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 is isoelectronic.

Step 1: Understanding Isoelectric Species

Isoelectronic species are atoms, molecules, or ions that have the same number of electrons. To determine if species are isoelectronic, count the total number of electrons for each species.

Step 2: Counting Electrons for Option A

Calculate the number of electrons for each species in option A: - H extsubscript{2} : 2 electrons (1 per hydrogen atom) - CO extsubscript{2} : 22 electrons (6 from C and 16 from 2 O atoms) - CN extsuperscript{-} : 14 electrons (7 from N, 6 from C, +1 for charge) - O extsuperscript{-} : 9 electrons (8 from O, +1 for charge) These have different electron counts and are not isoelectronic.

Step 3: Counting Electrons for Option B

Calculate the number of electrons for each species in option B: - N : 7 electrons - H extsubscript{2}S : 10 electrons (2 from H atoms, 8 from S) - CO : 14 electrons (6 from C, 8 from O) These are not isoelectronic.

Step 4: Counting Electrons for Option C

Calculate the number of electrons for each species in option C: - N extsubscript{2} : 14 electrons (7 from each N atom) - CO : 14 electrons (6 from C, 8 from O) - CN extsuperscript{-} : 14 electrons (7 from N, 6 from C, +1 for charge) - O extsubscript{2} extsuperscript{+2} : 14 electrons (16 from 2 O atoms, -2 for the charge) All have the same number of electrons, so they are isoelectronic.

Step 5: Counting Electrons for Option D

Calculate the number of electrons for each species in option D: - Ca : 20 electrons - Mg : 12 electrons - Cl : 17 electrons These have different electron counts and are not isoelectronic.

Key Concepts

Electron Counting

To determine if chemical species are isoelectronic, electron counting is essential. This involves calculating the total number of electrons in each atom, molecule, or ion.
Isoelectronic species have identical electron counts, despite potential differences in their atomic composition or charge.
When counting electrons:

• Note that each element contributes a number of electrons equal to its atomic number. For example, nitrogen ( N) has 7 electrons because its atomic number is 7.
• When dealing with ions, adjust the electron count according to the charge. For example, a negative charge means adding electrons, while a positive charge means subtracting.
• Combine the electron counts from individual atoms for molecules. For instance, H_{2} has 2 electrons (1 per hydrogen atom).

Applying these rules allows us to identify sets of atoms, ions, or molecules that share identical electron totals.

Chemical Species

The term 'chemical species' encompasses a broad range of forms, including atoms, molecules, and ions. Each type has distinct properties:

• Atoms: Basic units of matter, composed of protons, neutrons, and electrons.
• Molecules: Formed when two or more atoms bond together. For instance, carbon monoxide (CO) consists of a carbon atom and an oxygen atom.
• Ions: Charged particles that result when atoms gain or lose electrons. An example is the cyanide ion (CN^-), which has an extra electron, giving it a negative charge.

When we talk about isoelectronic species, we're often comparing these different types to see if they have the same electron count. This can include combinations of atoms, neutral molecules, and charged ions.

Molecular Ions

Molecular ions are species that originate from molecules but possess a net charge. These ions can be either positive or negative.
The charge results from the molecule gaining or losing electrons.
For example, the nitrogen molecule ( N_{2} ) can become the N_{2}^+ ion by losing one electron.

Molecular ions maintain the structure of the molecule but alter its electronic properties. This change can affect how the ion engages with electric fields and interacts with other species. When evaluating isoelectronic properties, such ions must be carefully assessed to include any electron changes due to the charge. By using electron counts before and after ionization, we determine the similarity with other species.