The atomic weight of `Cu` is `63.546`. There are only two naturally occurring isotopes of copper `.^(63)Cu` and `.^(65)Cu`. The natural abundance of the`.^(63)Cu` isotope must be approximately.

Explanation

To find the natural abundance of the copper-63 isotope, we can follow these steps:

### Step 1: Define Variables
Let the natural abundance of the isotope copper-63 be \( X \) (in percentage). Consequently, the natural abundance of the isotope copper-65 will be \( 100 - X \).

### Step 2: Set Up the Equation
The atomic weight of copper is given as \( 63.546 \) g/mol. The average atomic weight can be expressed using the natural abundances and atomic weights of the isotopes:
\[
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