Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The isoelectronic set of ions is (a) \(\mathrm{N}^{3-}, \mathrm{O}^{2-}, \mathrm{F}^{-}\)and \(\mathrm{Na}^{+}\) (b) \(\mathrm{N}^{3-}, \mathrm{Li}^{+}, \mathrm{Mg}^{2+}\) and \(\mathrm{O}^{2-}\) (c) \(\mathrm{F}^{-}, \mathrm{Li}^{+}, \mathrm{Na}^{+}\)and \(\mathrm{Mg}^{2+}\) (d) \(\mathrm{Li}^{+}, \mathrm{Na}^{+}, \mathrm{O}^{2-}\) and \(\mathrm{F}^{-}\)

Step-by-Step Solution

Answer

The correct isoelectronic set is (a) \(\mathrm{N}^{3-}, \mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{Na}^{+}\).

Step 1: Understand what isoelectronic means

Isoelectronic species are atoms or ions that have the same number of electrons and therefore the same electronic configuration. For this task, we need to identify which set of ions have the same number of electrons in them.

Step 2: Determining number of electrons in each species

Let's calculate the number of electrons each ion has:- \(\mathrm{N}^{3-}: 7 ext{{ (atomic number of nitrogen)}} + 3 = 10\text{{ electrons}}\)- \(\mathrm{O}^{2-}: 8 + 2 = 10\text{{ electrons}}\)- \(\mathrm{F}^{-}: 9 + 1 = 10\text{{ electrons}}\)- \(\mathrm{Na}^{+}: 11 - 1 = 10\text{{ electrons}}\)- \(\mathrm{Li}^{+}: 3 - 1 = 2\text{{ electrons}}\)- \(\mathrm{Mg}^{2+}: 12 - 2 = 10\text{{ electrons}}\)

Step 3: Compare each set to find the isoelectronic set

Now we check each of the sets to find a group with the same number of electrons:(a) \(\mathrm{N}^{3-}\), \(\mathrm{O}^{2-}\), \(\mathrm{F}^{-}\), \(\mathrm{Na}^{+}\): All have 10 electrons.(b) \(\mathrm{N}^{3-}\), \(\mathrm{Li}^{+}\), \(\mathrm{Mg}^{2+}\), \(\mathrm{O}^{2-}\): 10, 2, 10, 10 electrons respectively; \(\mathrm{Li}^{+}\) does not match.(c) \(\mathrm{F}^{-}\), \(\mathrm{Li}^{+}\), \(\mathrm{Na}^{+}\), \(\mathrm{Mg}^{2+}\): 10, 2, 10, 10 electrons respectively; \(\mathrm{Li}^{+}\) does not match.(d) \(\mathrm{Li}^{+}\), \(\mathrm{Na}^{+}\), \(\mathrm{O}^{2-}\), \(\mathrm{F}^{-}\): 2, 10, 10, 10 electrons respectively; \(\mathrm{Li}^{+}\) does not match.

Step 4: Choose the correct isoelectronic set

In set (a), all ions have 10 electrons each making them isoelectronic. Therefore, option (a) is the correct answer.

Key Concepts

Electron Configuration

Electron configuration is like an address, detailing where electrons reside in an atom or ion. It's the arrangement of electrons in energy levels or orbitals, and it follows specific principles based on quantum mechanics. Each electron shell has a maximum number of electrons it can hold, and electrons fill these from lowest to highest energy:

• First shell: 2 electrons
• Second shell: 8 electrons
• Third shell: 18 electrons, and so forth

When atoms gain or lose electrons to become ions, their electron configuration changes. For instance, oxygen has an atomic number of 8, so it naturally has an electron configuration of 1s² 2s² 2p⁴. When it gains two electrons to form \( ext{O}^{2-} \), its configuration becomes the same as neon, known as isoelectronic with neon.

Ionic Charges

The charge of an ion arises from the loss or gain of electrons. Positive ions (cations) form when an atom loses electrons, while negative ions (anions) form when an atom gains electrons. Understanding ionic charges is crucial when discussing isoelectronic species, as it impacts their electron counts.

• An atom's tendency to lose or gain electrons is often linked to achieving a more stable electron configuration, usually resembling a nearest noble gas.
• For instance, sodium ( ext{Na}\( ^+ \)) loses one electron to have the same electron configuration as neon, therefore it carries a +1 charge.
• On the other hand, a nitrogen atom gains three electrons to form ext{N}\( ^{3-} \), reaching the same electron configuration with neon.

These changes in electron counts, governed by the atomic number and the resulting ionic charges, allow us to determine if species are isoelectronic.

Atomic Number

The atomic number is fundamental to understanding elements and their electron configurations. It represents the number of protons in an atom's nucleus, which is also equal to the number of electrons in a neutral atom.

• For accurate electron configurations and understanding ionic species, always refer to the atomic number.
• The atomic number determines the overall identity of an element. For example, oxygen has an atomic number of 8, meaning it has 8 protons and, in its neutral state, 8 electrons.
• When an atom becomes an ion, the electron count changes but the atomic number remains constant. This crucial detail helps us calculate the total number of electrons, aiding in identifying isoelectronic species.

Understanding atomic numbers not only aids in electron configurations but also provides insights into the chemical behavior of ions and atoms across the periodic table.