Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

Set of isoelectronic species is (a) \(\mathrm{H}_{2}, \mathrm{CO}_{2}, \mathrm{CN}^{-}, \mathrm{O}^{-}\) (b) \(\mathrm{N}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{CO}\) (c) \(\mathrm{N}_{2}, \mathrm{CO}, \mathrm{CN}^{-}, \mathrm{O}_{2}^{+2}\) (d) \(\mathrm{Ca}, \mathrm{Mg}, \mathrm{Cl}\)

Step-by-Step Solution

Answer

Option (c) \\(\mathrm{N}_{2}, \mathrm{CO}, \mathrm{CN}^{-}, \mathrm{O}_{2}^{2+}\\) is the set of isoelectronic species, all having 14 electrons.

Step 1: Understanding Isoelectronic Species

Isoelectronic species are atoms, molecules, or ions that have the same number of electrons. To identify the set of isoelectronic species, we must calculate the total number of electrons in each option and determine which set shares the same electron count.

Step 2: Calculate Electrons in Option (a)

Let's calculate the electrons for each species in option (a):- \(\mathrm{H}_2\): 2 electrons (2 × hydrogen, each with 1 electron)- \(\mathrm{CO}_2\): 22 electrons (carbon 6 + oxygen 16)- \(\mathrm{CN}^-\): 14 electrons (carbon 6 + nitrogen 7 + extra electron due to negative charge)- \(\mathrm{O}^-\): 9 electrons (oxygen 8 + extra electron due to negative charge) Thus, option (a) does not consist of isoelectronic species.

Step 3: Calculate Electrons in Option (b)

Now calculate the electrons for option (b):- \(\mathrm{N}\): 7 electrons (nitrogen)- \({\mathrm{H}_2\mathrm{S}}\): 10 electrons (2 × hydrogen 2 + sulfur 8)- \({\mathrm{CO}}\): 14 electrons (carbon 6 + oxygen 8)Thus, option (b) does not consist of isoelectronic species.

Step 4: Calculate Electrons in Option (c)

Consider option (c):- \({\mathrm{N}_2}\): 14 electrons (2 × nitrogen, each with 7 electrons)- \({\mathrm{CO}}\): 14 electrons (carbon 6 + oxygen 8)- \({\mathrm{CN}^-}\): 14 electrons (carbon 6 + nitrogen 7 + extra electron)- \({\mathrm{O}_2^{2+}}\): 14 electrons (2 × oxygen total 16 - 2 electrons removed for 2+ charge)All species in option (c) have 14 electrons each, making them isoelectronic.

Step 5: Check Option (d) for Confirmation

Verify option (d):- \({\mathrm{Ca}}\): 20 electrons (calcium)- \({\mathrm{Mg}}\): 12 electrons (magnesium)- \({\mathrm{Cl}}\): 17 electrons (chlorine)There is no match in electron count, so option (d) does not consist of isoelectronic species.

Key Concepts

Electron Count

Electron count is actually the core foundation when determining isoelectronic species. It refers to the total number of electrons present in atoms, molecules, or ions.
One can simply count the electrons by knowing the atomic number (which equals the number of protons and electrons in a neutral atom) and then adjust for ionic charges (adding electrons for negative charges and subtracting for positive charges).

When examining molecules and ions to find isoelectronic species, follow these steps:

• Identify the number of electrons in each uncharged atom using the periodic table. For example, carbon has 6 electrons, while nitrogen has 7.
• Calculate the effect of any charges on the species. A negative charge means adding electrons, while a positive charge means subtracting electrons.

For example, the ion (\(\text{CN}^-)\) has an extra electron making its electron count 14.
This process allows us to determine which combinations of elements and charges result in the same total electron count, leading us straight to identifying isoelectronic species.

Molecular Ions

Molecular ions are molecules that have gained or lost electrons, resulting in a net charge.
This charge can either be positive or negative, indicating whether electrons have been removed or added. Molecular ions, much like individual atoms, abide by the rule of maintaining balance in their structure.

There are two main types of molecular ions:

• Cationic molecular ions have a positive charge, formed by removing electrons. An example is \(\text{O}_2^{2+}\), where two electrons are removed, altering the electron count and making it unique from its neutral form.
• Anionic molecular ions possess a negative charge, formed by gaining electrons. An example is \(\text{CN}^-)\), which has one extra electron compared to its neutral form.

Understanding the formation and characteristics of molecular ions is crucial in isoelectronic analysis because the charge and resulting electron count need to match for species to be isoelectronic.

Atomic Structure

The concept of atomic structure is essential for understanding how electrons are arranged in atoms and molecules.
Atoms consist of a nucleus made up of protons and neutrons, with electrons orbiting this nucleus in defined energy levels known as shells or electron clouds.

Here are the key points of atomic structure that relate directly to isoelectronic species:

• Electrons reside in specific energy levels around the nucleus. The arrangement of these electrons in shells is what gives an element its chemical properties.
• Atoms seek stable configurations, often trying to match the electron configuration of the nearest noble gas. This drive for stability influences how atoms bond and form molecules.
• Isoelectronic species have the same number of electrons and often mimic each other’s relative stability and behavior due to similar electron configurations.

Understanding these configurations helps identify why different species, though composed of different elements, can exhibit similar chemical behavior when they're isoelectronic.