Final answer:
When elements combine to form compounds, the resulting substance has different properties from the individual elements. Compounds are formed in fixed whole numbers proportions and have their own distinct chemical and physical properties, such as sodium chloride being safe to eat despite being made from reactive sodium and poisonous chlorine.
Step-by-step explanation:
When elements combine to form compounds, their properties indeed change. This is because a compound is a substance made up of atoms from two or more different elements joined together by chemical bonds. Atoms of one element differ in properties from atoms of all other elements, and when these atoms come together, they produce a compound with its own unique set of chemical and physical properties.
For example, sodium is a highly reactive metal and chlorine is a poisonous gas; however, when these two elements combine in a 1:1 ratio, they form sodium chloride (table salt), which is neither reactive nor poisonous and safe for consumption. This occurs because the atoms of different elements react with each other in a fixed whole numbers proportion to produce a new substance with different properties from the elements it is composed of.
Furthermore, in a chemical reaction, atoms are neither created nor destroyed; they are simply rearranged to form new compounds or mixtures. This means that in nature, where elements rarely occur in isolation, we find them mostly as part of compounds with precise compositions and definite proportions, like the aforementioned glucose molecule which always contains six carbon, twelve hydrogen, and six oxygen atoms in a fixed ratio.