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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
49
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
5
Practice Problems
13. Using noble-gas notation, write the electron configurations of
the following elements.
a. fluorine
b. phosphorus
c. calcium
d. cobalt
e. selenium
f. technetium
g. iodine
h. holmium
i. iridium
j. radium
Valence electrons When elements combine chemically, only the
electrons in the highest principal energy level of each atom are
involved. Therefore, these outermost electrons, called valence electrons, determine most of the chemical properties of an element.
Later in your chemistry course, you will study the way in which elements form chemical bonds. Because bonding involves an atom’s
valence electrons, it is useful to be able to sketch a representation of
an element’s valence electrons. The American chemist G. N. Lewis
devised the electron-dot structure to show an atom’s valence electrons by writing dots around the symbol of the element.
In writing electron-dot structures, a single dot is used to represent each valence electron. One dot is placed on each of the four
sides around the symbol before any two dots are paired together.
The following examples illustrate the process. The electron-dot
structure for hydrogen, which has one electron, is
. The electrondot structure for helium, 1s 2 , is
. The electron configuration of
lithium is 1s 2 2s 1 , but the two 1s electrons are in a stable inner
energy level and do not participate in chemical changes. Only the
outermost 2s electron is a valence electron, so the electron-dot
structure for lithium is
. The electron configuration of beryllium
Li
He
H
▲