How is the atomic mass of an atom determined and why are electrons not used in the calculation?

Why does earth rotate? because earth is formed from cold gases collapsing due to gravity because the matter in the nebula that formed earth was spinning because earth forms more than 99% of the mass of the solar system because the hydrogen atoms inside the nebula fused to form helium

What is a difference between a mixture of elements and a mixture of compounds

What is the measured amount of a product obtained from a chemical reaction?