What is the difference between community and ecosystem?
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Ecosystems are composed of all of the various living organisms, along with the abiotic factors that impact them. The difference between community and ecosystem in ecology is that the community only refers to living things within an ecosystem. A sample of soil can provide a small-scale example.:

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.