Information about atoms and elements can be found
p erio d ic table o f the elem ents
, or simply the
(see Appendix). Here, the elements are arranged
in rows called
and columns, in order of increasing
atomic number. In each element’s square are its name, sym-
bol (one- or two-letter abbreviation), atomic number, and
mass number. (The atomic mass number in the table is a
is the difference between matter and
are the major chemical elements in the
weighted average of the atomic masses of the element’s
isotopes. This is why you see decimal numbers in the mass
numbers of the table.) The period number indicates the
number of energy levels in the element’s atom, and the
column number indicates the number of valence electrons.
Chemists use the information in the periodic table to pre-
dict how atoms of different elements will react chemically.
particles are found in the nucleus of an
is an isotope of an element?
are radioisotopes used in nuclear medicine?
The Building Blocks of Matter Fit Together
to Make Ions and Compounds
between an atom, an ion, a mol-
ecule, and a compound.
and contrast an ionic bond with a
hen atoms interact by exchanging electrons,
ch em ical b o n d s
, which bind them
together in molecules and compounds and re-
sist separation. Let’s take a look at two types
of important bonds: ionic and covalent.
Atoms Combine to Form Compounds:
Ionic and Covalent
Atoms of various elements can interact with each other
by exchanging electrons of their valence shells in one of
Give up one or more valence electrons to another atom
Take one or more valence electrons from another atom
Share one or more valence electrons with one or more
between a polar covalent bond and
a non-polar covalent bond.
the various types of chemical
The chance that an atom will form a chemical bond
with another atom depends on the number of electrons
in its valence shell. An atom with an outer shell holding
eight electrons is
which means it is un-
likely to form chemical bonds with other atoms. Neon,
for example, has eight electrons in its outer shell, and
for this reason it rarely forms bonds with other atoms.
In contrast, the atoms of most biologically important
elements do not have eight valence electrons. However,
atoms tend to interact in such a way as to produce a
chemically stable arrangement of eight electrons in
the outer shell of each atom. (Note that hydrogen can
hold only two electrons in its valence shell.) This ten-
dency toward achieving a full valence shell is called the
Let’s look at the various ways that atoms can interact.
26 CHAPTER 2