Isoelectronic Species - Important Concepts with Practice Questions

The term isoelectronic means “same charge”. We know electrons are one of the subatomic particles of an atom. Every atom consists of a different number of electrons. However, the number of electrons of two different atoms(in case of isotopes), molecules, or ions can be equal. Such species are called isoelectronic species or isoelectronic structures.

The phenomenon of two species having the same isoelectronic structure is called Isoelectronicity. We’ll learn what isoelectronic species are, their examples, and their structures in this topic.

Isoelectronic Species Examples

Atoms (in case of isotopes), molecules, or ions with the same number of electrons and identical electronic configurations are considered to be isoelectronic species. Let’s have a look at some of the examples of isoelectronic species.

One of the examples of pairs of isoelectronic species is Na⁺ and F^-. The electronic configuration of both the ion is identical (1s² 2s² 2p⁶). This is because both the ions Na+ and F- have the same number of electrons which is equal to 10. 

As we know the atomic number of fluorine is 9, and the total number of electrons in an F atom is 9. But, F^- has one negative charge on fluorine, which means that it has one additional electron in its shell. Thus, the number of electrons in a fluorine ion is ten electrons (9+1=10). 

The atomic number of sodium is 11, and the total number of electrons in the Na atom is 11. But Na⁺ has one positive charge on sodium, which means that it has one less electron than the Na atom in its shell. Thus, the number of electrons in sodium ion is (11−1=10). 

Similarly, N^3-, Al³⁺ and Mg²⁺ are all three are parts of the isoelectronic species list, these ions have the same number of electrons and thus are isoelectronic species, the total number of electrons is 10 in all three ions.

Isoelectronic Molecules

Two molecules having the same number of electrons are considered isoelectronic molecules. To find out the number of electrons of a molecule we simply have to add up the electrons possessed by the atoms constituting that molecule.

Let’s consider carbon monoxide (CO) and nitrogen (N₂) molecules as examples. Carbon monoxide is made up of carbon (6 electrons) and oxygen (8 electrons) atoms, while N₂ is made up of two nitrogen atoms (7 electrons). Let’s count the total number of electrons in both the molecules

For CO: 6 + 8 = 14 electrons
For N₂: (2 x 7) = 14 electrons.

As we can see both the molecules have the same number of electrons despite having different constituent atoms. Such molecules are isoelectronic molecules.

Finding isoelectronic species

To find isoelectronic pairs we simply have to add the number of electrons in each atom in a given molecule. In the case of an ion along with the individual electrons of each atom, the charge of the ion is added. For example, one such isoelectronic pair is $CO_3^{2-}$ and Let's calculate the number of electrons in both the ions and see why they are isoelectronic. 

We know carbon has 6 electrons and oxygen has 8 electrons, the number of electrons in $CO_3^{2-}$ are $=6+(3\times8)+2=32$

Let’s count the number of electrons for $NO_3^-$. Now, nitrogen has 7 electrons and oxygen has 8. 

$NO_3^-=7+(3\times8)+1=32$

Both the ions have 32 electrons and that’s why they are called isoelectronic.

Let’s understand this better with help of a question

Ques: Which among K⁺, Cl^-, Na⁺ is a pair of isoelectronic species?

Ans: To find out the isoelectronic species among given ions let’s first find the number of electrons in each ion.

K⁺= Potassium (K) has atomic number 19 and therefore, elemental potassium has 19 electrons. However, there is one positive charge on potassium ion (K⁺) which means it has one electron less than a K atom. 

Therefore, electrons in K⁺= 19-1 = 18 electrons.

Cl^-= Atomic number of chlorine is 17, which means an uncombined Cl atom has 17 electrons. One negative charge represents one extra electron.

Therefore, electrons in Cl^-= 17+1 = 18 electrons.

Na⁺= Sodium atom has 11 electrons as its atomic number is 11. One positive charge represents one less electron in Na⁺

Conclusion

Different atoms(in case of isotopes), molecules, or ions having the same number of electrons and electronic configuration are called isoelectronic species. These species show similarities in their chemical properties. Na⁺ and F^- are examples of isoelectronic pairs having 10 electrons each. 

Molecules comprising different atoms having an equal number of electrons are isoelectronic molecules. Carbon monoxide (CO) and nitrogen (N₂) molecules are one such example, both the molecules have a total of 14 electrons.