124 PART 2^ |^ THE STARS
a comb through your hair creates a static charge by removing a
few electrons from their atoms. An atom that has lost or gained
one or more electrons is said to be ionized and is called an ion.
A neutral carbon atom has six electrons that balance the positive
charge of the six protons in its nucleus. If you ionize the atom by
removing one or more electrons, the atom is left with a net posi-
tive charge. Under some circumstances, an atom may capture
one or more extra electrons, giving it more negative charges than
positive. Such a negatively charged atom is also considered an
ion.
Atoms that collide may form bonds with each other by
exchanging or sharing electrons. Two or more atoms bonded
together form a molecule. Atoms do collide in stars, but the high
temperatures cause violent collisions that are unfavorable for
chemical bonding. Only in the coolest stars are the collisions
gentle enough to permit the formation of chemical bonds. You
will see later that the presence of molecules such as titanium
oxide (TiO) in a star is a clue that the star is very cool. In later
chapters, you will see that molecules also can form in cool gas
clouds in space and in the atmospheres of planets.Electron Shells
So far you have been thinking of the cloud of the whirling elec-
trons in a general way, but now it is time to be more specifi c
about how the electrons behave within the cloud.
Electrons are bound to the atom by the attraction between
their negative charge and the positive charge on the nucleus. Th is
attraction is known as the Coulomb force, after the French
physicist Charles-Augustin de Coulomb (1736–1806). To ionize
an atom, you need a certain amount of energy to pull an electron
completely away from the nucleus. Th is energy is the electron’s
binding energy, the energy that holds it to the atom.
Th e size of an electron’s orbit is related to the energy that
binds it to the atom. If an electron orbits close to the nucleus, it
is tightly bound, and a large amount of energy is needed to pull
it away. Consequently, its binding energy is large. An electron
orbiting farther from the nucleus is held more loosely, and less
energy is needed to pull it away. Th at means it has less binding
energy.
Nature permits atoms only certain amounts (quanta) of
binding energy, and the laws that describe how atoms behave are
called the laws of quantum mechanics (How Do We Know?
7-1). Much of this discussion of atoms is based on the laws of
quantum mechanics.
Because atoms can have only certain amounts of binding
energy, your model atom can have orbits only of certain sizes,
called permitted orbits. Th ese are like steps in a staircase: You
can stand on the number-one step or the number-two step, but
not on the number-one-and-one-quarter step–there isn’t one.
Th e electron can occupy any permitted orbit, but there are no
orbits in between.of a sphere 4.5 football fi elds in diameter, you can see that an
atom is mostly empty space.
Now you can consider a Common Misconception.
Most people, without thinking about it much, imagine that mat-
ter is solid, but you have seen that atoms are mostly empty space.
Th e chair you sit on, the fl oor you walk on, are mostly not there.
When you study the deaths of stars in a later chapter, you will see
what happens to a star when most of the empty space gets
squeezed out of its atoms.
Different Kinds of Atoms
Th ere are over a hundred chemical elements. Th e number of
protons in the nucleus of an atom identifi es which element it is.
For example, a carbon atom has six protons in its nucleus. An
atom with one more proton than this is nitrogen, and an atom
with one fewer proton is boron.
Although an atom of a given element always has the same
number of protons in its nucleus, the number of neutrons is less
restricted. For instance, if you added a neutron to a carbon
nucleus, it would still be carbon, but it would be slightly heavier.
Atoms that have the same number of protons but a diff erent
number of neutrons are isotopes. Carbon has two stable isotopes.
One contains six protons and six neutrons for a total of 12 particles
and is thus called carbon-12. Carbon-13 has six protons and
seven neutrons in its nucleus.
Th e number of electrons in an atom of a given element can
vary. Protons and neutrons are bound tightly into the nucleus,
but the electrons are held loosely in the electron cloud. Running
■ Figure 7-2
Magnifying a hydrogen atom by 10^12 makes the nucleus the size of a grape
seed and the diameter of the electron cloud about 4.5 times larger than a
football fi eld.
Football fieldNucleus
(grape seed)Electron
cloud