How does the average atomic mass of an element differ from the mass of an individual atom of the same element?

In periodic tables, the relative atomic mass is usually not a whole number, even though the mass of both protons and neutrons is 1. This is due to isotopes, which are atoms of the same element, but they have different amounts of neutrons in their nuclei. So the relative mass of an individual atom is always a whole number, and the relative atomic mass is an average of all of the isotopes of the element.

answers: to make sure that all of the broken pieces of tablet were added to the water, to allow for fair comparison of all three trials, with different particle sizes to avoid experimental error

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.