272
Chapter Eleven:
Compounds and Chemical Change
A
FIGURE 11.15
As you can see by studying these two charts, there is a relationship
between the number of bonding electron pairs and the number of
lone electron pairs and the shape of a molecule.
SUMMARY
B
FIGURE 11.14
Once you understand chemical names and formulas, you can figure
out what chemical compounds are contained in different
household products. For example, (A) washing soda is sodium
carbonate (Na2CO3), and (B) oven cleaner is sodium hydroxide
(NaOH), which is also known as lye.
Elements are basic substances that cannot be broken down into anything simpler, and an atom is the smallest unit of an element. Compounds are combinations of two or more elements and can be broken down into simpler substances. Compounds are formed when
atoms are held together by an attractive force called a chemical
bond. A molecule is the smallest unit of a compound, or a gaseous
element, that can exist and still retain the characteristic properties
of a substance.
A chemical change produces new substances with new properties,
and the new materials are created by making or breaking chemical
bonds. The process of chemical change in which different chemical
substances are created by forming or breaking chemical bonds is
called a chemical reaction. During a chemical reaction, different
Chapter Eleven:
chemical substances with greater or lesser amounts of internal potential energy are produced. Chemical energy is the change of internal
potential energy during a chemical reaction, and other reactions
absorb energy. A chemical equation is a shorthand way of describing a
chemical reaction. An equation shows the substances that are
changed, the reactants, on the left side, and the new substances produced, the products, on the right side.
Chemical reactions involve valence electrons, the electrons in the
outermost shell of an atom. Atoms tend to lose or acquire electrons
to achieve the configuration of the noble gases with stable, filled
outer orbitals. This tendency is generalized as the octet rule, that
atoms lose or gain electrons to acquire the noble gas structure of
eight electrons in the outer orbital. Atoms form negative or positive
ions in the process.
A chemical bond is an attractive force that holds atoms together in
a compound. Chemical bonds that are formed when atoms transfer
electrons to become ions are ionic bonds. An ionic bond is an electrostatic attraction between oppositely charged ions. Chemical bonds
formed when ions share electrons are covalent bonds.
Ionic bonds result in ionic compounds with a crystalline structure. The energy released when an ionic compound is formed is called
the heat of formation. It is the same amount of energy that is required
to decompose the compound into its elements. A formula of a compound uses symbols to tell what elements are in a compound and in
what proportions. Ions of representative elements have a single, fixed
charge, but many transition elements have variable charges. Electrons
are conserved when ionic compounds are formed, and the ionic compound is electrically neutral. The formula shows this overall balance
of charges.
Covalent compounds are molecular, composed of electrically neutral groups of atoms bound together by covalent bonds. A single covalent
bond is formed by the sharing of a pair of electrons, with each atom
contributing a single electron to the shared pair. Covalent bonds
formed when two pairs of electrons are shared are called double bonds.
A triple bond is the sharing of three pairs of electrons. If a shared electron pair comes from a single atom, the bond is called a coordinate covalent bond. Coordinate covalent bonding sometimes results in a group of
atoms with a charge that acts together as a unit. The charged unit is
called a polyatomic ion.
The electron-pulling ability of an atom in a bond is compared
with arbitrary values of electronegativity. A high electronegative value
means a greater attraction for bonding electrons. If the absolute difference in electronegativity of two bonded atoms is 1.7 or more, one
atom pulls the bonding electron away, and an ionic bond results. If
the difference is less than 0.5, the electrons are equally shared in a
covalent bond. Between 0.5 and 1.7, the electrons are shared
unequally in a polar covalent bond. A polar covalent bond results in
electrons spending more time around the atom or atoms with the
greater pulling ability, creating a negative pole at one end and a positive pole at the other. Such a molecule is called a dipole, since it has
two poles, or centers, of charge.
Compounds are named with systematic rules for ionic and covalent compounds. Both ionic and covalent compounds that are made
up of only two different elements always end with an -ide suffix, but
there are a few -ide names for compounds that have more than just
two elements.
The modern systematic system for naming variable-charge ions
states the English name and gives the charge with Roman numerals in
parentheses. Ionic compounds are electrically neutral, and formulas
must show a balance of charge. The crossover technique is an easy way to
write formulas that show a balance of charge.
Compounds and Chemical Change
273
Covalent compounds are molecules of two or more nonmetal
atoms held together by a covalent bond. The system for naming covalent compounds uses Greek prefixes to identify the numbers of atoms,
since more than one compound can form from the same two elements
(CO and CO2, for example).
Summary of Equations
11.1
Ionization of a metal atom:
energy
+
Na
Na+
•
+
e–
11.2
Ionic bonding reaction (single charges):
••
Na
+
•
•
••
• •)
Na+ ( ••Cl
••
Cl ••
••
11.3
Ionic bonding reaction (single and double charges):
••
•
••
( •• F ••)••
F ••
••
•
Ca •
Ca2+
+
••
•
••
( •• F ••)••
F ••
••
11.4
Covalent bonding (hydrogen-hydrogen):
H• +
H•
H •• H
11.5
Covalent bonding (hydrogen-fluorine):
••
H• +
•
F ••
••
••
H •• ••
F ••
11.6
Covalent bonding (fluorine-fluorine):
••
•
••
F •• +
••
•
F ••
••
•
•
••
••
•
•
F
• F•
•• ••
11.7
Coordinate covalent bonding (ammonium ion):
H
+
+
•
•
H
••
N •• H
+
H
••
•
H N•H
•
•
••
••
H
H