Question

Arrange the following isoelectronic ions in the increasing order of their ionic size. Give their correct electron configuration. Use Zeff as a tool to explain the trend in their ionic size. O 2- , F- , Na+ , Mg2+, Al3+. Ionic radii for the same ions are given in the text

Answer 1

each ion in the isoelectronic series O2 - , F-, Na +, Mg2+, and Al3+ has 10 electrons. In any isoelectronic series we can list the members in order of increasing atomic number; therefore, the nuclear charge increases as we move through the series. Because the number of electrons remains constant, ionic radius decreases with increasing nuclear charge as the electrons are more strongly attracted to the nucleus:

each electron in a many-electron atom is screened from the nucleus by the other electrons. It therefore experiences a net attraction that is less than it would in the absence of other electrons. the electron experiences a net attraction that is the result of the nuclear attraction decreased by the electron–electron repulsions. We call this partially screened nuclear charge the effective nuclear charge, Zef. For valence-electrons,The effective nuclear charge (Zeff) increases from left to right across any period of the periodic table

Increasing nuclear charge order: O^{2- ,} F^{- ,} Na ^{+ ,}Mg^2+, Al³⁺

So the incresing order of ionic size: Al^{3+ ,}Mg^{2+ ,}Na ^{+ ,.}F^{- ,}O^2-

Electron configurations:

O^2-: 1s² 2s² 2p⁶

F^{- :} 1s² 2s² 2p⁶

Na ⁺ : [Ne] 3s⁰

Mg^{2+ :} [Ne] 3s⁰

Al^{3+ :} [Ne] 3s⁰ 3p⁰