The air we breathe has about 10 19 atoms in each

The air we breathe has about 10
19
atoms in each cubic centimeter—and we usually think of air as empty space. In the
interstellar gas of the Galaxy, there is about one atom in every cubic centimeter. Intergalactic space is filled so sparsely that
to find one atom, on average, we must search through a cubic meter of space. Most of the universe is fantastically empty;
places that are dense, such as the human body, are tremendously rare.
Even our most familiar solids are mostly space. If we could take apart such a solid, piece by piece, we would eventually
reach the tiny molecules from which it is formed. Molecules are the smallest particles into which any matter can be divided
while still retaining its chemical properties. A molecule of water (H
2
O), for example, consists of two hydrogen atoms and
one oxygen atom bonded together.
Molecules, in turn, are built of atoms, which are the smallest particles of an element that can still be identified as that
element. For example, an atom of gold is the smallest possible piece of gold. Nearly 100 different kinds of atoms (elements)
exist in nature. Most of them are rare, and only a handful account for more than 99% of everything with which we come in
contact. The most abundant elements in the cosmos today are listed in Table 2.1 ; think of this table as the “greatest hits”
of the universe when it comes to elements.
The Cosmically Abundant Elements
Element
[1]
Symbol Number of Atoms per
Million Hydrogen Atoms
Hydrogen H 1,000,000
Helium He 80,000
Carbon C 450
Nitrogen N 92
Oxygen O 740
Neon Ne 130
Magnesium Mg 40
Silicon Si 37
Sulfur S 19
Iron Fe 32
Table 2.1
All atoms consist of a central, positively charged nucleus surrounded by negatively charged electrons. The bulk of the matter
in each atom is found in the nucleus, which consists of positive protons and electrically neutral neutrons all bound tightly
together in a very small space. Each element is defined by the number of protons in its atoms. Thus, any atom with 6 protons
in its nucleus is called carbon , any with 50 protons is called tin , and any with 70 protons is called ytterbium . (For a list of
the elements, see Appendix F .)
1. This list of elements is arranged in order of the atomic number, which is the number of protons in each
nucleus.
Chapter 2 | The Universe at Its Limits 59

The distance from an atomic nucleus to its electrons is typically 100,000 times the size of the nucleus itself. This is why
we say that even solid matter is mostly space. The typical atom is far emptier than the solar system out to Neptune. (The
distance from Earth to the Sun, for example, is only 100 times the size of the Sun.) This is one reason atoms are not like
miniature solar systems.
Remarkably, physicists have discovered that everything that happens in the universe, from the smallest atomic nucleus to
the largest superclusters of galaxies, can be explained through the action of only four forces: gravity, electromagnetism
(which combines the actions of electricity and magnetism), and two forces that act at the nuclear level. The fact that there
are four forces (and not a million, or just one) has puzzled physicists and astronomers for many years and has led to a quest
for a unified picture of nature.