Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

The set representing the correct order of ionic radius is: \(\quad\) [2009] (a) \(\mathrm{Na}^{+}>\mathrm{Li}^{+}>\mathrm{Mg}^{2+}>\mathrm{Be}^{2+}\) (b) \(\mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{Mg}^{2+}>\mathrm{Be}^{2+}\)(c) \(\mathrm{Mg}^{2+}>\mathrm{Be}^{2+}>\mathrm{Li}^{2+}>\mathrm{Na}^{+}\) (d) \(\mathrm{Li}^{+}>\mathrm{Be}^{2+}>\mathrm{Na}^{+}>\mathrm{Mg}^{2+}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Na}^{+} > \mathrm{Li}^{+} > \mathrm{Mg}^{2+} > \mathrm{Be}^{2+}\) is correct.

Step 1: Identify Charge and Electron Configuration

Assess the ionic charge and electron configuration of each ion: - \(\mathrm{Na}^{+}\) has lost one electron and has the same electronic configuration as Ne: \(1s^2 2s^2 2p^6\).- \(\mathrm{Li}^{+}\) has lost one electron and has the electronic configuration: \(1s^2\).- \(\mathrm{Mg}^{2+}\) has lost two electrons and has the electronic configuration: \(1s^2 2s^2 2p^6\).- \(\mathrm{Be}^{2+}\) has lost two electrons and has the electronic configuration: \(1s^2\).

Step 2: Understand Concept of Ionic Radius

The ionic radius generally decreases with increasing positive charge due to the higher effective nuclear charge attracting the electrons more strongly towards the nucleus.

Step 3: Compare Effective Nuclear Charge

- Both \(\mathrm{Na}^{+}\) and \(\mathrm{Mg}^{2+}\) have full shells with the electronic configuration of neon (2,8), but \(\mathrm{Mg}^{2+}\) has a greater nuclear charge (12 protons vs. 11 protons), pulling its electrons closer.- \(\mathrm{Li}^{+}\) and \(\mathrm{Be}^{2+}\) both have the \(1s^2\) configuration, but \(\mathrm{Be}^{2+}\) has a nuclear charge of 4 while \(\mathrm{Li}^{+}\) has 3, pulling its electrons closer.

Step 4: Arrange Ions by Increasing Radius

From the highest effective nuclear charge: - \(\mathrm{Be}^{2+}\) (smallest) due to the highest effective nuclear charge per electron.- \(\mathrm{Mg}^{2+}\) next as it has a full 2,8 shell and a slight larger, but still small radius.- \(\mathrm{Li}^{+}\) followed, slightly larger.- \(\mathrm{Na}^{+}\) has the largest size among these due to its shell configuration.

Key Concepts

Effective Nuclear Charge

The effective nuclear charge is a core concept that helps us understand how tightly electrons are held by an atom. When considering the ionic radius, effective nuclear charge plays a crucial role. It is the net positive charge experienced by electrons in an atom and is much less than the actual nuclear charge because of shielding, where inner electrons partly block the charge of the nucleus
. As the effective nuclear charge increases, electrons are pulled closer to the nucleus, resulting in a smaller ionic radius. This principle explains why ions with the same electron configuration can have different sizes, depending on the number of protons in their nucleus
. In the context of ionic radius:

• A higher effective nuclear charge corresponds to a smaller ionic radius.
• Ions with the same electron configuration, like \(\text{Na}^+\) and \(\text{Mg}^{2+}\), are smaller when they have more protons.
• This concept helps to understand the decrease in ionic radius across a period and increase down a group on the periodic table.

Electron Configuration

Electron configuration is a description of the arrangement of electrons in an atomic or molecular structure. This configuration defines the chemical properties and the ionic size of an element
. Each ion in the original exercise has a different electron configuration depending on how many electrons it has lost relative to its neutral atom form, impacting its ionic radius
. For example:

• \(\text{Na}^+\) has 10 electrons, same as neon, which provides a \(1s^2 2s^2 2p^6\) configuration.
• \(\text{Li}^+\) has only 2 electrons, like helium, with a configuration of \(1s^2\).
• \(\text{Mg}^{2+}\) also adopts the full shell configuration of neon.
• \(\text{Be}^{2+}\) has the same configuration as helium.

This arrangement affects their ionic radii. With fewer electrons and more protons, each ion's radius is influenced by the attraction within its configuration.

Cation Radius

The cation radius refers to the size of a positively charged ion, or cation. In the exercise given, all ions are cations since they've lost one or more electrons
. When cations lose electrons, their atomic radius decreases. This occurs because the remaining electrons experience a higher effective nuclear charge, pulling them closer to the nucleus
. Key points regarding cation radius include:

• The more electrons lost, the smaller the radius, due to reduced electron-electron repulsion and increased nuclear pull.
• Cations with the same number of electrons can have different sizes due to differing numbers of protons.
• \(\text{Na}^+\) is larger than \(\text{Mg}^{2+}\) even though both have the same electron count, because \(\text{Mg}^{2+}\) has more protons.

Understanding cation radius helps in determining trends in the periodic table and justifying the size order of ions.

Periodic Trends

Periodic trends refer to patterns seen in the periodic table as one moves across periods or down groups. These trends help predict atomic and ionic properties
. When analyzing ionic radii, several trends come into play:

• Ionic radii decrease across a period from left to right as effective nuclear charge increases.
• Ionic radii increase down a group because of more electron shells being added.
• For cations, radii again decrease from left to right across a period due to increasing effective nuclear charge drawing electrons closer.

In the context of the exercise: \(\text{Be}^{2+}\) and \(\text{Mg}^{2+}\) demonstrate decreasing radii across a period, whereas larger sets like \(\text{Na}^+\) increase in size down the group. These trends assist in understanding and predicting chemical behavior and reactivity.