Why isn't the atomic mass of most of the elements on the periodic table a whole number?

What pressure will be exerted by 0.675 moles of a gas at 25*c if it is in a 0.750-l container?

Plz choose one of the compounds from the table and explain how you know the numbers of atoms in your formula. is it possible for two different compounds to be made from the exact same two elements? why or why not? with a limited number of elements (less than 120 are known), does this mean we also have a small number of compounds or do we have a large number of compounds in this world?

What is i fracture in the crust called when land move up, down or sideways