Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

The pair of elements that have similar chemical properties is (a) beryllium and boron (b) lithium and magnesium (c) carbon and nitrogen (d) aluminium and magnesium

Step-by-Step Solution

Answer

Lithium and magnesium have similar chemical properties due to their diagonal relationship in the periodic table.

Step 1: Understanding Periodic Table Trends

Chemical properties of elements in the periodic table are guided by their positions. Elements in the same group (vertical columns) usually have similar properties because they have the same number of valence electrons.

Step 2: Element Group Analysis

Analyze the given pairs: (a) Beryllium is in group 2, Boron in group 13. (b) Lithium is in group 1, Magnesium in group 2. (c) Carbon is in group 14, Nitrogen in group 15. (d) Aluminium is in group 13, Magnesium in group 2. Elements in the same group are more likely to share chemical properties.

Step 3: Consider Diagonal Relationships

There are specific cases where elements in adjacent groups but different periods show similarities, known as diagonal relationships. Lithium and Magnesium are often considered to exhibit this due to their similar ionic radii and charge-to-size ratio.

Step 4: Identify the Pair with Similar Properties

Based on periodic trends and diagonal relationship, the expected pair with similar chemical properties is in option (b): lithium and magnesium. Their similarity stems from the diagonal relationship across the periodic table.

Key Concepts

Chemical Properties

Chemical properties of elements define how they behave in chemical reactions, encompassing their reactivity and types of bonds they form. These properties typically align with an element's position in the periodic table because each vertical column, or group, shares elements with the same number of valence electrons.
This similarity in valence electrons means elements within a group tend to form the same number and type of bonds and react similarly with other substances. For instance, all alkali metals in Group 1 are highly reactive, usually forming +1 ions. Similarly, halogens in Group 17 are eager to gain one electron to complete their outer shell, resulting in high electronegativity and reactivity. Overall, this group-based predictability is foundational in facilitating the understanding of chemical reactions and compound formation.

Diagonal Relationship

In the realm of periodic trends, diagonal relationships refer to the similarities in properties between certain elements in adjacent groups and periods. This is not a widely observed trend and occurs due to a delicate balance between increasing atomic number and size. For instance, lithium exhibits significant chemical similarities to magnesium.
The unique cross-periodic similarities these elements share are due to their:

• Similar ionic radii: The size of ions can affect physical and chemical properties significantly.
• Comparative charge-to-size ratio: Both elements making ions of comparable charge, leading to similar bonding behavior.

Therefore, despite being in adjacent groups and differing periods, lithium and magnesium reveal common behaviors, confirming the diagonal relationship between them. This concept helps broaden our understanding of elemental properties and their exceptions to the simpler group and period trends.

Group Analysis

Group analysis in the periodic table is the study of trends and properties that elements show within the same group. Understanding these trends helps understand why certain elements are grouped together. For example, analyzing the properties of elements such as beryllium and magnesium, both of which belong to Group 2.
Elements within the same group:

• Share the same valence electron count, influencing their chemical behavior greatly.
• Have increasing atomic radius and a decreasing ionization energy as you move down the group.
• Tend to form similar types of compounds (e.g., salts or oxides).

This analysis becomes pertinent particularly when predicting how an element will behave in a chemical reaction. By examining a group's characteristics, it's easier to predict properties such as melting and boiling points, hardness, and overall reactivity. Group analysis is a critical tool in chemistry that enhances the predictive power of the periodic table.