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Electronic Configuration of Atoms and Ions

Electronic Configuration: An Introduction

Last updated date: 23rd Mar 2023
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Have you ever considered how atoms are represented? The number of orbitals for atoms with a high atomic number is also high. For instance, the atomic number of mercury (Hg) is 80. For example, mercury (Hg) has an atomic number of 80. The filling of electrons is somewhat complicated as it has 80 electrons to fill around the nucleus. We must deal with a few electronic configuration prediction rules in order to specify the position of an electron in an atom.

Electronic configuration is defined as the layered arrangement of shells, where a specific number of electrons orbit around the nucleus. An atom has a central core where the entire positive charge concentrates; this is the nucleus. The nucleus contains neutrons and protons huddled closely right at the centre of the atom. Atomic shells have a layered arrangement. Each shell has a fixed number of electrons revolving around the nucleus.

A scientist named Charles Bury gave a set of simple rules that made it very easy to understand how electrons are arranged in Bohr's atomic model. This scheme of electron arrangement is known as the Bohr-Bury electron arrangement.

Rules for Filling Electrons

Rules for filling electrons based on Bohr-Bury electron arrangement are given as follows:

  1. An electron will always fill an orbit or a shell with lower energy and then occupy the higher energy ones. Therefore, electron filling starts from the K shell, then the L, M, and N accordingly.

  2. Depending on the orbit number, each orbit can only hold a finite number of electrons. The formula can determine this.

Number of electrons = 2n2

where n is the orbit number. Thus, this rule is frequently referred to as the "2n2 rule". The number of electrons that each orbit can hold increases with respect to its 'n' value.

Shells and Subshells

The shells are the paths around the nucleus where the electrons move. The shell nearest to the nucleus has lower energy. The energy increases as it travels to the outermost shell. It can be denoted as K, L, M, N, etc. Subshells are seen within the shells, which serve as the paths of electrons. The names of the subshells are based on the quantum number of angular momentum. A shell has four different subshell types: s, p, d, and f. The lowest energy subshell is the s, followed by p, d, and f. The formula 2(2l+1) can be used to determine the maximum number of electrons that can be occupied by each subshell. As a result, the s, p, d, and f subshells should be able to hold 2, 6, 10, and 14 electrons, respectively.

Number of Electrons Occupied in Each Shell


Shell Number (n)

Number of Electrons (2n2)

Subshells Involved

















The maximum number of electrons that the outermost shell of an atom can possess is eight. This is called the octet rule. Having the eight electrons in the outermost shell makes the atom stable. The outermost shell is also termed as the valence shell. The electrons occupied in the valence shell are called valence electrons.

Electronic Configuration of Atoms

Now let's put these rules to work and figure out the electronic configurations for some elements. For example, magnesium has an atomic number of 12. According to the first rule, the K shell is filled first as it is the lowest in energy. Only 2 out of 12 electrons are filled in the K shell. As a result, the second rule is also followed, because the K shell can only hold a fixed number of electrons, namely two. Now there is a balance of 10 electrons. As per the third rule, the valence shell, which is the L shell here, cannot hold more than 8 electrons. So out of 10, 8 electrons enter the L shell and the remaining 2 electrons enter the M shell. Hence, the M shell becomes the valence shell.

Electronic Configuration of Some Elements



Atomic Number

Electronic Configuration

Valence Shell














1s2 2s1





1s2 2s2





1s2 2s2 2p1





1s2 2s2 2p2





1s2 2s2 2p3





1s2 2s2 2p4





1s2 2s2 2p5





1s2 2s2 2p6





1s2 2s2 2p6 3s1


Electronic Configuration of Ions

The electronic configuration for ions is similar to that of atoms. The electronic configuration of a cation can be found out by removing electrons from the valence shells. Similarly, the electronic configuration of anions can be found by adding electrons to the valence shells.

For example, Mg has an atomic number of 12. The electronic configuration of Mg is

Mg: 1s2 2s2 2p6 3s2

For Mg2+, the electronic configuration can be written by removing 2 electrons from the valence shell, that is, 3s. Hence, the electronic configuration becomes

Mg2+: 1s2 2s2 2p6

Similarly, for Cl-, the atomic number is 17, with the electron configuration is

Cl: 1s2 2s2 2p6 3s2 3p5

To write the electronic configuration of cl-anion, we have to add one electron to the outermost shell, 3p.

Cl-: 1s2 2s2 2p6 3s2 3p6.

In this way, we can predict the electronic configuration of any ion.

Electronic Configuration Diagram

Electrons are arranged into energy levels or shells around the nucleus of an atom. The orbital radius increases as the energy level increases. We depict the shell by drawing a circle. A dot or a cross represents each electron and represents the nucleus by the chemical symbol. Each electron in an atom is in a particular shell, and the electron must occupy the lowest available shell nearest the nucleus. So, when we draw the electronic configuration, we have to fill up each shell in turn, starting with the lowest.

Let’s take lithium as an example.

Lithium Electron Structure

Lithium Electron Structure

Lithium has an atomic number of 3. It has 3 electrons and 3 protons - first electron into the first shell. And the second electron goes to the same shell. However, this shell can only contain a maximum of 2 electrons. So, the third element must fill in the next shell. This same process of filling electrons applies to even larger atoms too. The filling of electrons should be from the lowest to the highest energy levels.

Interesting Facts

Have you ever wondered why atomic shells are represented by the letters K, L, M, N, etc. instead of using A, B, C, etc. This is because of the fact that, at that point of time, only some orbitals were discovered. Scientists wanted to be certain that there was room to add more orbits inside and outside of the existing ones.

Key Features

  • An electronic configuration is the arrangement of electrons in an atom in a particular manner in the shells.

  • It follows the Bohr-Bury scheme to fill the electrons in the orbits.

  • A lower-energy orbit or shell will always be filled by an electron before moving on to a higher-energy orbit or shell.

  • According to the 2n2 rule, only a fixed number of electrons can be filled in each orbit, where n is the number of the orbit.

  • The maximum number of electrons that the outermost shell of an atom can contain is 8.

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FAQs on Electronic Configuration of Atoms and Ions

1. Explain Aufbau principle.

According to the Aufbau principle, electrons are filled into the empty orbitals of an atom from the lowest to the highest energy orbital. He explains the filling of electrons using principal quantum number (n) and azimuthal quantum number (l). Hence, this principle is also known as the (n+l) principle. The energy of each subshell depends on its (n+l) value. Subshells having a lower energy value have a smaller value of (n+l). If two subshells have the same (n+l) value, the subshell with the higher "n" value has the higher energy. For example, consider 3d and 4p subshells. They both have the same (n+l) value, that is 5. Hence, 4p will have greater energy than 3d because its 'n' value is greater for 4p.

2. Write down the electronic configuration of Ca2+ ions and predict their magnetic behaviour.

Calcium has an atomic number of 20. So, the electronic configuration can be written as 1s2 2s2 2p6 3s2 3p6 4s2. To write the electronic configuration for the ca2+ ion, we have to remove 2 electrons from the valence shell, that is, 4s. Hence, its electronic configuration becomes 1s2 2s2 2p6 3s2 3p6. From the electronic configuration, the number of unpaired electrons can be calculated. Here, for Ca2+, the number of unpaired electrons is zero. Hence, the Ca2+ ion is diamagnetic and has no magnetic moment.

3. State Hund's rule.

Hund’s rule states that pairing of electrons takes place only after each degenerate orbital is singly occupied. The electrons will be filled in such a way that the spin multiplicity value is maximised. The spin multiplicity is given by an expression, (2s+1), where s is the total spin value. When electrons are released into orbitals, they first try to fill every orbital with equal energy before combining with another electron in an orbital that is only half filled. Atoms in their ground states typically have a large number of unpaired electrons. Moreover, the spins of unpaired electrons in orbitals that are only partially filled are similar. That is, a subshell's initial electron may be spinning up or spinning down.