Chemistry Reference
In-Depth Information
Summary
C
hemical formulas identify compounds, ions, or mole-
cules. The formula implies that the atoms are held to-
gether by some kind(s) of chemical bond(s). When they
are not combined with other elements, hydrogen, nitro-
gen, oxygen, fluorine, chlorine, bromine, and iodine exist
as diatomic molecules (Figure 5.2).
In formulas for binary compounds, the more elec-
tropositive element is written first. A formula unit repre-
sents the collection of atoms in the formula. Subscripts in
a formula indicate the numbers of atoms of the elements in
each formula unit. For example, the formula unit H
2
O has
two hydrogen atoms and one oxygen atom. Formula units
of uncombined elements, such as Ne, are atoms. Formula
units of covalently bonded atoms are called molecules.
Formula units of ionic compounds do not have any special
name. In formulas, atoms bonded in special groups may
be enclosed in parentheses. A subscript following the clos-
ing parenthesis multiplies everything within the parenthe-
ses. For example, a formula unit of Ba(ClO
4
)
2
contains
one barium atom, two chlorine atoms, and eight oxygen
atoms. Formulas for hydrates have a centered dot pre-
ceding a number and the formula for water, such as
The number multiplies everything follow-
ing it to the end of the formula (Section 5.1).
Atoms of main group elements tend to accept, do-
nate, or share electrons to achieve the electronic structure
of the nearest noble gas. Metal atoms tend to donate elec-
trons and thereby become positive ions. When combining
with metals, nonmetal atoms tend to accept electrons and
become negative ions. The number of electrons donated
or accepted by each atom depends to a great extent on
the periodic group number; each atom tends to attain a
noble gas configuration. The attraction of oppositely
charged ions is called an ionic bond. Transition and in-
ner transition metal atoms donate their valence electrons
first but ordinarily do not achieve noble gas configura-
tions. Most of them can also lose electrons from an inner
shell and thus can form cations with different charges
(Section 5.2).
Electron dot diagrams can be drawn for atoms, ions,
and molecules, using a dot to represent each valence
electron. These diagrams are most useful for main group
elements. The diagrams help in visualizing simple reac-
tions and structures of polyatomic ions and molecules
(Section 5.3).
Formulas for ionic compounds may be deduced from
the charges on the ions, since all compounds have zero
net charge. Given the constituent elements, we can pre-
dict the formula for binary compounds of most main
group metals. We cannot do so for most transition metals
because of their ability to form ions of different charges.
(Given the specific ions, we can write a formula for any
ionic compound.) Conversely, given the formula of an
ionic compound, we can deduce the charges on its ions.
Writing correct formulas for compounds and identifying
the ions in compounds from their formulas are two
absolutely essential skills (Section 5.4).
Nonmetal atoms can share electrons with other non-
metal atoms, forming covalent bonds. In electron dot dia-
grams, the shared electrons are counted as being in the
outermost shell of
each
of the bonded atoms. A single
bond consists of one shared electron pair; a double bond
consists of two shared electron pairs; a triple bond con-
sists of three shared electron pairs. Macromolecules
result from covalent bonding of millions of atoms or
more into giant molecules.
Drawing electron dot diagrams for structures con-
taining only atoms that obey the octet rule can be eased
by subtracting the number of valence electrons
available
from the number
required
to get an octet (or duet) around
each nonmetal atom. The difference is the number of
electrons to be shared in the covalent bonds. For an ion,
we must subtract 1 available electron for each positive
charge on the ion or add 1 available electron for each neg-
ative charge. Main group metal ions in general require no
outermost electrons; but each hydrogen atom requires 2;
and each other nonmetal atom requires 8. Atoms in some
compounds do not follow the octet rule (Section 5.5).
CuSO
4
#
5H
2
O.
Items for Special Attention
Because formulas are used to represent unbonded atoms,
covalently bonded molecules (Section 5.5), and ionically
bonded compounds (Section 5.2), a formula unit can rep-
resent an atom, a molecule, or the simplest unit of an
ionic compound (Figure 5.8). For example, He repre-
sents an uncombined atom; F
2
represents a molecule of
an element; CO
2
represents a molecule of a compound;
and NaCl represents one pair of ions in an ionic
compound.
■
The seven elements that occur in the form of diatomic mol-
ecules (Figure 5.2) form such molecules
only when these
elements are uncombined with other elements.
When
combined in compounds, they may have one, two, three,
■
