Do the properties of elements change when combined to form compounds?

The chemical properties of atoms are significantly changed when they form compounds.

When elements combine to form compounds it is called a chemical reaction.

The compound is then a collection of molecules and each molecule has in it atoms from the elements used in the formation.

The atoms in the molecule are still the same atoms as one started with, but the arrangement of the electrons in the atoms has changed. It is the electronic properties of atoms and molecules that really determine the properties they have and which chemical reactions occur and which do not.

The chemical properties of an atom or molecule are determined by the arrangement of the electrons of that atom or molecule. Once atoms have gone through a chemical reaction, the electronic structure is changed. The atoms are the same identity, but the properties of the atoms have been changed by the chemical reaction.