Chemistry Reference
In-Depth Information
The electronic structure of an uncombined atom, first discussed in Chapter 4,
determines the ability of that atom to combine with other atoms to produce
molecules or ionic compounds. In this chapter, the fundamentals of chemical
bonding are covered. To discuss compounds, chemical formulas are required.
Moreover, when symbols for atoms are combined in a chemical formula, some
type of bonding is implied. Therefore, chemical formulas are introduced first,
in Section 5.1. (More information about chemical formulas will be presented in
Chapter 7.) Ionic bonding, which occurs when electrons are transferred from
one atom to another, is treated in Section 5.2. A convenient way to picture atoms
with their outermost electrons—the electron dot diagram—is presented in Sec-
tion 5.3. The number of electrons transferred from one atom to another, or the
charges on the resulting ions, enable us to deduce the formulas for binary ionic
compounds (Section 5.4). Atoms held together solely by covalent bonds form
units called molecules (or much larger units called macromolecules). Covalent
bonding, in which the sharing of electrons is the primary method of bonding,
is introduced in Section 5.5, which also discusses compounds with both ionic
and covalent bonding.
Writing a chemical formula
implies bonding of some type.
Ionic bonding involves transfer
of electrons. Covalent bonding
involves electron sharing. In
many ternary compounds, both
ionic and covalent bonding
occur.
5.1
Chemical Formulas
Just as a symbol identifies an element, a
formula
is a combination of symbols
that identifies a compound, an ion, or a molecule of an element. However, chem-
ical formulas do much more. A formula also indicates the relative quantities of
the elements contained in the compound or ion and implies some kind of chem-
ical bonding between the atoms.
Molecules of Elements
Formulas are used to identify molecules of free elements. A
molecule
contains
two or more nonmetallic atoms bonded together. Many free (uncombined) non-
metallic elements exist as molecules, such as H
2
, N
2
, O
2
, F
2
, Cl
2
, Br
2
, and I
2
,
as well as P
4
and S
8
(Figure 5.1). The formula P
4
indicates four phosphorus
atoms bonded together. This formula does not represent a compound, because
only one kind of atom is present.
Elemental
phosphorus in its lowest energy
form occurs in such molecules.
Seven elements occur as
diatomic molecules
(molecules with two atoms)
when they are not combined with other elements
. Fortunately, these elements
are easy to remember because, except for hydrogen, they form a shape like a seven
in the periodic table, starting at the element with atomic number 7 (Figure 5.2).
Hydrogen molecules (H
2
) are so much more stable than separated hydrogen
atoms that the reaction of the atoms to form molecules produces a lot of heat:
Seven elements occur as
diatomic molecules when they
are not combined with other
elements.
2 H £ H
2
heat
Production of a given number of H
2
molecules from hydrogen atoms produces
more heat than the production of the same number of CO
2
molecules from burn-
ing carbon (charcoal) in oxygen. Construction workers take advantage of the
reaction of atomic hydrogen to weld steel pieces together in the absence of oxy-
gen. That condition is desirable because oxygen might make the steel rust.
