How many atoms and elements in mg?

In 1 milligram (mg) of a substance, the number of atoms depends on the substance's molecular or atomic weight. For example, if you consider water (H₂O), which has a molar mass of about 18 g/mol, 1 mg contains approximately 3.34 × 10²² molecules, which equates to about 10²³ atoms. The number of different elements in that milligram would depend on the composition of the substance; pure elements contain only one type of atom, while compounds contain multiple types.