The group having isoelectronic species is
(a) O2-, F-, Na+, Mg2+
(b) O-, F-, Na, Mg+
(c) O2-, F-, Na

Q: Why do we add electrons when the charge is negative?

Electrons themselves carry a negative charge. So, if an atom has a net negative charge (like $-2$), it means it has accumulated extra negative particles (electrons) beyond its normal neutral state.

Q: Do isoelectronic species have the same size?

No! Even though they have the same number of electrons, they have different numbers of protons pulling on those electrons. A higher number of protons (higher atomic number) will pull the same 10 electrons closer, making the ion smaller. For example, $\text{Mg}^{2+}$ is much smaller than $\text{O}^{2-}$.

Q: Why do these specific atoms form these specific ions?

They are all trying to achieve the stable electron configuration of the nearest noble gas, which is Neon (10 electrons). Oxygen needs 2 more, Fluorine needs 1 more, Sodium needs to lose 1, and Magnesium needs to lose 2 to reach that stable state.

Q: What happens to the protons when an ion forms?

Absolutely nothing! The number of protons (the atomic number) is the identity of the element and remains completely unchanged during ordinary chemical reactions and ion formation.

Q: Can a neutral atom be part of an isoelectronic series with ions?

Yes, absolutely. In this specific series, the neutral noble gas Neon ($\text{Ne}$), which has an atomic number of 10 and 10 electrons, is isoelectronic with $\text{O}^{2-}$, $\text{F}^{-}$, $\text{Na}^{+}$, and $\text{Mg}^{2+}$.

Question

The group having isoelectronic species is

Options:

(A) $	ext{O}^{2-}, 	ext{F}^{-}, 	ext{Na}^{+}, 	ext{Mg}^{2+}$

(B) $	ext{O}^{-}, 	ext{F}^{-}, 	ext{Na}, 	ext{Mg}^{+}$

(C) $	ext{O}^{2-}, 	ext{F}^{-}, 	ext{Na}, 	ext{Mg}^{2+}$

(D) $	ext{O}^{-}, 	ext{F}^{-}, 	ext{Na}^{+}, 	ext{Mg}^{2+}$

Sigma Percentile · Accepted Answer

### Decoding Isoelectronic Species

When you encounter the term **isoelectronic**, the secret is hidden right in the name. The prefix *iso-* means "equal" or "same," and *electronic* refers to the electrons. Therefore, an isoelectronic series is simply a group of atoms or ions that possess the exact same number of electrons.

To determine the number of electrons in any given ion, we use a very straightforward formula:

$$ 	ext{Number of Electrons} = Z - 	ext{Charge} $$

Here, $Z$ represents the atomic number (the number of protons), which tells us how many electrons a neutral atom of that element would have. The charge tells us how many electrons have been gained or lost. Remember, electrons are negatively charged, so a negative ion (anion) has *gained* electrons, while a positive ion (cation) has *lost* electrons.

### The Electron Counting Game

Let's put our formula to the test by analyzing the species given in the first option.

First, we have the oxide ion, $	ext{O}^{2-}$. Oxygen has an atomic number of $Z = 8$. The $-2$ charge indicates it has gained two extra electrons to complete its octet.

$$ 	ext{Electrons in } 	ext{O}^{2-} = 8 - (-2) = 10 $$

Next is the fluoride ion, $	ext{F}^{-}$. Fluorine's atomic number is $Z = 9$. With a $-1$ charge, it has accepted one additional electron.

$$ 	ext{Electrons in } 	ext{F}^{-} = 9 - (-1) = 10 $$

Now, let's look at the cations. The sodium ion, $	ext{Na}^{+}$, comes from sodium, which has an atomic number of $Z = 11$. The $+1$ charge means it has lost one of its valence electrons.

$$ 	ext{Electrons in } 	ext{Na}^{+} = 11 - (+1) = 10 $$

Finally, we evaluate the magnesium ion, $	ext{Mg}^{2+}$. Magnesium starts with $Z = 12$. The $+2$ charge tells us it has donated two electrons.

$$ 	ext{Electrons in } 	ext{Mg}^{2+} = 12 - (+2) = 10 $$

### Analyzing the Winning Group

Look at the beautiful symmetry we just uncovered! Every single ion in this group—$	ext{O}^{2-}$, $	ext{F}^{-}$, $	ext{Na}^{+}$, and $	ext{Mg}^{2+}$—has exactly **10 electrons**. They all share the stable, closed-shell electron configuration of the noble gas Neon ($1s^2 2s^2 2p^6$). Because they all have the same number of electrons, they perfectly fit the definition of an isoelectronic series.

### Why the Others Fail

If we quickly glance at the other options, it becomes obvious why they are incorrect. For instance, options (b) and (c) include neutral Sodium ($	ext{Na}$), which has $11$ electrons, breaking the pattern of $10$. Option (d) includes $	ext{O}^{-}$, which only has $8 + 1 = 9$ electrons.

This is a classic, high-yield concept in chemistry. Always be meticulous with your signs when counting electrons—it is the easiest place to make a silly mistake!

Animated Solution for Chemistry - Periodicity in Properties: The group having isoelectronic species is

Select Answer:

Visualized Solution

\text{Concept of Isoelectronic Species}

\text{Electrons in } O^{2-}

\text{Electrons in } F^{-}

\text{Electrons in } Na^{+}

\text{Electrons in } Mg^{2+}

\text{Checking other options}

The Sigma Insight: Periodic Table and Periodic Properties

Decoding Isoelectronic Species

The Electron Counting Game

Analyzing the Winning Group

Why the Others Fail

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