Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

The incorrect statement/s among the following is /are (a) \(\mathrm{NCl}_{5}\) does not exist while \(\mathrm{PCl}_{5}\) does (b) Lead prefers to form tetravalent compounds (c) The three \(\mathrm{C}-\mathrm{O}\) bonds are not equal in the carbonate ion (d) Both \(\mathrm{O}_{2}^{+}\)and \(\mathrm{NO}\) are paramagnetic

Step-by-Step Solution

Answer

The incorrect statements are (b) and (c).

Step 1: Analyzing (a) NCl 5 and PCl 5

Consider the elements nitrogen (N) and phosphorus (P). Nitrogen cannot form NCl 5 due to its inability to expand its octet (it lacks d-orbitals for bonding beyond four atoms). Phosphorus, however, can expand its octet and form PCl 5 .

Step 2: Evaluating (b) Lead's Valency Preferences

Lead (Pb) commonly forms both tetravalent (+4 oxidation state) and divalent (+2 oxidation state) compounds. However, due to the inert pair effect, Pb prefers forming compounds in the +2 state rather than the tetravalent state.

Step 3: Investigating (c) Carbonate Ion's C-O Bonds

In the carbonate ion CO 3 2− , the three C-O bonds are equal due to resonance. The bond character is between a single bond and a double bond, resulting in three equivalent C-O bonds.

Step 4: Assessing (d) Paramagnetism of O 2 + and NO

Determine the magnetic property based on the unpaired electrons: O2+ has one unpaired electron and is paramagnetic, while NO also has one unpaired electron, making it paramagnetic as well.

Key Concepts

Octet Rule

In the realm of chemical bonding, the octet rule is a fundamental principle. It states that atoms tend to bond in such a way that they each have eight electrons in their valence shell, resembling the electron configuration of a noble gas. This configuration represents stability.
- For most elements, achieving an octet involves gaining, losing, or sharing electrons through ionic or covalent bonds. - Nitrogen ( N ), for instance, can share three electrons to meet the octet rule's requirement, forming compounds like NH _3. - However, nitrogen cannot expand its octet as it lacks available d-orbitals, preventing compounds such as NCl _5. On the contrary, PCl _5 can exist because phosphorus ( P ) has access to d-orbitals, allowing it to exceed the octet rule when necessary. This difference highlights how the octet rule can have exceptions, often influenced by the presence of d-orbitals in heavier elements.

Paramagnetism

Paramagnetism is a property of materials that are attracted to magnetic fields. This phenomenon arises because of the presence of unpaired electrons in a substance's electron configuration. Electrons are like tiny magnets due to their spin, and when these tiny magnets are unpaired, they create a magnetic field.
- An example is the dioxygen cation ( O_2^+ ), which has one unpaired electron. This unpaired electron makes O_2^+ paramagnetic, meaning it will be drawn toward a magnetic field. - Similarly, nitric oxide ( NO ) also has an unpaired electron, which leads to its paramagnetic nature. Since paramagnetism results from unpaired electrons, it’s a useful property to determine electron configurations and understand the bonding in molecules. When assessing magnetic properties, counting unpaired electrons gives crucial insights into how substances will behave in a magnetic environment.

Resonance

Resonance is a fascinating concept crucial to understanding the true nature of certain molecules, like the carbonate ion ( CO_3^{2-} ). Unlike a static structure, resonance describes a situation where the actual electron structure of a molecule is a hybrid of multiple possible structures.
- In the case of the carbonate ion, resonance explains why the C-O bonds are equal. - Rather than having one double bond and two single bonds, each C-O bond shares the same bond length and strength because the electrons are delocalized. This delocalization results in an average bond character that is intermediate between a single and a double bond, ensuring all bonds are equivalent. Resonance thus provides a more accurate depiction of electron distribution, which directly affects physical and chemical properties.

Oxidation States

Oxidation states are numbers assigned to elements in a compound to indicate the distribution of electrons among the atoms. This concept helps chemists understand electron transfer processes that take place during chemical reactions.
- For example, lead ( Pb ) exhibits different oxidation states, predominantly +2 and +4. Though it can form both, Pb prefers the +2 state due to the inert pair effect, which involves electrons in the 's' orbital being less reactive. - This preference is crucial when predicting the behavior of lead compounds in reactions. Understanding oxidation states enables the identification of which atom is oxidized and which is reduced in redox reactions. It also assists in predicting formulas and structures of compounds, providing a clearer picture of elemental interactions.