Isoelectronic Species, Molecules and Structure for JEE

VSAT 2022

Isoelectronic Definition

“Iso” means equal whereas Isoelectronic as the name suggests means the equal charge or equal electric. Two or more atoms, molecules, or ions that have the same number of electrons and electronic structure are known to be isoelectronic. Isoelectronic species typically have the same chemical properties. Thus, the atoms or ions with the same electronic configurations are said to be isoelectronic to one another. In other words, it can also be stated as the ions or atoms that have the same number of electrons are isoelectronic species. The elements might be different when the number of electrons is equal which is why they are termed as isoelectronic species. 

The isoelectronic species are said to have the property of isoelectronic. Isoelectronicity is a phenomenon between two or more molecules with the same structure and electronic configuration. Isoelectronicity has a few applications. It is a phenomenon that can be used to find rare or unknown compounds by comparing their configuration with the known compounds. 

Isoelectronic Species

Any two or more atoms, ions, or molecules that have the same number of electrons are termed isoelectronic species. Isoelectronic species do not necessarily have to be the same elements. The elements may differ.

Isoelectronic Species Examples

Some of the examples of isoelectronic species are as follows:

  • Na+ and F- is one isoelectronic pair.

The electronic configuration of the isoelectronic pair is 1s² 2s² 2p6 .

  • N3-, Al3+, Mg2+, for N3- the number of electrons would be 10, that is 7 + 3. For Al3+ would be 10, and for Mg2+ it’s 10.

  • OH-, H2O both have 18 electrons.

How to Find Isoelectronic Species

To find whether a pair of species is isoelectronic or not look at the calculations below:

C=6e, O=8e, and 2e from negative charge.

So, CO32−​⇒ 6 + (3×8) + 2 = 8 + 24 = 32 electrons

In the same way, for NO3− ​ , N = 7e, O = 8e

NO3​(total electrons) =7 + (3×8) + 1 = 32 electrons

Both CO32−​and NO3−  ​have 32 electrons each; therefore, they are isoelectronic. 

Another example:


PO43−​⇒15+ (8×9) +3=15+3+32=50 electrons 

and SO42−​⇒S has 16 electrons

SO42−​⇒16+ (4×8) +2=16+2+32=50 electrons

Thus, PO43−​and SO42−​  are isoelectronic.

CO and N2​    

Here, the atomic number for C=6, O=8, and N=7.

6+8 and 7+7 =14.

As both have 14 electrons, therefore, they are isoelectronic.

Isoelectronic Ions

Two or more ions that have the same number of electrons are termed isoelectronic ions. Some of the examples of isoelectronic ions are the K+ ion which is isoelectronic with the Ca2+ ion. Also, carbon monoxide CO is isoelectronic to nitrogen N2 and NO+. 

Isoelectronic Molecules

Two or more molecules or a group of two or more atoms that have the same electronic configuration and the same structure but differ by some specific elements in the structure are known as isoelectronic molecules.

Some examples of isoelectronic molecules include CO, NO+, and Nitrogen (N2).

Isoelectronic Series

The atoms, ions, or molecules form an isoelectronic series when they all have the same number of electrons. Examples of an isoelectronic series include:

  • O2−, F, Na+, Mg2+, Al3+ all have 10 electrons.

Another example of an isoelectronic series is: 

  • N3-, Al3+, Mg2+: All these have an equal number of electrons, i.e., 10.

  • He, H-, and Li+ are all isoelectronic species as all of them have 2 electrons (i.e. have the same electronic configuration).

It must also be noted here that all isoelectronic species have the same chemical properties as well.

Isoelectronic Pairs List

Listed below are some isoelectronic pairs.

  • CO32−​and NO3−​

  • PO43- and SO42-

  • CO and N2

More Examples on Isoelectronic Elements and Their Electron Configuration

Now that we have gone through the various isoelectronic elements, let us look at some of the other isoelectronic pairs, their electron configuration, and the number of electrons.


Isoelectronic Elements

Electronic Configuration

Number of Electrons


Na+ and F-

1s2 2s2 2p6.



N3-, Al3+

1s² 2s²2p⁶



Ne, F-

1s² 2s²2p⁶



Na+, Mg2+

1s² 2s²2p⁶



S2-, P3-

1s² 2s²2p⁶3s2 3p6



Ar, S2-

1s² 2s²2p⁶3s2 3p6



He, Be2+




The article presents a thorough understanding of concepts such as isoelectronicity. The article traverses through the various ions, molecules, series, and the pair list that is isoelectronic and depicts isolectronicity. An insight into the structure of isoelectronic elements is also studied. The article also summarises the steps on how to measure whether the given species are isoelectronic or not. In addition, to summarise a few uses of isoelectronicity, it is a phenomenon used to predict the properties of certain species.

It is also used to look over at the reaction of species with one another. Isoelectronicity can also be used to identify hydrogen-like atoms that are isoelectronic to hydrogen. One of the major uses of isoelectricity is that the phenomenon can be used to predict or identify unknown or rare compounds or elements on the earth. Their electronic resemblance can be compared with those of known species.

Competitive Exams after 12th Science

FAQs on Isoelectronic Species, Molecules and Structure for JEE

1. Are O2- and F- isoelectronic?

NO, O2-, and F- are not isoelectronic. Iso as we know means equal whereas isoelectronic means elements with the same number of electrons. O and F are two consecutive atoms in the periodic table, and they differ structurally by one proton. Also, the atomic number of Oxygen is 8 and Fluorine is 9. It concludes that O has to have 8 electrons and F will have nine electrons to compensate for those protons.

Now, in nature, oxygen exists as the O2 molecule; fluorine is far too reactive to exist naturally as F2, it is only found in nature as the fluoride F-. The F2 molecule has 14 electrons, while the oxygen molecule has 12 electrons. These two are not isoelectronic as the numbers of electrons are unequal for the pair.

2. What are ions?  Which ion is isoelectronic with Ar?

The atoms which gain or lose electrons to give them an overall charge are termed ions. We know for a neutral atom, the number of protons is equal to the number of electrons. Therefore, an atom does not have a charge as the positive balances out the negative. 

For example, Sodium (Na) has a +1 charge as it has lost 1 electron, therefore, making it slightly more positive. Normally, it has 11 electrons, but it loses one electron. Chlorine (Cl) has a -1 charge as it has gained 1 electron thereby making it slightly more negative. Chlorine normally has nine electrons but gains one electron to make it ten. Both chlorine and sodium are considered isoelectronic to Argon (Ar) as both have the same number of electrons. Negative ions gain electrons thereby giving them a negative charge, whereas all positive ions lose electrons thereby giving them a positive charge. As both have the same number of electrons, therefore they are isoelectronic to Argon. A detailed overview of how to find out the isoelectronic species is given above.