Which of the following is the correct order of siz – Class 11 Chemistry

NCERT Exemplar · Q10

Q. Which of the following is the correct order of size of the given species:

(i) I > I^- > I^+

(ii) I^+ > I^- > I

(iii) I > I^+ > I^-

(iv) I^- > I > I^+

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Concept understanding — Ionic Radii Trends

Ionic Radii: What It Means and Why It Matters

Imagine an atom as a tiny, fuzzy sphere. When it loses an electron to become a positive ion (cation), or gains an electron to become a negative ion (anion), its size changes. That new size is the ionic radius — the distance from the nucleus to the outermost electron in the ion.

The key question: Why does the size change at all?

The Core Intuition: Two Forces at Play

Every electron in an atom is pulled toward the nucleus by electrostatic attraction. But electrons also repel each other. The balance between these two forces determines how "big" the electron cloud is.

When an atom loses an electron (becomes a cation), two things happen:

The number of protons stays the same, but there are fewer electrons.
The remaining electrons feel a stronger pull from the nucleus because there's less electron-electron repulsion to push them apart.

Result: The cation shrinks compared to the neutral atom.

When an atom gains an electron (becomes an anion):

The number of protons stays the same, but there are more electrons.
The extra electron adds more repulsion, pushing the electron cloud outward.
The nucleus can't pull the extra electrons in as tightly.

Result: The anion expands compared to the neutral atom.

Note

This is why, for the same element, the cation is always smaller than the neutral atom, and the anion is always larger. For example, a sodium atom (Na) has a radius of about 186 pm, but Na⁺ has a radius of only about 102 pm — nearly half the size.

The Precise Trend Across the Periodic Table

Now let's look at how ionic radii change as you move across a period and down a group.

Across a Period (Left to Right)

Consider the elements of Period 3: Na, Mg, Al, Si, P, S, Cl.

As you move right, the nuclear charge (number of protons) increases. Electrons are added to the same shell (n=3). The increasing positive charge pulls the electron cloud inward more strongly.

But here's the twist: cations and anions form at different places. The trend isn't smooth like atomic radii.

On the left, elements form cations (Na⁺, Mg²⁺, Al³⁺). These are much smaller than their neutral atoms.
On the right, elements form anions (P³⁻, S²⁻, Cl⁻). These are much larger than their neutral atoms.

So across a period, you see a sharp drop from the neutral atom to the cation, then a sharp rise to the anion, then a gradual decrease as you move further right among the anions.

Watch out

A common mistake is to think ionic radii decrease smoothly across a period like atomic radii do. They don't — the change from cation to anion creates a huge jump. Always check whether you're comparing cations, anions, or neutral atoms.

Down a Group (Top to Bottom)

This is straightforward: ionic radii increase down a group.

Why? Each step down adds a new electron shell (n increases). The outermost electrons are farther from the nucleus, so the ion gets bigger.

For example:

Li⁺: ~76 pm
Na⁺: ~102 pm
K⁺: ~138 pm
Rb⁺: ~152 pm
Cs⁺: ~167 pm

The same trend holds for anions: F⁻ < Cl⁻ < Br⁻ < I⁻.

Important

The increase down a group is the most reliable trend for ionic radii. It holds for all ions — cations, anions, and even transition metal ions.

The Isoelectronic Series: A Special Case

Sometimes you compare ions that have the same number of electrons (isoelectronic). For example: O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ all have 10 electrons (like neon).

Here, the trend is determined entirely by nuclear charge. More protons = stronger pull = smaller radius.

Ion	Protons	Electrons	Radius (pm)
O²⁻	8	10	140
F⁻	9	10	133
Na⁺	11	10	102
Mg²⁺	12	10	72
Al³⁺	13	10	53.5

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Gaining an electron increases electron-electron repulsion and decreases effective nuclear charge, making an anion larger than its neutral atom. Losing an electron reduces repulsion and increases effective nuclear charge, making a cation smaller. Thus, the order of size is $\text{I}^- > \text{I} > \text{I}^+$ .

The size of an atom or ion is primarily determined by the balance between the attractive force of the nucleus on the electrons and the repulsive forces between the electrons themselves. When an atom gains or loses electrons to form an ion, this balance shifts, leading to a change in its overall size. Understanding these changes is key to comparing the radii of different species derived from the same element.

Here's how the formation of anions and cations affects atomic size:

Formation of an Anion (e.g., $\text{I} \rightarrow \text{I}^-$ ):

When a neutral atom gains one or more electrons, the total number of electrons increases while the number of protons in the nucleus remains constant.
1. Increased Electron-Electron Repulsion: The added electron(s) increase the electron density in the valence shell, leading to greater repulsive forces between the electrons. This increased repulsion pushes the electron cloud outwards.
2. Decreased Effective Nuclear Charge ( $Z_{eff}$ ): Although the nuclear charge is constant, the increased number of electrons means that each electron experiences a slightly lower effective nuclear charge due to enhanced shielding by other electrons. A lower $Z_{eff}$ means the nucleus pulls less strongly on the valence electrons. Both these factors contribute to an expansion of the electron cloud, making the anion larger than its parent neutral atom.
Formation of a Cation (e.g., $\text{I} \rightarrow \text{I}^+$ ):

When a neutral atom loses one or more electrons, the total number of electrons decreases while the number of protons in the nucleus remains constant.
1. Decreased Electron-Electron Repulsion: With fewer electrons, the repulsive forces between the remaining electrons are reduced.
2. Increased Effective Nuclear Charge ( $Z_{eff}$ ): The nuclear charge remains the same, but with fewer electrons, the remaining electrons experience a stronger pull from the nucleus. This is because there is less shielding from other electrons, effectively increasing the $Z_{eff}$ experienced by each remaining electron. These factors cause the electron cloud to contract, making the cation smaller than its parent neutral atom. In some cases, losing electrons can even lead to the removal of an entire valence shell, drastically reducing the size.

Important

For a given element, the general trend in size is:

$\text{Cation} < \text{Neutral Atom} < \text{Anion}$

Let's apply this concept to the given species: $\text{I}$ , $\text{I}^-$ , and $\text{I}^+$ .

Identify the species:
- $\text{I}$ is the neutral iodine atom.
- $\text{I}^-$ is the iodide anion, formed by gaining one electron.
- $\text{I}^+$ is the iodinium cation, formed by losing one electron.
Compare $\text{I}$ and $\text{I}^-$ : …

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Q.Which of the following is the correct order of size of the given species:

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