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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
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SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 6
6.2 Classification of the Elements
Chemists today understand that the repetition of properties of elements occurs because the electron configurations of atoms exhibit
repeating patterns. Thus, the arrangement of elements in the periodic
table reflects the electron structures of atoms.
For example, the group number of a representative element gives
the number of valence electrons in an atom of that element. The
group 13 element aluminum, for example, has the electron configuration 1s 2 2s 2 2p 6 3s 2 3p 1 , or [Ne]3s 2 3p 1 . The three electrons in the
third energy level (3s 2 and 3p 1 ) are the valence electrons of the aluminum atom. In a similar way, the period number of a representative
element indicates the energy level of the valence electrons.
Aluminum is in the third period, and aluminum’s valence electrons
are in the third energy level.
Practice Problems
Use the periodic table to answer the following questions.
8. How many valence electrons are in an atom of each of the
following elements?
a. magnesium
d. arsenic
b. selenium
e. iodine
c. tin
9. In which energy level are the valence electrons of the elements
listed in question 8?
10. Identify each of the following elements.
a. an electron configuration of [Kr]5s 2 4d 10 5p 2
b. five valence electrons in the sixth energy level
c. two valence electrons in the first energy level
d. three fewer electrons in the fourth energy level than
krypton
e. an electron configuration ending in 4p 2