Answer to Question #70719 in General Chemistry for dante Flamminio

Question

what are isotope notations and how are they found?

Answer 1

To specify one or another isotope from each other the subscripts and superscripts can be added to an element’s symbol to specify a particular isotope of the element and provide other important information. For example:
(_92^238)U
It is Uranium-238 notation where 238 is the mass number, 92 is the atomic number and U is a element’s symbol.
All isotopes of one element have the same atomic number, that is why it is often left away from the notation, for example Cerium-140:
(_ ^140)Ce
The isotopes was found around one hundred year ago by the chemists who was investigated radioactivity decay chain and found more than 40 species. He tried to separate them but found that a lot of them have the same atomic mass. Other scientist found variations between the atomic weight of lead from different mineral sources, attributable to variations in isotopic composition due to different radioactive origins. That is why have been proposed that several elements can occupy the same place in the periodic table. So, verbatim, the word isotope translates from Greek like “at the same place”.