When elements combine to form compounds what happens to the properties of the original elements?

The properties of the original elements are completely changed.

For example, in the compound called "iron (II) sulphide", it is composed of iron and sulphur, that are chemically combined together. The element iron is attracted to magnets. Meanwhile, iron (II) sulphide is not attracted to magnets.

Another example is where sulphur has a yellow colour, but iron (II) sulphide has a brownish colour. As we can see, even the physical properties (colours) are completely different.

Therefore, the properties of the original elements are completely changed when they're combined into compounds.