Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following questions two statements (Assertion) (A) and Reason (R) are given. Mark (a) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\). (b) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct but \(\mathrm{R}\) is not the correct expalnation of \(\mathrm{A}\). (c) \(\mathrm{A}\) is true but \(\mathrm{R}\) is false. (d) A is false but \(R\) is true. (e) \(\mathrm{A}\) and \(\mathrm{R}\) both are false. Assertion: The electron gain enthalpies have large negative values toward the upper right of the periodic table preceding the noble gases. Reason: The effective nuclear charge increases from left to right across a period and consequently it will be easier to add an electron to a smaller atom since the added electron on an average would be closer to the positively charged nucleus.

Step-by-Step Solution

Answer

(a) Both A and R are correct and R is the correct explanation of A.

Step 1: Understanding Assertion (A)

The assertion (A) states that towards the upper right of the periodic table, the electron gain enthalpies are large negative values, especially near the noble gases. This is generally true as elements like fluorine, oxygen, and others in upper right want to achieve a stable noble gas configuration, hence they have high electron affinity which means they release energy when gaining an electron.

Step 2: Understanding Reason (R)

The reason (R) provided states that as you move across a period from left to right, the effective nuclear charge increases, making it easier to add an electron to a smaller atom because the added electron is closer to the nucleus. This explanation is factual as the increasing effective nuclear charge does make it more favorable for atoms to gain an extra electron.

Step 3: Assessing Relationship Between A and R

Now, we need to check if the reason (R) explains the assertion (A). The increasing effective nuclear charge, which results in higher electron gain enthalpy (more negative value), is a direct cause for the trends described in the assertion. Therefore, not only are both A and R true, but R also adequately explains A.

Step 4: Choosing the Correct Option

Given that both A and R are true and R is the correct explanation for A, the correct option is (a).

Key Concepts

Assertion and Reason questions

Assertion and Reason questions are a common type in chemistry problem-solving, where two statements are provided. They require the student to assess whether each statement is true or false and to determine if the reason correctly explains the assertion. This involves critical thinking and contextual understanding.
In these questions, comprehending the assertion requires the student to understand the basic statement about a chemical property or trend. Meanwhile, the reason provides a supporting explanation, and it's essential to evaluate the logic and correctness of this explanation.

• If both the assertion and the reason are true and the reason explains the assertion, the answer would typically be option (a).
• If both are true but the reason doesn't explain the assertion, option (b) is chosen.
• Option (c) applies if the assertion is true but the reason is false.
• If the assertion is false and the reason is true, option (d) is correct.
• Finally, if both are false, option (e) would be the answer.

By breaking down each part and analyzing the connection between the statements, students can confidently tackle these types of questions. This approach enhances understanding by forcing the learner to engage deeply with each concept.

Periodic table trends

The periodic table is structured to show repeating patterns or trends in elemental properties. One important trend is how electron gain enthalpy changes across the table.
Moving from left to right across a period, elements exhibit increasing effective nuclear charge. This trend is significant because:

• The increase in nuclear charge tends to make the atom smaller, as electrons are pulled closer to the nucleus.
• Elements become more electronegative from left to right, meaning a stronger desire to attract and gain extra electrons to complete their outer shells.

The vertical trend shows that as you move up a group, the atoms have fewer electron shells, leading to higher electron gain enthalpy. This occurs due to less electron shielding and a stronger attraction between the nucleus and incoming electrons. These patterns help predict the behavior of elements in chemical reactions, especially those involving electron transfer.

Electron gain enthalpy

Electron gain enthalpy refers to the energy change when an electron is added to an isolated atom in the gaseous state. Typically expressed in kilojoules per mole ( ext{kJ/mol}), it tells us whether an atom will release or absorb energy when gaining an electron.

• A more negative electron gain enthalpy value indicates a higher energy release, signifying a greater likelihood of an atom gaining an electron.
• Elements like fluorine and chlorine have high negative electron gain enthalpy values due to their strong ability to attract additional electrons.
• The large negative values are often found in elements near the top right of the periodic table, excluding noble gases, who already have a stable electron configuration.

This concept is essential to understanding why some elements like to gain electrons while others prefer to lose them during chemical reactions. Analyzing these tendencies provides insights into reactivity and bonding behavior, crucial for mastering chemical concepts.