Page 82 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 5] CHEMICAL BONDING 71

In the solid state, an ionic compound will not conduct. However, if the compound is heated until it melts or if it
is dissolved in water, the resulting liquid will conduct electricity because in the liquid state the ions are free to
move.
Periodic group IA and IIA metals always form ions with 1+ and 2+ charges, respectively. Most other metals
form more than one type of ion, for example, iron forms Fe 2+ and Fe 3+ ions. Without further information (such
as is given in Sec. 8.3), it is impossible to tell which of these ions will be formed in a given situation. The charges
on these types of ions will be more fully presented in Sec. 6.3 and in Chap. 14.

5.5. ELECTRON DOT NOTATION
To represent the formation of bonds between atoms, it is convenient to use a system known as electron dot
notation. In this notation, the symbol for an element is used to represent the nucleus of an atom of the element
plus all the electrons except those in the outermost (valence) shell. The outermost electrons are represented by
dots (or tiny circles or crosses). For example, the dot notations for the first 10 elements in the periodic table are
as follows:

H He Li Be B C N O F Ne

Using electron dot notation, the production of sodium fluoride, calcium fluoride, and calcium oxide may be
pictured as follows: A sodium atom and a fluorine atom react in a 1 : 1 ratio, since sodium has one electron to
lose from its outermost shell and fluorine requires one more electron to complete its outermost shell.

Na + F Na + + F −

To lose its entire outermost shell, a calcium atom must lose two electrons. Since each fluorine atom needs only
one electron to complete its octet, it takes two fluorine atoms to react with one calcium atom:

−
F F
Ca + Ca 2+ +
F F −

The calcium atom has two electrons in its outermost shell. Each oxygen atom has six electrons in its outermost
shell and requires two more electrons to attain its octet. Each oxygen atom therefore requires one calcium atom
from which to obtain the two electrons, and calcium and oxygen react in a 1 : 1 ratio.

Ca + O Ca 2+ + O 2–

EXAMPLE 5.6. With the aid of the periodic table, use electron dot notation to determine the formula of the ionic compound
formed between lithium and oxygen.
Ans. Lithium is in group IA and oxygen is in group VIA, and so each lithium atom has one outermost electron and each
oxygen atom has six. Therefore, it takes two lithium atoms to supply the two electrons needed by one oxygen atom.

+
Li Li
+ O + O 2–
Li Li +

The formula of lithium oxide is Li 2 O.