Problem 127 - 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) 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 5 th period of periodic table contains 18 elements not 32 . Reason: \(\mathrm{n}=5,1=0,1,2,3\). The order in which the energy of available orbitals \(4 \mathrm{~d}, 5 \mathrm{~s}\) and \(5 \mathrm{p}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding Statements

Let's first restate and analyze both given statements for clarity. **Assertion (A):** The 5th period of the periodic table contains 18 elements, not 32. **Reason (R):** For the 5th period, when n=5, the available energy levels include orbitals 4d, 5s, and 5p. We need to determine the truthfulness of these statements and if Reason (R) correctly explains Assertion (A).

Step 2: Evaluating The Assertion

In the periodic table, the 5th period indeed contains 18 elements. Each element corresponds to the filling of the 5s, 5p, and 4d orbitals, which align with the contemporary understanding of periodicity where only 18 elements are expected in that period. Therefore, Assertion (A) is correct.

Step 3: Evaluating The Reason

The Reason states that for n=5, the orbitals are 4d, 5s, and 5p. This accurately describes the filling of electron shells and the associated orbitals, which contributes to the placement of elements in the 5th period. Thus, Reason (R) is also correct.

Step 4: Checking Explanation

Now, let's evaluate if Reason (R) correctly explains Assertion (A). As described, the 5th period is composed of elements filling the 4d, 5s, and 5p orbitals, resulting in 18 elements rather than 32. The sequence of energy levels affects the number of elements in a period, thereby making Reason (R) a valid explanation for Assertion (A).

Key Concepts

Periodic Table

The Periodic Table is a powerful tool for understanding chemical elements and their properties. It organizes elements by increasing atomic number and groups them into columns based on similar chemical behaviors. Each row in the periodic table is called a "period," and as we move across a period, the atomic number and number of electrons increase by one for each element.
In the table, elements are also organized into "groups," which are the columns running vertically. Elements within the same group typically contain the same number of electrons in their outermost shell, which explains their similar chemical properties.
The periodic table not only allows us to identify relationships between elements but also helps predict how elements will behave in chemical reactions. Understanding its structure is fundamental to grasping various concepts in chemistry.

Electron Orbitals

Electron orbitals describe the regions in an atom where electrons are most likely to be found. These are mathematically defined regions arising from the solutions to the Schrödinger equation for atoms, and they provide a visual representation of an electron's behavior.
There are different types of orbitals - s, p, d, and f - each with different shapes and capacities for holding electrons:

• s-orbitals: Spherical in shape and can hold up to 2 electrons.
• p-orbitals: Dumbbell-shaped and can accommodate up to 6 electrons across three orbitals.
• d-orbitals: More complexly shaped and can contain up to 10 electrons distributed among five orbitals.
• f-orbitals: Even more complex with space for up to 14 electrons spread over seven orbitals.

Electrons fill orbitals based on increasing energy levels, which follows the Madelung rule (also known as the Aufbau principle). This principle dictates the order of occupancy by the electrons amongst the available orbitals.

5th Period Elements

The 5th period of the periodic table comprises 18 elements ranging from rubidium (Rb) to xenon (Xe). This period includes a variety of chemical elements with different characteristics and properties.
Elements in this period showcase a transition from metallic to non-metallic behavior. The sequence includes:

• The alkali metal rubidium (Rb) and the alkaline earth metal strontium (Sr), both highly reactive metals.
• A series of transition metals, including yttrium (Y) and zirconium (Zr), characterized by partially filled d-orbitals.
• Non-metals such as iodine (I) and xenon (Xe), which possess distinct chemical properties.

The presence of different types of orbitals (4d, 5s, 5p) gives flexibility in electron occupancy, allowing the 5th period to host this diverse range of elements.

Periodic Trends

Periodic trends refer to patterns observed in the periodic table as you move across periods or down groups. Understanding these trends is crucial for predicting the behavior and properties of elements.
Some fundamental periodic trends include:

• Atomic Radius: Generally decreases across a period due to increasing nuclear charge drawing electrons closer. It increases down a group because of the additional electron shell.
• Ionization Energy: Generally increases across a period due to tighter electron binding but decreases down a group as outer electrons are further from the nucleus.
• Electronegativity: Tends to increase across a period and decrease down a group, reflecting the atom's ability to attract and bond with electrons.
• Melting Points: Variable, but often higher for transition metals.

These trends aid in predicting and explaining the reactivity and bonding behavior of elements based on their position in the periodic table.