You need to find the abundance. Then, multiply the abundance by 100, and add that to the mass for each isotope. Basically, for each isotope, take the percentage abundance and add it to the mass. Multiply each calculation of these together to get your average atomic mass,
The accepted model of an atom was made by Schrodinger, which is known as the quantum mechanical model. Unlike Bohr's model, electrons are represented as a cloud and not small balls orbiting the nucleus. There is no define path of electrons in Schrodinger's model, but it can predict the odds of where an electron is located.
The answer to your question is then Electrons form a cloud around the nucleus.
