Page 85 - Theory and Problems of BEGINNING CHEMISTRY

74 CHEMICAL BONDING [CHAP. 5

Ans. (a) C O Total (b) C O Total
Electrons needed 8 8 16 8 2 × 8 24
Available 4 6 10 4 2 × 6 16
To be shared 6 8
C O O C O
Add the rest of the electrons: C O O C O

(c) C H Total (d ) Mg Cl Total
Electrons needed 8 4 × 2 16 0 2 × 8 16
Available 4 4 × 1 8 2 2 × 7 16
To be shared 8 0
H
–
H C H Mg 2+ 2 Cl
H
Note that the main group cation needs no
valence electrons. No covalent bonds exist
in MgCl 2 ; there is no electron sharing.
In polyatomic ions, more or fewer electrons are available than the number that comes from the valence shells
of the atoms in the ion. This gain or loss of electrons provides the charge.

EXAMPLE 5.8. Draw an electron dot diagram for CO 3 .
2−
Ans. 1. Valence electrons available C 4
3O 18
charge 2 (2− charge means 2 extra electrons)
Total 24
2. Valence electrons required C 8
3O 24
Total 32
3. To be shared 32 − 24 = 8
2–
O C O
5. With shared electrons
O
2–
O C O
6. With all electrons
O

Scope of the Octet Rule
It must be emphasized that the octet rule does not describe the electronic configuration of all compounds.
The very existence of any compounds of the noble gases is evidence that the octet rule does not apply in all
cases. Other examples of compounds that do not obey the octet rule are BF 3 ,PF 5 , and SF 6 . But the octet rule
does summarize, systematize, and explain the bonding in so many compounds that it is well worth learning
and understanding. Compounds in which atoms attain the configuration of helium (the duets) are considered to
obey the octet rule, despite the fact that they achieve only the duet characteristic of the complete first shell of
electrons.

5.7. DISTINCTION BETWEEN IONIC AND COVALENT BONDING
The word bonding applies to any situation in which two or more atoms are held together in such close
proximity that they form a characteristic species which has distinct properties and which can be represented by a
chemical formula. In compounds consisting of ions, bonding results from the attractions between the oppositely
charged ions. In such compounds in the solid state, each ion is surrounded on all sides by ions of the opposite