Isoelectronic ions are those which have a same number of electrons and hence same electronic configuration but the different magnitude of nuclear charge. A set of species (cations or anions) having the same number of electrons is known as iso-electronic series, e.g. N3-, O2-, F-, Na+, Mg+, Al3+ are isoelectronic species.
Within the series of isoelectronic ions, as the atomic number or nuclear charge increases, the attractive force between the nucleus and a same number of electrons also increases. This results in the decrease of ionic radius.
For example,
Ion =N3- O2- F- Na+ Mg+ Al3+
No. of electrons = 10 10 10 10 10 10
At. No = 7 8 9 11 12 13
Since, the size of the iso-electronic ion decreases with the increase in atomic number, the order of decreasing size is
N3- > O2- > F- > Na+ > Mg2+ > Al3+
Give the formula of a species which will be isoelectronic with the following atoms or ions:
(i) Na2+
(ii) Cl-
(iii) Ca2+
(iv) Rb+.
Which of the following species will have the largest and the smallest size?
Mg, Mg2+, Al, Al3+.
In each of the following pairs, which species has a larger size? Explain.
(i) K or K+
(ii) Br or Br-
(iii) O2- or F-
(iv) Li+ or Na+
(v) P or As
(vi) Na+ or Mg2+
Consider the following species : N3-, O2-, F-, Na+, Mg2+ and Al3+
(a) What is common in them?
(b) Arrange them in the order of increasing ionic radii.
What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions.
(i) F- (ii) Ar (iii) Mg2+ (iv) Rb+
