classic example that is often
used is that of water and hydrogen peroxide.
Both are composed of only hydrogen and
oxygen. The difference is in the mass ratios
of these atoms in the final compound. In the
case of water, the ratio of water, the
compound is made up of two atoms of
hydrogen and one atom of oxygen. This
amounts to a mass ratio of 11.2%
hydrogen and 88.8% oxygen. In the case
of hydrogen peroxide this mass ratio
corresponds to two atoms of hydrogen
for every two atoms of oxygen. This
ratio is 5.9% hydrogen and 94.1%
oxygen by mass.
*Law of Multiple Proportion- Atoms
of two or more elements can combine to
form more than one compound in different
ratios.
5.4 The Nature of Electric Charge
1. Electric charge may be positive or negative.
2. Unlike charges attract, and like charges
repel.
3. Charge may be transferred by contact or
induction.
4. The greater the distance between charges
the weaker the force.
Michael Faraday discovered the conductivity
of ions and gave the particles the name "ion".
Svante Arrhenius reasoned positive ions travel
to the negative electrode and that negative
ions travel to the positive electrode. Stoney
named the fundamental unit of negative charge
the "electron". Thomson provided experimental
proof for the existence of the electron.
5.6 Subatomic Parts of the Atom
In this section we will confine our discussion
to the protons, electrons, and neutrons. There
are many other subatomic particles that are
known, however in terms of chemistry these are
the ones of greatest importance for this course.
The atom is composed of a nucleus with a
positive charge and the electrons surrounding
it with a negative charge(e-). With a rest mass
of 9.11 x 1028grams. The total of the positive
protons(p+) in the nucleus and the negative
electrons surrounding the nucleus is zero.
The rest mass of the proton is approximately
1.673 x 1024-grams.
In addition to the protons in the nucleus there
are neutrons which bear no charge. Neutrons
rest mass is approximately 1.675 x 1024-grams.
The discovery of radioactivity in certain
elements by Henri Becquerel in 1896 was closely
followed by the discovery of alpha particle
emissions and gamma ray emissions.
Earnest Rutherford had shown that the
positive charged alpha particles were ions of
helium and with his own information he
proceeded to establish that most of the mass
of an atom was in the nucleus which occupied
an extremely small volume of space. The density
of this has been established to be around a
trillion times the density of water i.e.
1 x 1012grams/cm3.
5.8 General Arrangement of Subatomic Particles
The atom consists of a dense nucleus
surrounded by a cloud of electrons. In the
neutral atom there are an equal number
of protons and electrons. Exp:
5.9 Atomic Numbers of the Elements
The atomic number is the number
of protons in an atom. The atomic number
defines each element. It also tells us how
many electrons there are in a neutral atom.
Exp:
Isotopes are atoms that have the same number
of protons but different number of neutrons.
Exp:
Atomic mass of an element is the average
relative mass of the isotopes that make up
the element. Exp: Copper occurs as 2 isotopes:
