How do I find an isotope symbol and the number of neutrons in one atom?

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caledon | High School Teacher | (Level 3) Senior Educator

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There are several ways of denoting atomic masses; you simply have to memorize what information is being shown in each depiction, and interpret it accordingly. 

Isotopes can be thought of as the different masses possible for a given element. This is determined strictly by the number of neutrons present in the nucleus, since the atom is always identified by the number of protons, and the number of protons cannot change without changing the identity of the atom. Thus isotopes for a hydrogen atom, which has 1 protons, might be Hydrogen 1 (only the proton), Hydrogen 2 (1 proton 1 neutron), Hydrogen 3 (1 proton 2 neutrons) etc.

The shorthand way of writing isotopes is writing the name or symbol of the element, then a dash, then the mass number. For example, U-238 is a common isotope of uranium. The number of neutrons is almost never written; it would have to be determined mathematically, by subtracting the protons from the mass number.

Another way of seeing this information in a more complete form is the traditional textbook nuclear notation, which shows the element symbol in the middle, the mass as a superscript on the left, and the atomic number (protons) as a subscript on the bottom left. Again, the number of neutrons would have to be determined by subtracting the atomic number from the atomic mass.