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HN S O S HS Cl
Cl
Q
Each atom shares a pair of electrons to achieve a noble gas con-
figuration. Hydrogen achieves the helium configuration, and
chlorine achieves the neon configuration. All the halogens have
seven valence electrons, and all need to gain one electron (ionic
A bond) or share an electron pair (covalent bond) to achieve a no-
ble gas configuration. This also explains why the halogen gases
occur as diatomic molecules. Two chlorine atoms can achieve a
noble gas configuration by sharing a pair of electrons:

Cl
Cl
N O S N O S S Cl O S
O SCl
Q
Q
Q
Q
B Each chlorine atom thus achieves the neon configuration by
bonding together. Note that there are two types of electron
FIGURE 9.8 (A) Two hydrogen atoms, each with its own prob-
pairs: (1) orbital pairs and (2) bonding pairs. Orbital pairs are
ability distribution of electrons about the nucleus. (B) When the
hydrogen atoms bond, a new electron distribution pattern forms not shared, since they are the two electrons in an orbital, each
around the entire molecule, and both electrons occupy the molecu- with a separate spin. Orbital pairs are also called lone pairs, since
lar orbital. they are not shared. Bonding pairs, as the name implies, are the
electron pairs shared between two atoms. Considering again the
Cl 2 molecule,
Bonding pair
Consider how the covalent bond forms between two hy-
drogen atoms by imagining two hydrogen atoms moving toward
each other. Each atom has a single electron. As the atoms move Lone O Lone
Cl Cl
Q Q
closer and closer together, their orbitals begin to overlap. Each pairs S O S S pairs
electron is attracted to the oppositely charged nucleus of the other
atom, and the overlap tightens. Then the repulsive forces from Often, the number of bonding pairs that are formed by an
the like-charged nuclei will halt the merger. A state of stability atom is the same as the number of single, unpaired electrons in the
is reached between the two nuclei and two electrons, and an H 2 atomic electron dot notation. For example, hydrogen has one un-
molecule has been formed. The two electrons are now shared by paired electron, and oxygen has two unpaired electrons. Hydrogen
both atoms, and the attraction of one nucleus for the other elec- and oxygen combine to form an H 2 O molecule, as follows:
tron and vice versa holds the atoms together (Figure 9.8).
HN

Covalent Compounds and Formulas N O O N S O O S
Q
Q
Electron dot notation can be used to represent the formation of HN H H
covalent bonds. For example, the joining of two hydrogen atoms
to form an H 2 molecule can be represented as
Unpaired
HN HN HSH electrons

Since an electron pair is shared in a covalent bond, the two elec-
The diatomic hydrogen (H 2 ) and chlorine (Cl 2 ), hydrogen
trons move throughout the entire molecular orbital. Since each
chloride (HCl), and water (H 2 O) are examples of compounds
hydrogen atom now has both electrons on an equal basis, each
held together by covalent bonds. A compound held together
can be considered to now have the noble gas configuration of
by covalent bonds is called a covalent compound. In general,
helium. A dashed circle around each symbol shows that both
covalent compounds form from nonmetallic elements on the
atoms share two electrons:
right side of the periodic table. For elements in families IVA
through VIIA, the number of unpaired electrons (and thus the
HN HN HSH number of covalent bonds formed) is 8 minus the family num-
ber. You can get a lot of information from the periodic table
Hydrogen and chlorine react to form a covalent molecule, from generalizations like this one. For another generalization,
and this bond can be represented with electron dots. Chlorine is compare Table 9.4 with the periodic table. The table gives the
in the VIIA family, so you know an atom of chlorine has seven structures of nonmetals combined with hydrogen and the re-
valence electrons in the outermost energy level. The reaction is sulting compounds.

9-9 CHAPTER 9 Chemical Bonds 237