Final answer:
An element has a fixed, uniform composition due to the law of definite proportions, which ensures that a chemical compound contains the same elements in the same proportions by mass in every sample.
Step-by-step explanation:
The reason an element has a fixed, uniform composition is because of the law of definite proportions, also known as the law of constant composition. This law states that a given chemical compound always contains its component elements in the same proportions by mass.
For instance, water (H2O) will always consist of hydrogen and oxygen in a mass ratio of approximately 1:8, reflecting the fact that each molecule is made up of two hydrogen atoms and one oxygen atom. Regardless of the source or method of preparation, these proportional relationships remain consistent across all samples of a pure compound.
Furthermore, the atomic mass of an element, which accounts for the average mass of its naturally occurring isotopes, also plays a role in the fixed composition of compounds formed. So, while an element might have isotopes with different masses, the atomic mass used in composition calculations is a weighted average that reflects their natural abundance, thereby ensuring a consistent composition when elements combine to form compounds.