Electron Configuration

What is the Electron Configuration?

The electron configuration is the summary of where the electrons are located around a nucleus. Each neutral atom has a number of electrons equal to its number of protons. Therefore, these electrons are located in orbitals in an arrangement around the nucleus. The notation indicates that indicates their energy and the type of orbital in which they are located. The types of orbitals and how many electrons each can accommodate is summarized as follows up to the n=4 level.

Then, according to the above table, it needs 2 electrons to fill an orbital s, 6 electrons to fill an orbital p, 10 electrons to fill an orbital d and 14 electrons to fill the orbital f.

However, in order to determine the electron configuration of every element in the periodic table, we will need to know the order of filling of these orbitals.

Order of filling

The order in which the electrons are placed in orbitals is based on the energy of these. Thus gradually filling up by order of this energy according to the Aufbau principle.

The Aufbau principle derived from the Schrödinger equation, which describes the behavior of electrons in an atom.

According to the Aufbau principle, electrons fill the lowest energy orbitals first before occupying higher energy orbitals. This means that in a ground state atom, the electrons will always occupy the orbitals with the lowest energy levels first, and then fill higher energy levels as needed.

The orbitals of lowest energy are filled first, and is determined in the following way:

1s²→2s²→2p⁶→3s²→3p⁶→4s²→3d¹⁰→4p⁶→5s²→4d¹⁰→5p⁶→6s²→4f ¹⁴→5d¹⁰→6p⁶→7s²→5f ¹⁴→6d¹⁰→7p⁶ (6f ¹⁴ 7d¹⁰ 7f ¹⁴) → 8s² …

The order of filling of the last three layers are described, has not been able to determine. This is mainly due to non-availability of the necessary amount of the corresponding items to make experimental measurements of physico-chemical properties.

Keep in mind that this order is an estimate, and for certain elements varies as there are configurations that are more stable when the orbitals are filled, semillenos, or empty.

To write the electron configuration, we start with the layer number (n) followed by the type of orbital and finally the superscript indicates how many electrons are in the orbital. For example, an atom of carbon has 6 electrons would be:

Carbon: 1s² 2s² 2p⁴

Often abbreviated notation, using the noble gas preceding in order of atomic number in square brackets. For the carbon would be:

Carbon: [He] 2s²2p⁴

The order of filling orbitals follows a specific pattern, which is based on the periodic table. The first two electrons occupy the 1s orbital, followed by two electrons in the 2s orbital. The 2p orbitals are then filled with six electrons, before the 3s orbital is filled with two electrons. This pattern continues until all of the electrons are accounted for.

The Aufbau principle is a useful tool for predicting the electron configurations of atoms and molecules. By following the order of filling orbitals based on the principle, we can determine the electron configuration of an atom or ion, which is essential for understanding its chemical and physical properties.

However, it is important to note that the Aufbau principle is a simplified model that does not account for some exceptions and anomalies that occur in certain elements. For example, the electron configurations of some transition metals and lanthanides are not strictly predictable based on the Aufbau principle alone. Nonetheless, the principle remains a useful starting point for understanding the behavior of electrons in atoms and molecules.

Electron configuration table

The following table shows the electron configuration of the elements, ordered by their atomic number (Z).

Electron Configuration Video

Frequently asked questions about electronic configuration

• What is the electron configuration?

The electron configuration is a description of the electronic structure of an atom. The orbitals are filled with electrons in all shells following three rules: Aufbau’s principle, Pauli’s exclusion principle and Hund’s rule.

• What is the electron configuration of the chromium atom (Cr Z = 24)?

The expected electron configuration for chromium is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s². However, half-filled and fully filled subshell present extra stability, and, thus, the correct electron configuration of crhomium atom [Z = 24] is [Ar] 3d⁵ 4s¹.