Lesson 7.6 Electron Configurations II
Suggested Reading
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Zumdahl Chapter 7 Section 7.11
Essential Question


How is the periodic table used to write electron?
What are the magnetic properties of atoms?
Learning Objectives
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
Determine electron configurations based on position in the periodic
table.
Describe the magnetic properties of atoms.
Introduction
Atomic configurations help you to understand bonding and the chemistry of
the elements. Therefore, it is helpful if you can reproduce atomic
configurations with ease using the building up principle. This can be
accomplished by recalling the building up order of the subshells using the
periodic table.
Writing Electron Configurations Using the Periodic Table
The structure of the periodic table offers a sound way to remember the
building-up order. There is a definite pattern to the order in which the
subshells are filled with electrons, and from this you can write the buildingup order. The periodic table is divided into blocks representing the different
subshells that are filled as one moved across the periodic table.
For example, in the orange area a ns subshell is being filled. In the purple
area a np subshell is being filled. The value for n is obtained from the
period (row) number. In the d-block, an (n-1)d subshell is being filled.
You read the building up order by starting the the first period, which which
the 1s subshell is being filled. In the second period, the 2s subshell is filled
and then you jump across to the 2p subshell. In the 3rd period you have 3s
and 3p. In the 4th period you have 4s, 3d, and 4p. This pattern should
become clear enough to visualize with a periodic table that is not labeled
with the subshells, such as the one provided for your exams. Take a
moment to see if you can identify the blocks and building-up order on an
unmarked periodic table.
Ex: Determining the Configuration of an Atom Using the Building-Up
Principle
Use the building-up principle to obtain the configuration for the ground state
of the gallium atom (Z = 31). Give the configuration in complete form (do
not abbreviate for the core). What is the valence shell configuration?
Solution
Find Gallium on the periodic table. Note that is sits in period 4 of the pblock. Now use the periodic table to identify the filling up order up until
Gallium is reached.
Now fill the subshells with electrons until you have a total of 31:
1s22s22p63s23p64s23d104p1
The subshells are completely filled until the 4p orbital. Recall that electrons
are added as you move across the periodic table. Thus, you can see from
gallium's position in the p-block that it only has one electron in the porbital.
If you what to write the configuration so that the valence (outermost) shells
appear last you rearrange the configuration giving:
1s22s22p63s23p63d104s24p1
Finally, the valence shell configuration
is 4s24p1
Here is an important point that many students overlook:
In many cases, you need the configuration for the outer (valence)
electrons. Remember Lewis diagrams? You can determine this from the
position of the element in the periodic table. The valence-shell configuration
of a main-group element is nsa npb, where n is the principal quantum
number for the outer shell, also equals the period number for the element.
The total number of valence electrons, which equals a + b can be obtained
from the group (column) number. For example, gallium is in Period 4, so n
= 4. It is in Group IIIA, so the number of valence electrons is 3. This give
the valence-shell configuration of 4s24p1. The configuration for all elements,
whether Main Group or Transition, are obtained in a similar way.
Please make sure you have a complete understanding of valence
electrons. This is a key concept for much of chemistry.
Ex. Determining the valence shell configuration from the periodic
table.
What is the valence configuration for tellurium (Z = 52)?
Solution:
Tellurium (Te) is located in Period 5, Group VIA. Because it is a main-group
element, the outer shells are 5s and 5p. These subshells contain 6
electrons because the Group is VIA. The valence-shell configuration is
5s25p4.
Magnetic Properties of Atoms
Occasionally, questions about the magnetic properties of atoms crop up on
AP Chemistry exams. Here are the basic concepts. You learned that
electrons behave like small magnets by virtue of their spin. However, when
two electrons of opposite spins are placed in an orbital according to Hund's
rule, the magnetic attractions cancel each other. As a result, an atom that
has only doubly occupied orbitals has no net spin magnetism. Conversely,
atoms with unpaired electrons do exhibit magnetic properties. Atoms that
do not exhibit magnetic properties are called diamagnetic substances while
those that do are called paramagnetic substances.
By placing atoms in a magnetic field we can determine is a substance is
diamagnetic or paramagnetic.
A paramagnetic substance is a substance that is weakly attracted by a
magnetic field, and this attraction is generally the result of unpaired
electrons.
A diamagnetic substance is a substance that is not attracted by a magnetic
field. This property generally means that the substance has only paired
electrons.
You should be able to identify diamagnetic and paramagnetic substances
from their electron configurations. You must also be able to state why a
substance is either diamagnetic and paramagnetic based on electron
configuration.
Homework:
Book questions pg. 323 questions 79, 81, 83