Achemical element can have multiple isotopes. Each of those isotopes has
the same atomic number but different mass number. As you’ll recall from earlier
in this chapter, the mass number is the sum of protons and neutrons in the
nucleus. Each isotope of the same chemical element has a different number of
neutrons but the same number of protons.
Around They Go: Electronic Configuration
Previously in this chapter you learned that electrons of an atom move around the
atom’s nucleus in a pattern called an orbital. An orbital of an atom is organized
into one or more energy levels around the nucleus. The lowest energy level
orbital is closest to the nucleus. The highest energy level is in the outermost
orbital. The outermost orbital is called the valence shell.Each orbital holds a
maximum number of electrons.
An atom with completely filled shells is called an inert atom and ischemi-
cally stable.An inert atom tends not to react with other atoms. However, an atom
that has an incomplete set of electrons in its valence shell is chemically unstable
and tends to react with other atoms in an effort to become stable. Atoms want to
be stable, so they either empty or fill their valence shell.
If an atom’s valance shell is not filled, it is considered unstable. In order to
become stable the atom must undergo a chemical reaction to acquire one or more
electrons from another atom, give up one or more electrons to another atom, or
share one or more electrons with another atom.
Achemical reactionis a chemical change in which substances called reactants
change into substances called productsby rearrangement, combination, or separa-
tion of elements. Chemical reactions occur naturally, sometimes taking a relatively
long time to complete. Acatalystcan be used to speed up a chemical reaction.
Enzymes are chemical substances that act like catalysts to increase the rate of reac-
tion, without changing the products of the reaction or by being consumed in the
reaction. A catalyst remains unaffected by the chemical reaction and does not
affect the result of the reaction. It simply speeds up the reaction.
Before James There Was Bond...Chemical Bond
An atom stabilizes by bonding with another atom in order to fill out its outer set
of electrons in its valence shell. When two atoms of the same chemical element
CHAPTER 2 Chemical Elements of Microorganisms^29