There are many things in our surroundings that we know exist, yet are not matter.
They are forms of energy. Heat, light, and electricity are examples of energy. Energy
may be simply defined as the ability to do work or overcome resistance.
Early scientists felt that all matter must be built from some basic unit, just as a
wall may be constructed from a basic unit, the brick. In trying to find this basic unit, they
separated matter by all the methods (chemical and physical) available to them until they
could not separate it any further. They felt this separation must result in the building
block of matter, which they called the atom (from the Greek word for indivisible). They
also observed that the basic units or atoms for various elements differed in their
properties, as iron was certainly different from carbon. This led them to try to find the
structure of the atom. The difficulty of this problem can be seen when you consider that
one cubic centimeter of gold contains as many as 59,000,000,000,000,000,000,000
atoms. The atom is so small that it defies conception. Through ingenious methods,
particularly in the last 100 years, we have discovered many facts about this tiny particle,
which enables us to understand many of the changes that occur around us.
a. Atomic Model. In order for us to picture what an atom looks like, we can use
a description with which most people are familiar--the solar system model. In this
model, the atom is thought of as a tiny solar system in which there is a central core (like
the sun) with other particles traveling in circular paths or orbits (like the planets). While
more complex and exact models have been developed, this is the best approximation
for general use.
b. The Nucleus. The central core from the solar system model is called the
nucleus (which is derived from the Latin word nucis meaning nut or kernel). The
nucleus contains two types of particles, the proton and the neutron.
(1) The proton. The proton is a particle that has a mass (or weight) of one
amu (atomic mass unit) and a positive one (+1) electrical charge. The symbol for the
proton is p, p+ or H+.
(2) The neutron. The neutron has a mass of one amu (atomic mass unit)
but has no electrical charge; that is, it is a neutral particle. In an atom that has more
than one proton, the positive charges tend to repel each other. The neutrons serve to
bind the protons so that this electrical repulsion does not cause them to fly off into
space. The symbol for the neutron is n.