The idea that a chemical element can actually be a mixture of atoms with different radioactive behavior and different atomic masses, but all with the same chemical properties, meant that one of the postulates would have to be changed basic principles of Dalton's atomic theory, namely, the postulate that the atoms of a pure element are equal in all respects.
According to Soddy, the atoms of a given element are identical only in chemical properties . The different species of physically different atoms that make up a particular element occupy the same place in the periodic table that is, they have the same atomic number Z . For this reason, Soddy called them isotopes of the element, from the Greek same-place referring to the fact that they occupy the same place in the periodic table.
Thus, uranium-238 ( 238 U) and uranium-234 ( 234 U) are isotopes of uranium ( 92 U); Lead-214 ( 214 Pb) and lead-206 ( 206 Pb) are isotopes of lead ( 82 Pb). They are chemically the same; they occupy the same place on the periodic table and have the same atomic number Z, which is usually written as a subscript. But they are physically different, because they have different atomic masses A, which is usually written as a superscript in atomic mass units .
With this idea in mind, chemical analysis soon showed that the many species of radioactive atoms in the radioactive series were isotopes of one or other of the last 11 natural elements of the periodic table, from lead (Z = 82) to uranium (Z = 92). For example, the second and fifth members of the uranium series (see table) were shown to be thorium isotopes, with Z = 90; members 8, 13, and 17 were found to be isotopes of polonium (Z = 84). The old names and symbols given to the members of the radioactive series after their discovery were substituted to represent both the chemical similarity and the physical difference between isotopes .
It is important to emphasize that when writing the symbol of a nuclide  the atomic mass is always given as a positive natural number (eg U-238); but consulting any periodic table will give us a value for the atomic mass of an element that is a rational number (for uranium, 238.02891). This is because the atomic mass given in the periodic table refers to the mass of the element in its natural state, which is a mixture of the various natural isotopes of the element, with their respective electrons . The atomic mass of the natural element is therefore an average of the atomic masses of the individual isotopes, weighted according to their abundance relative to those of the other isotopes.
 This is not entirely true in the lighter elements, but it can be accepted as a general rule.
 Any kind of atom, called nuclide is represented as, for example, 234 90 Th and 230 90 Th for two of the isotopes of thorium. The subscript (90 in both cases for thorium) is the atomic number Z, the number assigned by the place in the periodic table; the superscript (234 or 230) is the mass number A, the approximate atomic mass in atomic mass units.
 For example, uranium X 1 and ionium became thorium-234 and thorium-230.