Animated Solution for Chemistry - Periodicity in Properties: The isoelectronic set of ions is

Have you ever wondered what a nitrogen ion, an oxygen ion, and a sodium ion could possibly have in common? On the surface, they belong to completely different groups in the periodic table. They have different masses, different numbers of protons, and vastly different chemical behaviors. Yet, under the right conditions, they can join an exclusive chemical club known as an isoelectronic series.

Let's decode the word itself. The prefix iso- comes from Greek, meaning "equal" or "same." The electronic part, naturally, refers to electrons. Therefore, isoelectronic species are simply atoms or ions that possess the exact same total number of electrons.

To determine if a group of ions is isoelectronic, we need to count their electrons. In a neutral atom, the number of electrons is perfectly balanced by the number of protons, which is given by the atomic number ($Z$).

However, ions are formed when atoms gain or lose electrons. The formula to find the total number of electrons in an ion is beautifully simple:

Remember the golden rule of charges: a negative charge means the atom has gained negatively charged electrons (so subtracting a negative number mathematically adds to the total). A positive charge means the atom has lost electrons.

Let's put our formula to the test by examining the ions presented in the correct option.

First up is the nitride ion, $N^{3-}$. Nitrogen has an atomic number ($Z$) of 7. The $-3$ charge indicates it has gained 3 extra electrons to complete its octet.

Next is the oxide ion, $O^{2-}$. Oxygen sits at atomic number 8. With a $-2$ charge, it has gained 2 electrons.

Then we have the fluoride ion, $F^{-}$. Fluorine's atomic number is 9. A $-1$ charge means it grabbed 1 extra electron.

Finally, let's look at the sodium ion, $N{a}^{+}$. Sodium is a metal with an atomic number of 11. The $+1$ charge tells us it has lost 1 electron.

Look at that perfect alignment! Every single ion in the set $\{N^{3-},O^{2-},F^{-},N{a}^{+}\}$ has exactly 10 electrons. They have all achieved the stable electron configuration of the noble gas Neon ($1s^{2}2s^{2}2p^6$).

If you glance at the other options, you'll spot the lithium ion ($L{i}^{+}$). Lithium has an atomic number of 3, so $L{i}^{+}$ only has $3-1=2$ electrons. Its presence immediately disqualifies any set from being isoelectronic with our 10-electron club. Thus, we can confidently conclude that our chosen set is the correct answer.