Do the atoms in a mixture have a set ratio?

Answer 1

Atoms can interact in different manners. Compounds can have the same ratios of atoms combined in different ways producing different properties (cis and trans forms of compounds come to mind).

Additionally, many compounds consist of the same elements but in different ratios. Compare Carbon dioxide, a inert waste product of respiration, to carbon monoxide, a gas deadly to humans. Both use carbon and oxygen, but CO2 has an extra oxygen molecule, greatly changing its properties.

So in short, no. Atoms can combine in multiple ratios, and even compounds of equal ratios can come together in different manners to produce different compounds.

Answer 2

Yes: Part of the definition of a compound is that any size sample of it contains the same elements in the same atomic ratios.

Answer 3

To form different atoms.