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Atomic Mass Unit
An atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom. As carbon-12 has 12 nucleons, one atomic mass unit roughly corresponds to the mass of one nucleon. Nucleon is the general term for a proton or neutron. They are grouped together in this way because they have almost identical masses. By definition, carbon has 12 atomic mass units.
Convenience
Atomic mass is a convenient way of measuring and recording the masses of atoms. Atomic masses are relative masses. This means that they have no unit associated with them. For example, the atomic mass of hydrogen is one. Atoms are so small that their weight in the usual units, kilograms, would be cumbersome to use. A hydrogen atom weighs 1.67 x 10^-27 kilograms, which written in full is a decimal point followed by 26 zeroes before the digits 167. A relative mass solves this problem.
Isotopes
Atomic mass units are used to find the average atomic mass for each element. This may not be a round number if an element has isotopes. For example, chlorine has isotopes so it has 35.5 atomic mass units. An isotope is an atom that has the usual number of protons for that particular element but a different number of neutrons. Isotopes of the same element have different atomic masses. To find the average atomic mass for that element scientists take into account the atomic masses and natural abundances of the isotopes.
Atomic Structure
The number of atomic mass units an atom has tells you the number of nucleons in the nucleus of that atom. You can use this information to tell how big the atom is and to estimate the number of electrons in the atom. Atoms comprise nucleons and electrons. The number of electrons is always equal to the number of protons in a neutral atom and the number of protons is usually approximately half the total number of nucleons. Using this information, you can estimate the number of electrons in an atom if you have the number of atomic mass units.