Question

Do elements have a variable compostion?

Answer 1

No

Explanation:

Elements are pure substances BUT they can exist as isotopes which are different forms of the same atom; different in that they have a different number of neutrons.

Answer 2

Yes if you mean how they are composed
No if you mean what they are composed of

Explanation:

Elements are made of atoms with the same number of protons. The atoms can vary in their mass because of different isotopes having different numbers of neutrons. This is why the masses given on the periodic table are not whole numbers. Boron for example has approximately 80% Boron 11 ( 5 protons 6 neutrons) and approximately 20% Boron 10 ( 5 protons, 5 neutrons) .
The difference in the weight of the isotopes could be considered a difference in the composition but this doesn't affect the element being a pure substance or its chemical properties.

Elements can be composed or put together differently. Carbon for instance can be composed of Carbon atoms in a tetrahedral forming hard diamonds, in sheets `s^2P^2` forming graphite or in an amphorus form (soot) While all of these are made only of Carbon atoms they have different properties. A piece of coal may be pure carbon just like a diamond but it doesn't look as good in an engagement ring..

To sadly sound like Bill Clinton the answer depends on how you define composed.