Assertion`:` The number of electrons gained, lost or shared by the atom of an element so as to complete its octet is called the valency of the element.
Reason `:` Elements having the same number of valence electrons in their atoms possess different chemical properties.

Options

A. Both A and R are true, and R is correct explanation of the assertion.
B. Both A and R are true, but R is not the correct explanation of the assertion.
C. A is true, but R is false.
D. A is false , but R is true.

Explanation

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that the valency of an element is defined as the number of electrons it gains, loses, or shares to complete its octet. This is true because valency is fundamentally linked to how an atom interacts with others to achieve a stable electron configuration.

2. **Understanding the Reason**:
- The reason claims that elements with the same number of valence electrons have different chemical properties. This statement is incorrect. Elements that have the same number of valence electrons typically exhibit similar chemical properties because their reactivity and bonding behavior are determined by these electrons.

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