identifying properties of sugar. The only thing that changed was the physical size of the
sugar. If we continue this subdividing process by grinding the sugar into a fine powder, the
results are the same. Even dissolving the sugar in water does not change its identifying
properties, in spite of the fact that the particles of sugar are now too small to see even with
a microscope. Eventually, we end up with a quantity of sugar, which cannot be further
divided without it ceasing to be sugar. This quantity is known as a molecule of sugar. If the
molecule is further divided, it is found to consist of three simpler kinds of matter: carbon,
hydrogen, and oxygen. These simpler forms are called elements. Therefore, since elements
consist of atoms, then a molecule of sugar is made up of atoms of carbon, hydrogen, and
1-27. As we examine the atom, we find that it is basically composed of electrons,
protons, and neutrons. The electrons, protons, and neutrons of one element are identical to
those of any other element. However, there are different kinds of elements because the
number and the arrangement of electrons and protons are different for each element.
1-28. The electron is considered to be a small negative charge of electricity. The proton
has a positive charge of electricity equal and opposite to the charge of the electron.
Scientists have measured the mass and size of the electron and proton and they know how
much charge each possesses. The electron and proton have the same quantity of charge,
although the mass of the proton is approximately 1,837 times that of the electron. In some
atoms there exists a neutral particle called a neutron. The neutron has a mass
approximately equal to that of a proton, but it has no electrical charge.
1-29. According to a popular theory, the arrangement of electrons, protons, and neutrons
of an atom is similar to a miniature solar system. Notice the helium atom in Figure 1-2.
Two protons and two neutrons form the heavy nucleus with a positive charge around which
two very light electrons revolve. The path each electron takes around the nucleus is called
an orbit. The electrons are continuously being acted upon in their orbits by the force of
attraction of the nucleus. To maintain an orbit around the nucleus, the electrons travel at a
speed that produces a counterforce equal to the attraction force of the nucleus. Just as
energy is required to move a space vehicle away from the earth, energy is also required to
move an electron away from the nucleus. Like a space vehicle, the electron is said to be at
a higher energy level when it travels a larger orbit. Scientific experiments show that the
electron requires a certain amount of energy to stay in orbit. This quantity is called the
electron's energy level. By virtue of just its motion alone, the electron contains kinetic
energy. Due to its position, it also contains potential energy. The total energy contained by
an electron (kinetic energy plus potential energy) is the main factor that determines the
radius of the electron's orbit. In order for an electron to remain in this orbit, it must neither
gain nor lose energy.
1-30. The orbiting electrons do not follow random paths; instead they are confined to
definite energy levels. Picture these levels as shells with each successive shell being spaced
a greater distance from the nucleus. The shells, and the number of electrons required to fill
them, may be predicted by using Pauli's exclusion principle. Simply stated, this principle
specifies that each shell will contain a maximum of 2n2 electrons, where "n" corresponds to
the shell number starting with the one closest to the nucleus. By this principle, the second
shell, for example, would contain 2(2)2 or 8 electrons when full.
1-31. In addition to being numbered, the shells are also given letter designations starting
with the shell closest to the nucleus and progressing outward (see Figure 1-3). The shells
are considered to be full, or complete, when they contain the following quantities of
electrons: two in the K(1st) shell, eight in the L(2nd) shell, eighteen in the M(3rd) shell,
23 June 2005