CHAPTER 7 | ATOMS AND STARLIGHT 125
The So-Called Scientifi c Method
How can you understand nature if it depends
on the atomic world you cannot see? You can
see objects such as stars, planets, aircraft carri-
ers, and hummingbirds, but you can’t see indi-
vidual atoms. As scientists apply the principle
of cause and effect, they study the natural
effects they can see and work backward to fi nd
the causes. Invariably that quest for causes
leads back to the invisible world of atoms.
Quantum mechanics is the set of rules that
describe how atoms and subatomic particles
behave. On the atomic scale, particles behave
in ways that seem unfamiliar. One of the
principles of quantum mechanics specifi es that
you cannot know simultaneously the exact
location and motion of a particle. This is why
physicists prefer to go one step beyond the
simple atomic model that has electrons follow-
ing orbits and instead describe the electronsin an atom as if they were a cloud of negative
charge. That’s a better model.
This raises some serious questions about
reality. Is an electron really a particle at all?
If you can’t know simultaneously the position
and motion of a specifi c particle, how can you
know how it will react to a collision with a
photon or another particle? The answer is that
you can’t know, and that seems to violate the
principle of cause and effect.
Many of the phenomena you can see
depend on the behavior of huge numbers of
atoms, and quantum mechanical uncertainties
average out. Nevertheless, the ultimate causes
that scientists seek lie at the level of atoms,
and modern physicists are trying to under-
stand the nature of the particles that make
up atoms. That is one of the most exciting
frontiers of science.The world you see, including these neon signs, is
animated by the properties of atoms and subatomic
particles. (Jeff Greenberg/PhotoEdit)7-1
Quantum MechanicsSCIENTIFIC ARGUMENT
How many hydrogen atoms would it take to cross the head
of a pin?
This is not a frivolous question. In answering it, you will dis-
cover how small atoms really are, and you will see how powerful
physics and mathematics can be as a way to understand nature.
Many scientifi c arguments are convincing because they have the
precision of mathematics. To begin, assume that the head of a
pin is about 1 mm in diameter. That is 0.001 m. The size of a
hydrogen atom is represented by the diameter of the electron
cloud, roughly 0.4 nm. Because 1 nm equals 10−9 m, you can
multiply and discover that 0.4 nm equals 4 10 −10 m. To fi nd
out how many atoms would stretch 0.001 m, you can divide the
diameter of the pinhead by the diameter of an atom. That is,
divide 0.001 m by 4 10 −10 m, and you get 2.5 106. It would
take 2.5 million hydrogen atoms lined up side by side to cross
the head of a pin.
Now you can see how tiny an atom is and also how powerful a
bit of physics and mathematics can be. It reveals a view of nature
beyond the capability of your eyes. Now build an argument using
another bit of arithmetic. How many hydrogen atoms would you
need to add up to the mass of a paper clip (1 g)?Th e arrangement of permitted orbits depends primarily on
the charge of the nucleus, which in turn depends on the number
of protons. Consequently, each kind of element has its own pat-
tern of permitted orbits (■ Figure 7-3). Isotopes of the sameHydrogenHydrogen nuclei have
one positive charge; the
electron orbits are not
tightly bound.Only the innermost
orbits are shown.Boron nuclei have 5
positive charges; the
electron orbits are more
tightly bound.Helium Boron(^112)
2
3
4
5
3
4
3
2
6
1
■ Figure 7-3
The electron in an atom may occupy only certain permitted orbits. Because
different elements have different charges on their nuclei, the elements have
different, unique patterns of permitted orbits.
elements have nearly the same pattern because they have the same
number of protons. However, ionized atoms have orbital patterns
that diff er from their un-ionized forms. Th us, the arrangement of
permitted orbits diff ers for every kind of atom and ion.