Aufbau Principle
Ck12 Science
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Printed: August 8, 2014
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Aufbau Principle
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Aufbau Principle
• State the Aufbau principle.
• Use the Aufbau principle to determine the electron configuration of an atom.
How are buildings constructed?
Construction of a building begins at the bottom. The foundation is laid and the building goes up step by step. You
obviously cannot start with the roof since there is no place to hang it. The building goes from the lowest level to the
highest level in a systematic way.
Aufbau Principle
In order to create ground state electron configurations for any element, it is necessary to know the way in which the
atomic sublevels are organized in order of increasing energy. The Figure 1.1 shows the order of increasing energy
of the sublevels.
The lowest energy sublevel is always the 1s sublevel, which consists of one orbital. The single electron of the
hydrogen atom will occupy the 1s orbital when the atom is in its ground state. As we proceed with atoms with
multiple electrons, those electrons are added to the next lowest sublevel: 2s, 2p, 3s, and so on. The Aufbau principle
states that an electron occupies orbitals in order from lowest energy to highest. The Aufbau (German: “building up,
construction”) principle is sometimes referred to as the “building-up” principle. It is worth noting that in reality
atoms are not built by adding protons and electrons one at a time and that this method is merely an aid for us to
understand the end result.
As seen in the Figure 1.1, the energies of the sublevels in different principal energy levels eventually begin to
overlap. After the 3p sublevel, it would seem logical that the 3d sublevel should be the next lowest in energy.
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FIGURE 1.1
Electrons are added to atomic orbitals in
order from low energy (bottom of graph)
to high (top of graph) according to the Aufbau principle. Principal energy levels are
color coded, while sublevels are grouped
together and each circle represents an
orbital capable of holding two electrons.
However, the 4s sublevel is slightly lower in energy than the 3d sublevel and thus fills first. Following the filling of
the 3d sublevel is the 4p, then the 5s and the 4d. Note that the 4f sublevel does not fill until just after the 6s sublevel.
The Figure 1.2 is a useful and simple aid for keeping track of the order of fill of the atomic sublevels.
Summary
• The Aufbau principle gives the order of electron filling in an atom.
• It can be used to describe the locations and energy levels of every electron in a given atom.
Practice
Questions
Use the link below to answer the following questions:
http://ths.talawanda.net/~BrambleN/classroom/Chemistry/Notes/Section%202A/Exceptions&Shortcut.htm
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Chapter 1. Aufbau Principle
FIGURE 1.2
The Aufbau principle is illustrated in the
diagram by following each red arrow in
order from top to bottom: 1s, 2s, 2p, 3s,
etc.
1. Do the electrons in all atoms follow the Aufbau rule?
2. What happens to electrons in copper to make the atom more stable?
3. How does silver become more stable?
Review
Questions
1.
2.
3.
4.
What is the Aufbau principle?
Which orbital is filled after the 2p?
Which orbital is filled after 4s?
Which orbital is filled after 6s?
• Aufbau principle: An electron occupies orbitals in order from lowest energy to highest.
References
1. Gary Minnaert. http://commons.wikimedia.org/wiki/File:LACMA_BCAM02.jpg .
2. CK-12 Foundation - Christopher Auyeung. .
3. CK-12 Foundation - Christopher Auyeung. .
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