The first element of group different form its congeners, i.e. other members of the group in many ways. These differences may be
due to the following:
i. Small size of atom and ion.
ii. High electronegativity.
iii. Non-availability of low lying d-orbitals.
The first element of a group shows resemblance with the second element of the adjacent group on the right. This is known as
diagonal relationship.
Lithium exhibits many physical and chemical similarites with magnesium. The reason is:

Options

A. Both have the same size.
B. Both are found in native state.
C. Both have the same ionisation enthalpies.
D. Both have the same electronic configuration.

Explanation

To understand why lithium exhibits many physical and chemical similarities with magnesium, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Identify the Groups**:
- Lithium (Li) is the first element of Group 1 (alkali metals).
- Magnesium (Mg) is the second element of Group 2 (alkaline earth metals).

2. **Understand the Concept of Diagonal Relationship**:
- The diagonal relationship refers to the similarities between the first element of one group and the second element of the adjacent group. In this case, lithium and magnesium show such a relationship.

3. **Consider the Factors Leading to Differences in Group 1**:
- The first element of a group (like lithium) often differs from its congeners (other alkali metals) due to:
- Small atomic and ionic size.
- High electronegativity.
- Non-availability of low-lying d-orbitals.

4. **Compare Atomic and Ionic Sizes**:
- The atomic radius of lithium is about 152 picometers, while that of magnesium is approximately 160 picometers.
- The ionic size of Li⁺ is about 76 picometers, and Mg²⁺ is about 72 picometers.
- The similarity in sizes contributes to their comparable physical and chemical properties.

5. **Evaluate Electronegativity**:
- Both lithium and magnesium have similar electronegativity values, which further enhances their chemical similarities.

6. **Conclude the Reason for Similarities**:
- The primary reason for the physical and chemical similarities between lithium and magnesium is their comparable atomic and ionic sizes, leading to similar electronegativities.

### Final Answer:
Lithium exhibits many physical and chemical similarities with magnesium primarily due to the diagonal relationship, which is attributed to the similarity in their atomic and ionic sizes.

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