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The Aufbau Principle

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

The Aufbau principle explains how electrons fill up orbitals and shells inside an atom. It is used by chemists to predict the types of chemical bonds that an atom is likely to form. Learn more about it in this lesson.

Inside an Atom

Imagine for a minute that you are smaller than an insect, even smaller than a single celled organism. Keep going until you are the size of a single atom. How small is that? Atoms are the smallest particles of any material that can exist by themselves. They are so tiny that it takes about 100,000 of them to equal the width of a single human hair!

Even though atoms are small, there are even smaller particles that are found inside each atom. These are called protons, neutrons, and electrons. While protons and neutrons form the nucleus of the atom, electrons are outside the nucleus in a region called the electron cloud.

Do the electrons just randomly float around in the cloud? No, they certainly don't! Each electron is located in what we call an orbital, and each orbital can contain exactly two electrons. The orbitals are arranged into shells, that are based on the amount of energy that the electrons in each orbital have. Electrons in shells with higher numbers have more energy than electrons in lower numbered shells.

The Bohr Model of the Atom

In 1913, Niels Bohr first proposed the model of atomic structure, which we now call the Bohr Model. In the Bohr model, electrons orbit the atomic nucleus in defined orbitals, that together make up a series of energy levels. He called each energy level an electron shell, and he used this model to explain the chemical behavior of the elements.

Although quantum mechanics has given us more information about the nature of the electron cloud in the past 100 years, the Bohr model is still a good representation of what is actually happening inside an atom, and it still accurately predicts which elements are likely to bond with each other chemically and what the nature of those bonds will be.

The Aufbau Principle

In the Bohr model, electrons fill up the lowest available energy level before beginning to fill the next shell. Bohr called this the Aufbau principle. Aufbau means building up in German (Bohr's native language), so the Aufbau principle explains how the electron shells are built up, or filled with electrons.

In addition to the Aufbau principle, Hund's rule says that in a given energy level, electrons will go into unoccupied orbitals first before filling orbitals that already have one electron. Sort of like how a bunch of strangers getting onto a bus will tend to sit in only the empty seats until there are no empty seats left. Only then they will start sharing a seat with another. In atoms, if a shell has four orbitals that each can hold two electrons, each orbital will get one electron before any orbital will have two.

In the Bohr model, each electron shell is given a number, beginning with one. Within each shell there are different subshells that are designated by letters. The letter associated with each sub shell (s, p, d, f, etc.) tells you what shape the orbitals in that subshell will have and also how many orbital pairs can be in each shell. The two smallest subshells, labeled s and p, can hold a maximum of two and six electrons, respectively.

aufbau principle

So, how can you use the Aufbau principle to predict which electron shells will be filled in an atom? Let's use the chlorine atom, which has 17 electrons, as an example.

  • In this atom, the 1s shell will fill first with two electrons. This leaves 15 electrons.
  • The next two electrons will go into the 2s shell because it is the lowest energy subshell in shell two.
  • Six electrons will fill up the 2p subshell next and then two more in the 3s subshell.
  • This leaves a total of five electrons left that will go into the 3p subshell. One electron will be added to each orbital until all three have an electron, then electrons will start to fill the second spot in each 3p orbital, but there are only 2 electrons left.

One orbital will be left with only one electron instead of 2. So, this is the configuration of electrons in a chlorine atom:

1s: 2 electrons

2s: 2 electrons

2p: 6 electrons

3s: 2 electrons

3p: 5 electrons (one orbital will only have one, so the shell is not complete)

Note that 1s has two electrons, 2s and 2p total eight electrons, and the outer orbital 3s and 3p have 7 electrons.
bohr model

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