We will see that the volume of the unit cell is 144,070,699.06 pm^3
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How to get the volume of a body-centered cubic unit cell?</h3>
In a body-centered cubic unit cell, the side length of the cube is given as:

Where R is the radius of the atom.
And the volume of a cube is the side length cubed, then we can see that the volume of our cube will be:

Solving that we get:

This is the approximated volume of the unit cell.
If you want to learn more about unit cell structures, you can read:
brainly.com/question/13110055
The mass percentage is 15.1465%.
Both aluminum and chlorine have known charges, which are +3 and -1 respectively. To make them cancel each other out in charge, you would need 3 chlorine and for one aluminum, therefore

would be correct
Yes because the protons are the same but the neutrons change