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EXAMPLE
Problem 1
ELECTRON CONFIGURATION AND THE PERIODIC TABLE
Strontium, which is used to produce
red fireworks, has an electron configuration of [Kr]5s
2
. Without using the periodic table, determine
the group, period, and block of strontium.
1
ANALYZE THE PROBLEM
You are given the electron configuration of strontium.
Known
Unknown
Electron configuration
=
[Kr]5s
2
Group
=
?
Period
=
?
Block
=
?
2
SOLVE FOR THE UNKNOWN
The s
2
indicates that strontium’s valence electrons fill
the s sublevel. Thus, strontium is in
group 2
of the
s-block
.
The 5 in 5s
2
indicates that strontium is in
period 5
.
3
EVALUATE THE ANSWER
The relationships between electron configuration and
position on the periodic table have been correctly applied.
For representative elements, the number
of valence electrons can indicate the group
number.
The number of the highest energy level
indicates the period number.
As you move across a period, electrons fill the d orbitals. For example, scandium (Sc),
the first d-block element, has an electron configuration of [Ar]4s
2
3d
1
. Titanium (Ti), the
next element on the table, has an electron configuration of [Ar]4s
2
3d
2
. Note that titani-
um’s filled outermost s orbital has an energy level of
n
=
4, while the d orbital, which is
partially filled, has an energy level of
n
=
3.
As you learned previously, the aufbau principle states that the 4s orbital has a lower
energy level than the 3d orbital. Therefore, the 4s orbital is filled before the 3d orbital.
The five d orbitals can hold a total of 10 electrons; thus, the d-block spans 10 groups on
the periodic table.
f-Block elements
The f-block contains the inner transition metals. Its elements are characterized by a filled,
or partially filled outermost s orbital, and filled or partially filled 4f and 5f orbitals.
The electrons of the f sublevel do not fill their orbitals in a predictable manner.
Because there are seven f orbitals holding up to a maximum of 14 electrons, the
f-block spans 14 columns of the periodic table.
STEM CAREER Connection
Physical Chemist
Does the idea of using computers and sophisticated laboratory instruments to
model, simulate, and analyze experimental results appeal to you? Are you
someone who enjoys developing new theories? Physical chemists are interested
in how matter behaves at the molecular and atomic level. They have a strong
interest and background in chemistry, physics, and math.
Lesson 2 • Classification of the Elements
149