Video Transcript
Which of the following atoms or
ions is not isoelectronic with all of the others? (A) Ne, (B) Na, (C) Mg2+, (D) Al3+,
(E) F−.
Let’s start by defining the term
isoelectronic. Atoms and ions of different
elements are isoelectronic if they have the same number of electrons and the same
electronic configuration. To answer this question, we will
need to write the electronic configurations of all the given atoms and ions.
Let’s start with a neon atom, which
has a total of 10 electrons. The electronic configuration of a
neon atom is 1s2 2s2 2p6. A sodium atom has 11 electrons. Its electronic configuration is 1s2
2s2 2p6 3s1. Now, a magnesium atom has 12
electrons, and its electronic configuration is 1s2 2s2 2p6 3s2.
However, we need to determine the
electronic configuration of the Mg2+ ion. An ionic charge of two plus
indicates that the magnesium atom lost two electrons. These two valence electrons will be
lost from the 3s subshell. Therefore, the magnesium two plus
ion has only 10 electrons, and its electronic configuration is 1s2 2s2 2p6. If we pause here, we can already
see that the neon atom and the Mg2+ ion have the same number of electrons and the
same electronic configuration.
Now, let’s have a look at the next
two answer choices. An aluminum atom has 13 electrons
and has the electronic configuration shown. An ionic charge of three plus means
that the aluminum atom lost three electrons. These three electrons are lost from
the 3s and 3p subshells. Therefore, the Al3+ ion has 10
electrons, and its electronic configuration is 1s2 2s2 2p6.
Finally, we need to go through a
similar process for the last answer choice. A fluorine atom has nine electrons
and has the electronic configuration 1s2 2s2 2p5. A fluorine atom must gain one
electron in its valence shell to form the F− ion. This electron is added to the 2p
subshell. Therefore, the F− ion has 10
electrons, and its electronic configuration is 1s2 2s2 2p6.
Now we can see that the neon atom,
the magnesium ion, the aluminum ion, and the fluoride ion all have the same number
of electrons. And they have the same electronic
configurations. The sodium atom is the only species
that does not share this electronic configuration. Therefore, the atom or ion that is
not isoelectronic with all of the others is sodium, or answer choice (B).
