An atom with an abnormal amount of neutrons is called a(n)_?

Elements are made up of atoms. Atoms consist of sub-atomic particles: protons, electrons and neutrons. The number of protons in an atom is equal to its atomic number, while the sum of neutrons and protons is the mass number or atomic mass. 

Not all the atoms of an element contain the equal number of neutrons. Some atoms contain more and some less. These atoms that contain an abnormal number of neutrons are called isotopes. Thus, an isotope will have the same atomic number as all other copies of the atom, but a different mass number. The average atomic mass of an element is thus dependent on the relative abundance of different isotopes of that element. Average atomic mass is the sum of atomic mass of each isotope times its relative abundance.

For example, carbon has two isotopes: C-12 and C-13, with relative abundances of 98.91% and 1.1%, respectively. The average atomic mass of carbon is 12 x 0.9891 + 13 x 0.011 = 12.012 amu (atomic mass unit).

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